Agronomy of Irrigated Tea in Low Elevation Growing Areas of Sri Lanka
Shyamantha Nelum Bandara
Thesis submitted for the Degree of Doctor of Philosophy
School of Agriculture, Food & Wine
The University of Adelaide
2011 November Declaration
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ii Abstract
The low elevation tea growing region of Sri Lanka produces 60% of the national production and the most limiting factor affecting productivity in this region is the short but intense inter- monsoonal dry period. Irrigation, along with other methods like cultivar selection, is likely to be the best way to mitigate the impact of drought and generally improve productivity. However the dearth of knowledge of the response, and financial feasibility, of low elevation tea to irrigation retards its adoption. Therefore the Tea Research Institute of Sri Lanka (TRISL) funded a series of trials from 2006 to 2009 with the aim to understand the agronomic and physiological characteristics of low-grown tea in response to irrigation, and the financial practicality of introducing irrigation in the region. This aim was achieved through the following objectives to evaluate: (1) the changes in physiology and yield as affected by the drought and recovery by irrigation; (2) the environmental parameters that govern the water use of tea in low elevation growing areas; (Li, Yang et al.) the physiological and yield responses to different micro-irrigation methods; (4) the effect of soil moisture limitation on young tea plant growth and physiology; and (5) the practical financial feasibility of irrigating tea in the region.
Field trials were conducted at the TRISL’s low-country research station at Ratnapura, Sri Lanka using mainly drip, and to a lesser extent sprinkler, irrigation to evaluate the response using different tea cultivars. Irrigation was only applied during the inter-monsoonal dry period of the January to March. Glasshouse experiments with different water stress levels were conducted specially for evaluating the short term drought effect on the post-nursery growth stages, which is critical for long term productivity.
The key results to the research objectives were as follows:
1] Key physiological process underpinning yield (Pn, El, gs, Ψ) were depressed even under drip irrigation. Nevertheless, the drip-irrigated crop had an annual yield increase of 21% (p<0.001) over the rain-fed control. Higher yields were observed even during non-irrigated periods as tea plants irrigated from establishment had better root and canopy structure. Strong cultivar differences were observed. The benchmark high-yielding, but drought-susceptible, cultivar TRI 2023 yielded 16% more under drip irrigation than the benchmark drought- tolerant cultivar TRI 3025. Evaluation of more recently developed cultivars revealed that irrigation will be a more effective drought mitigation strategy than selection of drought- tolerant cultivars. Among a group of 5 leading new cultivars only TRI 4049 showed physiological characters preferred for irrigation.
iii 2] The environmental parameters measured were rainfall, ambient temperature, solar radiation, vapour pressure deficit and potential evapotranspiration. Temperature is the dominant parameter driving transpiration in the wet season (r2=0.62, P=<0.0001) and suppressing all the other physiological processes in the dry season. Average daily transpiration was 2.3(±0.3) and 1.3(±0.2) mm in wet and dry seasons respectively for rain-fed plants. Irrigated plants showed >100% increase in transpiration than rain-fed plants in dry season.
3] During a very wet year (2008) sprinkler irrigation resulted in 15% higher assimilation rates and produced 6% higher yield increase than drip irrigation on TRI 2023. This was due to the maintenance of 2-40C lower leaf temperature in midday hours than under drip irrigation. Between 6 to 8% higher water use efficiency was achieved under sprinkler than drip irrigation.
4] Glasshouse and field experiments emphasised the importance of irrigation for the establishment of young (<1 year) tea plants. Decreasing the irrigation frequency from daily irrigation to a 4 day interval reduced stem growth by 20% and root growth by 46%. Deficit irrigation of 50% for 20 days, reduced the stem and root growth by 34% and 45% respectively. In the field, short dry spells (<3 weeks), reduced LAI and root growth. Establishing plants on mounds, as opposed to conventional flat ground preparation, enhanced the effect of irrigation to facilitate the growth of more root (12 and 8% increase in fine and coarse root) and leaf (60% increase in leaf area).
5] The on-farm feasibility of irrigating tea in low elevation areas was evaluated using Net Present Value (NPV) analysis. The analysis was based on 10 years (1999-2009) of yield data of TRI 2023 and TRI 3025 under drip irrigation. Long term yield data was not available for sprinkler irrigation. Sensitivity analysis was applied using variables of wage rate, green leaf price and discount rate (2007 as base year). Under these assumptions it is highly feasible to establish TRI 2023 under drip irrigation but not TRI 3025 (NPVs Rs 391779 and Rs -57898 respectively. The threshold levels for feasibility of TRI 2023 are a discount rate of 15%, or a green leaf price below Rs48. Irrigating TRI 3025 is economically viable only if green leaf price reaches Rs 65. Cultivar selection is a key factor in the financial success of tea irrigation. The higher water demand of a sprinkler system may limit its practical application.
The study confirms that irrigation can be used as viable option to improve productivity and to mitigate short term drought effects in low elevation tea growing areas. It is crucial to irrigate from establishment of young plants as early growth determines later yield potential. Cultivars suitable for irrigation are those that are inherently faster growing and less drought tolerant.
iv Further studies are encouraged comparing sprinkler and drip irrigation, soil moisture based irrigation scheduling and different water application rates under drip irrigation.
v
vi Table of Contents Thesis Declaration ...... ii Abstract ...... iii List of Tables ...... xii List of Figures ...... xiii List of Acronyms ...... xv Acknowledgement ...... xvi Chapter 1 Study Overview ...... 17 1.0 Introduction ...... 17 1.2 The Aim ...... 20 1.3 Structure of the thesis ...... 21 Chapter 2 Sri Lanka Tea Industry–History, Production and Scope for Irrigated Cultivation .. 25 2.1 Introduction ...... 25 2.2 The Emergence of “Ceylon” Tea ...... 25 2.2 Cultivation of Tea ...... 25 2.2.1 Land suitability ...... 26 2.2.2 Land preparation and field planting ...... 26 2.2.2 Harvesting ...... 26 2.2.3 Other agronomic operations ...... 27 2.3 Tea Growing Areas of Sri Lanka and Productivity Constraints ...... 28 2.3.1 General climate of Sri Lanka ...... 28 2.3.2 Tea growing area classification ...... 30 2.3.3 Adapting technology to mitigate drought ...... 32 2.4 Conclusion ...... 34 Chapter 3 The Botany and Physiology of Tea and its Water Relations ...... 35 3.1 Introduction ...... 35 3.2 Botany of Tea Plant ...... 36 3.21 Origin and distribution ...... 36 3.2.2 Varietal difference ...... 37 3.2.3 Growth pattern ...... 39 3.3 Physiology of Tea Plant ...... 40 3.3.1 Photosynthesis ...... 40 3.3.2 Plant water relations ...... 42 3.4 Climatic Requirement ...... 43 3.4.1 Air temperature ...... 43 3.4.2 Soil temperature ...... 44 3.4.3 Vapor pressure deficit ...... 45 3.4.4 Rainfall ...... 45 3.5 Drought in Tea Cultivations ...... 46 3.5.1 Occurrence of the drought ...... 46 3.5.2 Drought in Sri Lankan tea plantations ...... 47 3.5.3 Tea plant response to drought ...... 49 3.5.4 Drought mitigation ...... 50 3.5.5 Impact of climate change in low elevation tea sector ...... 51 3.6 Irrigation in Tea Plantations ...... 52 3.6.1 Yield response to irrigation ...... 53 3.6.2 Effect on tea physiology ...... 54 3.6.3 Varietal difference in response to irrigation ...... 54 3.6.4 Growth response to irrigation ...... 55 3.6.5 Economic evaluation of tea irrigation ...... 55 3.7 Conclusion ...... 56 Chapter 4 Experimental Site Description and Location ...... 57 4.1 Introduction ...... 57 vii 4.2 Experimental Overview ...... 57 4.3 Field Experiment Site and Location ...... 59 4.3.1 Site and soil description ...... 59 4.3.2 Climate ...... 62 4.3.2 Rainfall pattern ...... 63 4.3.3 Seasonal aridity ...... 66 4.4 Significance of short rain-free periods in creating water stress Error! Bookmark not defined. 4.4.1 Daily rainfall intensity ...... 67 4.4.2 Water stress coefficient ...... 68 4.5 Conclusion ...... 70 Chapter 5 Physiology, Water Use and Yield in Low Elevation Tea–Analyzing with Special Reference to Dry Period of the Year ...... 71 5.1 Introduction ...... 71 5.2 Experiment 1: Physiological and Yield Performance of Two Contrasting Tea Cultivars in Response to Irrigation ...... 74 5.2.1 Introduction ...... 74 5.2.2 Method ...... 74 5.2.2.1 Experimental design and field layout ...... 74 5.2.2.2 Tea cultivar, planting and cultural practices ...... 75 5.2.2.2 Water application and moisture measurement ...... 76 5.2.2.3 Water potential measurements ...... 77 5.2.2.3 Gas exchange measurements ...... 77 5.2.2.4 Harvesting ...... 78 5.2.2.5 Stem canker infection...... 78 5.2.3 Results...... 78 5.2.3.1 Meteorological conditions during the study period ...... 78 5.2.3.2 Leaf water potential ...... 80 5.2.3.3 Photosynthesis ...... 81 5.2.3.4 Stomatal conductance ...... 83 5.2.3.5 Leaf transpiration ...... 84 5.2.3.6 Diurnal variation of photosynthesis, transpiration and leaf temperature ...... 84 5.2.3.7 Light response ...... 87 5.2.3.8 Response to ambient temperature ...... 89 5.2.3.9 Annual yield variation ...... 90 5.2.3.10 Yield response to climatic factors: ...... 92 5.2.3.11 Stem canker infection...... 93 5.2.4 Discussion ...... 94 5.2.5 Summary of Results of Experiment 1 ...... 97 5.3 Experiment 2: Water Use of Tea under Rain-fed and Irrigated Conditions ...... 98 5.3.1 Introduction ...... 98 5.3.2 Materials and Method ...... 98 5.3.2.1 General experiment details ...... 98 5.3.2.2 Instrumentation ...... 99 5.3.2.3 Meteorological data...... 100 5.3.2.4 Soil moisture ...... 100 5.3.3 Results...... 100 5.3.3.1 Transpiration during wet season ...... 100 5.3.3.1.1 Dry matter production in the wet season ...... 102 5.3.3.1.2 Response to environmental variables: ...... 103 5.3.3.2 Transpiration of during dry season ...... 104 5.3.3.2.1 Climate during the study period ...... 104 5.3.3.2.2 Transpiration rate of irrigated and rain-fed tea ...... 105 5.3.3.2.3 Dry matter production during dry season ...... 106 viii 5.3.3.2.4 Diurnal variation in transpiration ...... 106 5.3.4 Discussion ...... 107 5.3.4.1 Transpiration and dry matter production ...... 108 5.3.5 Summary of Results of Experiment 2 ...... 110 5.4 Experiment 3: Physiological Response of New Cultivars to Drought ...... 111 5.4.1 Introduction ...... 111 5.4.2 Method ...... 111 5.4.3 Results ...... 112 5.4.3.1 Climate during the study period ...... 112 5.4.3.2 Soil moisture ...... 112 5.4.3.3 Leaf Transpiration ...... 113 5.4.3.4 Photosynthesis ...... 114 5.4.3.5 Leaf temperature ...... 115 5.4.3.6 Water use efficiency ...... 116 5.4.4 Discussion ...... 118 5.4.4 Summary of Results of Experiment 3 ...... 118 5.5 Conclusion ...... 118 Chapter 6 Experiment 4: Evaluation of Irrigation Technology ...... 121 6.1 Introduction ...... 121 6.2 Materials and Method ...... 121 6.2.1 Irrigation application ...... 122 6.2.2 Harvesting and yield factors ...... 123 6.2.3 Physiological measurements ...... 123 6.2.4 Growth measurements ...... 124 6.2.5 Root measurements ...... 124 6.3 Results ...... 124 6.3.1 Weather during trial period ...... 124 6.3.2 Soil moisture ...... 126 6.3.3 Photosynthesis ...... 127 6.3.4 Stomatal conductance ...... 128 6.3.5 Transpiration ...... 129 6.3.6 Diurnal variation of leaf physiology ...... 130 6.3.7 Irrigation effect on shoot weight ...... 133 6.3.8 Shoot extension rate and shoot count ...... 134 6.3.9 Tea yield ...... 135 6.3.11 Irrigation water use efficiency ...... 138 6.3.12 Total plant growth ...... 138 6.3.14 Root Density ...... 139 6.4 Discussion ...... 140 6.4.1 Relationship between physiology and irrigation method ...... 142 6.4.2 Yield response to environmental variables during dry season 2009 ...... 143 6.4.3 Yield response to irrigation ...... 144 6.4.4 Soil moisture variation and plant growth ...... 145 6.5 Summary of results ...... 145 6.6 Conclusion ...... 146 Chapter 7 Effect of Short Term Water Stress and Raised Beds on Young Tea Plants ...... 147 7.1 Introduction: ...... 147 7.2 Experiment 5: Effect of Water Stress Duration on Young Tea Plant Growth ...... 150 7.2.1 Introduction ...... 150 7.2.2 Method ...... 150 7.2.2.1 Plant material and water application ...... 150 7.2.2.2 Measurements ...... 152 7.2.3 Results ...... 152 7.2.3.1 Soil moisture ...... 152 ix 7.2.3.2 Plant growth response ...... 152 7.2.3.3 Stem and root growth ...... 153 7.2.4 Discussion ...... 154 7.2.5 Summary of Results ...... 155 7.3 Experiment 6: Effect of Partial Irrigation on Young Tea Growth ...... 156 7.3.1 Introduction ...... 156 7.3.2 Method ...... 156 7.3.3 Results...... 157 7.3.3.1 Plant growth response ...... 157 7.3.3.2 Dry matter production and partition...... 158 7.3.4 Discussion ...... 158 7.3.5 Summary of Results ...... 159 7.4 Experiment 7: Effect of Irrigation and Raised Bed on Young Tea Growth ...... 160 7.4.1 Introduction ...... 160 7.4.2 Method ...... 160 7.4.3 Results...... 162 7.4.3.1 Rainfall and soil moisture during the study ...... 162 7.4.3.2 Soil bulk density...... 163 7.4.3.2 Plant height ...... 164 7.4.3.3 Growth of branch shoots ...... 165 7.4.3.4 Leaf area index ...... 166 7.4.3.5 Development of stem parts and roots ...... 167 7.4.4 Discussion ...... 168 7.4.5 Summary of results ...... 169 7.5 Conclusion ...... 169 Chapter 8 Financial Feasibility of Drip Irrigation in Low Elevation Tea Growing Area ...... 171 8.1 Introduction ...... 171 8.2 Methodology ...... 172 8.2.1 Analytical methods ...... 172 8.2.2 Irrigation system and cost estimation ...... 173 8.2.3 Green leaf price and wage rate ...... 174 8.3 Results and Discussion ...... 175 8.3.1 Drip irrigation system cost...... 175 8.3.2 Green leaf yield ...... 176 8.3.4 Net present value of installing a drip irrigation system ...... 176 8.3.5 Internal rate of return (IRR) ...... 178 8.3.6 Variation in capital cost ...... 178 8.3.7 Sensitivity to variation in green leaf price and wage rate ...... 179 8.4 Summary of Results ...... 181 8.5 Conclusion ...... 181 Chapter 9 Discussion ...... 183 9.1 Introduction ...... 183 9.2 Tea Plant Response to Water Stress and Irrigation ...... 187 9.2.1 Physiological response ...... 187 9.2.2 Yield response ...... 189 9.2.3 Tea Cultivars for irrigation and drought mitigation ...... 191 9.3 Water use of tea in low elevation area ...... 192 9.4 Irrigation System Selection ...... 193 9.5 Effect on Young Tea Growth ...... 195 9.5.1 Effect of water stress interval on young tea ...... 196 9.5.2 Effect of partial irrigation on young tea ...... 197 9.5.3 Raised beds to enhance irrigation in young tea ...... 198 9.5.4 Final comment on irrigation in young tea plants ...... 199 9.6 Further agronomic considerations ...... 199 x 9.6.1 Carry-over effects into the wet season ...... 199 9.6.2 Irrigation scheduling ...... 200 9.6.3 Shade trees ...... 201 9.7 Financial Evaluation ...... 202 9.8 Summary ...... 204 Chapter 10 Conclusion ...... 205 Appendix 1: Rain Partitioning in a Low Elevation Tea Field ...... 209 Appendix 2: Tea Plant Behavior under Water Stress on Different Temperature Regimes .... 217 References ...... 229
xi List of Tables
Table 2.1 Number of small holdings and extent in three census ...... 31 Table 2.2 Results of drought awareness survey ...... 34 Table 3.1 Characters of some popular tea cultivars cultivated in Sri Lanka ...... 38 Table 3.2 Characteristics of drought period in some tea areas ...... 48 Table 3.3 Characteristics of dry season and wet season in Ratnapura, Sri Lanka ...... 49 Table 4.1 An overview of the experimental parameters ...... 58 Table 5.1 Major soil physical and chemical properties of Experiment 1 site ...... 75 Table 5.2 Water application rates of two cultivars ...... 77 Table 5.3 Meteorological condition during first 10 weeks of 2007 ...... 79 Table 5.4 Total tea yield for year 2007 ...... 92 Table 5.5 Relationship between micrometeorological parameters and yield ...... 93 Table 5.6 Climate, soil moisture content, transpiration and crop coefficient TRI 2023 ...... 106 Table 5.7 Transpiration efficiency of TRI 2023 ...... 108 Table 5.8 Climate during February–March 2009 ...... 113 Table 5.9 Leaf temperature at mid-day ...... 116 Table 5.10 Relationship between maximum temperature and photosynthetic rate ...... 117 Table 6.1 Number of irrigation days, amount of water applied and rainfall ...... 123 Table 6.2 Total made tea production according to irrigation treatment ...... 137 Table 6.3 Water use productivity during 2008 (Jan-Feb) and 2009 (Jan-Mar) ...... 138 Table 6.4 Irrigation water use efficiency during 2008(Jan-Feb) and 2009(Jan-Mar) ...... 138 Table 7.1 Major soil chemical constituents of top 20cm soil layer in Field no 01 ...... 151 Table 7.2 Water application rate and total amount of water applied-Experiment 5 ...... 152 Table 7.3 Average volumetric moisture-Experiment 5 ...... 153 Table 7.4 Plant base girth, height, leaf number and branches-Experiment 5 ...... 153 Table 7.5 Plant stem and root weight and root:shoot ratio-Experiment 5 ...... 154 Table 7.6 Plant growth characters-Experiment 6 ...... 158 Table 7.7 Plant root and stem weight-Experiment 6 ...... 158 Table 7.8 New branch growth during short dry spell 2008 ...... 166 Table 7.9 Dry weight of main stem and twigs ...... 167 Table 7.10 Fine and coarse root dry weight-Experiment 7 ...... 168 Table 8.1 Land extend distribution among tea small holder farmers ...... 171 Table 8.2 Investment cost of drip irrigation system ...... 175 Table 8.3 Total annual operational cost ...... 175 Table 8.4 Yield response of two tea cultivars to drip irrigation ...... 176 Table 8.5 Net Present Value of installing drip irrigation system ...... 177 Table 8.6 Sensitive analysis of Net Present Value ...... 178 Table 9.1 Study objectives and relevant hypothesis ...... 184 Table 9.2 Experimental summary of results ...... 185
xii List of Figures
Figure 1.1 Use of portable gun sprinkler in low elevation tea field ...... 19 Figure 2.1 Agro-climatic zones of Sri Lanka with district boundaries...... 29 Figure 2.2 Total, high , medium and low elevation tea extent ...... 31 Figure 2.3 Total tea production according to elevation ...... 32 Figure 2.4 Farmer using small lawn sprinkler in Urubokka, Sri Lanka ...... 33 Figure 3.1 Development of tea shoot ...... 40 Figure 4.1 Geographical distribution of major plantation crops in Sri Lanka ...... 60 Figure 4.2 Aerial view of main research field at St. Joachim Estate, Ratnapura ...... 61 Figure 4.3 View of sprinkler operation at Experiment 4 ...... 61 Figure 4.4 View of drip irrigated and rain-fed TRI 3025 ...... 62 Figure 4.5 Monthly climate average of St.Joachim Estate, Ratnapura ...... 64 Figure 4.6 Annual rainfall, Ratnapura ...... 65 Figure 4.7 Percentile of annual rainfall distribution ...... 65 Figure 4.8 Monthly variation of rainfall Ratnapura, ...... 66 Figure 4.9 Monthly tea crop water requirement, total and effective rainfall ...... 67 Figure 4.10 Average rainless days and frequency of drought duration ...... 68 Figure 4.11 Water stress coefficient for young tea ...... 70 Figure 5.1 Average rainfall and potential evapotranspiration in 2007 ...... 79 Figure 5.2 Maximum potential soil water deficit from January to March, 2007 ...... 80 Figure 5.3 Leaf water potential of two tea cultivars-Experiment 1 ...... 81 Figure 5.4 Mid-day photosynthetic rate-Experiment 1 ...... 82 Figure 5.5 Mid-day stomatal conductance-Experiment 1 ...... 83 Figure 5.6 Leaf transpiration rate-Experiment Experiment 1 ...... 85 Figure 5.7 Diurnal air temperature, and solar radiation and physiological parameters ...... 86 Figure 5.8 Diurnal air and leaf temperature-Experiment 1 ...... 87 Figure 5.9 Light response of photosynthesis-Experiment 1 ...... 89 Figure 5.10 Relationship of maximum air temperature and photosynthesis ...... 90 Figure 5.11 Average made tea yield during 2007 ...... 91 Figure 5.12 Incidence of stem canker ...... 94 Figure 5.13 Sap flow sensor fixed to a drip irrigated TRI 2023 ...... 99 Figure 5.14 Rainfall and soil moisture-Experiment 2 (Aug-Nov, 2008) ...... 101 Figure 5.15 Transpiration and dry matter production-Experiment 2 (Aug-Nov, 2008) ...... 103 Figure 5.16 Relationships of transpiration and environmental parameters ...... 104 Figure 5.17 Rainfall, soil moisture and transpiration-Experiment 2 (Feb-Mar, 2009) ...... 105 Figure 5.18 Diurnal transpiration-Experiment 2 ...... 107 Figure 5.19 Rainfall and soil moisture-Experiment 3(Feb-March, 2009) ...... 113 Figure 5.20 Water use of efficiency-Experiment 3 ...... 117 Figure 6.1 Rainfall during January, 2008 to March, 2009 ...... 125 Figure 6.2 Daily rainfall and potential evapotranspiration (Janury-February, 2008) ...... 126 Figure 6.3 Monthly soil moisture-Experiment 4 ...... 127 Figure 6.4 Leaf photosynthesis vs irrigation method ...... 128 Figure 6.5 Stomatal conductance vs irrigation method ...... 129 Figure 6.6 Leaf transpiration vs irrigation method ...... 129 Figure 6.7 Diurnal climate and leaf physiology-Experiment 4 ...... 131 Figure 6.8 Diurnal air and leaf temperature-Experiment 4 ...... 132
xiii Figure 6.9 Shoot weight according to irrigation method ...... 134 Figure 6.10 Yield factors vs irrigation method ...... 135 Figure 6.11 Tea yield vs irrigation method...... 136 Figure 6.12 Plant growth vs irrigation method ...... 139 Figure 6.13 Root density vs irrigation method ...... 140 Figure 6.14 Maximum temperature vs photosynthesis-Experiment 4 ...... 143 Figure 6.15 Relationship of yield and climate-Experiment 4 ...... 144 Figure 7.1 Rainfall and potential evapotranspiration-Experiment 7 ...... 162 Figure 7.2 Soil water deficit-Experiment 7 ...... 163 Figure 7.3 Soil bulk density-Experiment 7 ...... 164 Figure 7.4 Plant height-Experiment 7 ...... 165 Figure 7.5 Leaf area index-Experiment 7 ...... 167 Figure 8.1 Variation of Net Present Value ...... 180 Figure 9.1 Sprinkler irrigation in commercial tea field ...... 197 Figure 9.2 Soil disturbance due to rain ...... 202
xiv List of Acronyms
Ψ Leaf water potential (MPa) Φ Quantum efficiency (µmol (PAR)-1) ADB Asian Development Bank
Dr root zone water depletion (mm) E Transpiration (mm/day) -2 -1 El Instantaneous leaf transpiration (mmol H2O m s )
ET0 Potential evapotranspiration (mm/day) -2 -1 gs Stomatal conductance (mol H2O m s ) I Incident light intensity (µmolm-2s-1) IRR Internal Rate of Return (%)
IRRi Irrigation water applied (mm) IWUE Irrigation water use efficiency (kg/ha/mm)
Kc Crop coefficient
Ks Water stress coefficient LCLWT Low country live wood termite (Glyptotermes dilatatus) MARR Minimum Attractive Rate of Return (%) N Sunshine hours NPV Net Present Value PAR Photosynthetically active radiation (Wm-2) -2 -1 Pn Photosynthesis (µmol CO2 m s )
Rd Dark respiration RF Rainfall (mm) Rn Solar radiation (MJm-2) SHB Shot hole borer (Xyleborus fornicatus) SLTB Sri Lanka Tea Board 0 Tavg Average air temperature ( C) 0 Tmax Maximum air temperature ( C) 0 Tmin Minimum air temperature ( C) TAW Total available water in root zone (mm) TE Transpiration efficiency (g/mm) TRISL Tea Research Institute of Sri Lanka TSHDA Tea Small Holder Development Authority
Wi Instantaneous water use efficiency VPD Vapor pressure deficit (kPa)
xv Acknowledgement
I would like to thank the Government of the Republic of Sri Lanka for supporting this research. In particular I would like to thank the Hon. Anura Yapa, Former Minister of Plantation Industries and Dr. Sunil Jayasekara, former Chairman TRISL who activated the scholarship. I wish to thank Dr. S.S.B.D.G. Jayawardana, Chairman and Research Board for continuing the scholarship, and Dr. I.S.B. Abeysighe, Director and Dr. K.G. Premathilka, Head/Agronomy Division, for their logistic and administrative support.
It is the great effort of my team of supervisors who ultimately delivered the product. Dr. Ian Nuberg with his vast knowledge ranging from highlands of Sri Lanka to the perfect tea cup, guided me well, and with great patience, particularly through the irrationalities and subtleties of the English language. I also thank Prof. Janendra de Costa who was always at my rescue during long field works in Sri Lanka; Dr. A. Ananthacumarasmamy, former scientist at TRISL helped me with his resources and expertise during field work; and Assoc Prof. Glenn McDonald for guiding me through critical points. I especially appreciate the moral support of Dr. Annie McNeill. She, together with Murray, was once offered fee accommodation for my rescue. I appreciate kind advices of Assoc Prof. Gurjeet Gill as well. A very special thank should be delivered to Ms Jane Copeland, University of Adelaide, for advising effectively how to recover from disastrous situations. Dr. John Golding, of the Gosford Horticultural Institute NSW, is also acknowledged for assistance with quality analysis of tea in a parallel experiment.
Without the kind patronage of Mr. Somapala Jayasinghe and Mr.Sunil Fernando of Hettipola, Mr.Shiyabdeen of Asanakotuwa the scholarship would simply not have materialized. Great thanks are also due to Mr.Palitha Jayasinghe (Dudley ayya)Ja of Chilaw High Court who voluntarily assisted in processing the bond agreement for the scholarship. Mr.K.G.Piyasena and Ms.Rohini Dissanayake, TRISL at Talawakalle were very helpful in administration and financial matters.
Dr. M.A.Wijeratna and Staff-TRILCS and Manager and Staff-St. Joachim Estate, Ratnapura, were always helpful during the hard field work time. Specially, DWV, Padmini, Noel, Shantha, Jaliya, Samanthi, Mahinda and all others who were at my call. Nevil B, Wasantha M and Randika gave much needed technical solutions. I am indebted specially to the field staff at Ratnapura: Suresh, Sanju, Anura, Siva, ChandraKumar, Devaraj and all who greatly assisted me in field even during mid night.
During my stay at Adelaide, my friends at Roseworthy Campus in South Australia – Ben, Mick, Chris, Hugh and Malinee assisted me in employing in their own research projects. In Adelaide, my friends Laxman, Lakshitha, Dr. Senaka, Nilmini, Pryantha, Rasika, gave much needed encouragement.
Finally, but most importantly, my two kids, Pasindu and Sandani and loving Manju, endured great suffering during my involvement in the research work. Manju and kids endured two long years isolation when I was in Australia. In such instances, I highly appreciate the assistance provided by both my parents and Manju’s parents.
xvi Chapter 1
Study Overview
1.0 Introduction
Tea cultivation is the most important plantation industry for Sri Lanka, which is ranked among the top three tea export nations in the world. Annual tea production of the country is sensitive to fluctuations in weather patterns. For it to maintain this position Sri Lanka needs to adapt to changes in climate and market. While world tea production is largely rain-fed, some countries including Sri Lanka, are attempting to use irrigation technology to maintain their competitive edge.
Sri Lanka’s total production of made tea was 291million Kg in 2009 while in 2010 it increased to 329 million Kg, entirely due to favourable weather (SLTB 2011). Total export income generated through tea export surpassed US $ 1.37 billion in 2010 (Anon. 2011). As such it contributes to 1.1% of Sri Lanka’s Gross Domestic Production of US $ 49.5 billion (Central Bank 2011) and provides direct employment for 220,000 people and their close dependants of nearly a million people (Feizal 2009). This is about 5% of the population (Central Bank 2011). Tea is grown over 222,000 hectares and the industry is broadly divided into high-grown (>1200m.a.s.l), mid-grown (600-1200m.a.s.l) and low-grown (<600m.a.s.l) areas, depending on the altitude of the production range. It is also divided into a plantation sector and a small-holder sector, based on the ownership of the land. The entire southern mountainous wet zone region of the Sri Lanka largely depends on low-grown tea as an employment provider and income generator. This region contributes 60% of the total tea production (Anon 2009).
The tea industry faces many uncertainties over its survival and sustainability in the face of the changing global climate, productivity decline in mountainous agriculture lands and shrinking worker population (Illukpitiya, Shanmugaratnam et al. 2004). Productivity decline is mainly related with decline in fertility and ageing plant population (De Costa and Sangakkara 2006).
Global climate change is increasingly threatening the survival of present agriculture systems. Compared with other perennial crops in Sri Lanka tea is particularly vulnerable to climate change, mainly due to the nature of its physiology and cultivation pattern (Eriyagama, Smakhtin et al. 2010). It is predicted under a medium global emissions scenario that the mean temperature during the northeast monsoon and southwest seasons will increase about 2.9°C and 2.5°C respectively, over the baseline by the year 2100 (IPCC 2001). More frequent and more severe drought occurrences are also anticipated as the result of climate change (Pandey,
17 Gupta et al. 2003). There are reports that already rainfall patterns have changed in tea growing areas of Sri Lanka with higher intensity rains and less rainy days (Herath and Ratnayake 2004). So the overall effect on the local tea industry could be quite negative.
Tea needs a fairly uniform rainfall throughout the year and is very sensitive to ambient temperature above 350C (Carr and Stephens 1992). Tea growing patterns and its geographical locations make it difficult to mitigate the effects of climate change. Tea cultivation occupies fragile mountain soils and is solely depend on rain for fulfilling its water requirement. Fragile soils and rain dependent soil moisture replenishment make tea one of the most vulnerable crops in the country in the face of climate change (Eriyagama, Smakhtin et al. 2010). Sri Lanka experiences a bimodal monsoonal climate with two distinct dry periods. In the southwest of the island average annual rainfall is in the order 4,000mm. However, the two dry periods of December to March and July to August can cause severe water stress to crops. Severe drought1 conditions devastated the Sri Lankan tea industry in 1983 and in 1992 (Fuchs 1989; Wijeratne 1994) and such conditions are expected to occur more frequently. Even without a scenario like climate change rain-fed cultivation of tea limits its ability to increase the crop productivity (Mongi, Majule et al. 2010).
Maintaining and improving the productivity of Sri Lanka’s tea sector has encountered hindrances for some time (Wijeratne and Shyamalie 2009). Average yield is low and falling (Akiyama and Trivedi 1987). While the focus of research has been in breeding programs and improved agronomic practices, the conversion of land from rain-fed to irrigated cultivation is recognized as pivotal for maintaining tea productivity both locally and internationally.
Consequently, the Tea Research Institute of Sri Lanka (TRISL) started preliminary trials on sprinkler and drip irrigation of tea in 1984 (Ananthacumarswamy, Herath et al. 1985). While the results were encouraging there was very little adoption of the technology or further development of scientific investigation. Renewed interest in irrigation arose with the arrival of many irrigation equipment manufacturers, like Rain Bird (USA), Netafim (Israel) and Jain Irrigation (India) to Sri Lanka in late 1990s seeking to introduce irrigation technology among perennial tree industries, especially among tea and coconut cultivations.
Irrigation has been part of TRISL’s long-term research agenda since 1998 (TRISL 1998). However, the spread of irrigation technology has not occurred among large plantation estates because of the high capital requirement, water availability and the lack of knowledge. In
1 In Australia the word “drought” is associated with rainless periods that are abnormally extended in the context of long term averages. Droughts occur over periods of months and years. However, in the Wet Zone of Sri Lanka the word “drought” is routinely used to refer to rainless periods of only 5 days and more Sumanasena, H. (2008). Effects of Short Dry Spells on Productivity of some Perennial Spice and Beverage Crop Species. 63rd Annual Sessions - Sri Lanka Association for the Advancement of Science Colombo, Sri Lanka Association for the Advancement of Science . 18 contrast, the small holder growers, especially in low elevation growing regions, show a keen interest to adopt irrigation. In particular, they seek to increase the yields of their crops in drier months and to protect young tea plants from drought damage (Mahinda 2009). Growers with extent of 2.0 ha or more and with adjacent water sources are faster in adopting this new technology due to the capital resources they have and due to the freedom of decision making (Kuehne, Bjornlund et al. 2010). Many such farmers, who also have the luxury of nearby water source, already practice irrigation manually. Nevertheless by 2009 less than 100 farmers had installed drip or sprinkler irrigation systems (Mahinda 2009). One of the simpler types of installations is of the portable gun irrigation systems for watering crops during dry seasons (Figure 1.1).
Figure 1.1 Use of portable gun sprinkler in low elevation tea field in Galle District, Sri Lanka. Farmers use nearby perennial river (Gin Ganga) for irrigation during dry season
So while there is a very positive groundswell of interest in irrigating tea, there has been no detailed scientific evaluation of its efficacy in Sri Lanka. Such evaluation has been made in other tea growing regions like India or East Africa (Hudson 1991; Panda, Stephens et al. 2003; Carr 2010), but to adapt irrigation technology to Sri Lankan conditions we need to undertake local research. Some of the issues to consider are: the physiological response of tea to irrigation under Sri Lankan conditions, how other agronomic factors interact with irrigation, and the financial viability of investing in irrigation technology.
It is already known that drought impacts photosynthesis and transpiration resulting in lower dry matter production and final made tea yield (Smith, Burgess et al. 1994; Ng'etich and Stephens 2001). The physiological response of the plant to irrigation will also depend on the 19 tea cultivar, and cultivar differences are a significant factor in determining the final yield (Burgess and Carr 1996). A wide range of drought tolerant cultivars of tea have been bred for the Sri Lankan industry (TRISL 2002). However, non-drought tolerant lines are still grown by the growers, because of faster growth rate and productivity under ideal climatic conditions. It is important for us to understand the physiological basis of the cultivar differences in response to irrigation.
Ground management practices are very important in ensuring that water, either as rain or applied as irrigation, gets to the crop. The root and shoot ratio of the tea plant is helpful in understanding optimal growth of the plant that ensure long sustainability (Bannerjee 1993). In this regard, ground management as well the type of irrigation used also needs evaluation to underline the strategies to achieve best optimal performance of the irrigation systems.
Financial considerations are very important when implementing irrigation in tea growing areas where bimodal rain pattern prevails (Carr 2010). Drought mitigation by irrigation may or may not be cost effective. Though there are many unpublished data and industry communications (TRISL 2003; TRISL 2004) in Sri Lanka claiming yield increases ranging from 20% to 60%, the return to the investment may vary according to the age of the bush, severity of the drought, tea cultivar, soil and other management practices.
The appropriate adaptation of irrigation systems to the low-grown tea sector requires a range of detailed studies. This thesis presents a collection of such studies in the attempt to understand the long term viability of establishing irrigation systems in low elevation tea growing regions of Sri Lanka.
1.2 The Aim
Therefore the aim of this study is to evaluate the effect of short-term water stress on the agronomic and physiological characteristics of low-grown tea, the responses to irrigation, and the financial practicality of introducing irrigation in the low elevation tea growing areas of Sri Lanka.
The specific objectives to achieve this aim are as follows: A. To quantify the changes in physiology and yield as affected by the water stress and recovery by irrigation B. To evaluate the water use of tea and environmental parameters that govern the water use in low elevation tea growing areas C. To evaluate plant performance in response to different micro-irrigation methods D. To quantify the effect of soil moisture limitation on young tea plant growth E. To undertake a simple valuation of the practical financial feasibility of irrigating tea. 20 1.3 Structure of the thesis
To achieve these objectives this thesis has the following structure.
Chapter 2 provides an essential background to the Sri Lankan tea industry and the development of interest in irrigation. In particular it presents the history, cultivation pattern, tea growing areas, productivity constraints and farmer efforts to adapt irrigation to there tea cultivation.
Chapter 3 provides a review of botany of tea, its physiology and international literature on tea irrigation. This review indicates that though there are studies related to water stress and irrigation in tea, there is still a gap in understanding of how tea behaves in hot humid condition, when subjected to short-term water stress and possible recovery through irrigation.
Chapter 4 describes the field site where the experimental work was undertaken, the St Joachim Tea Estate of the Sri Lanka Tea Research Institute at Ratnapura. It presents an overview of all the experiments; their timing, cultivars involved, and measurements undertaken. This chapter also presents information on long-term weather data at the site, particularly rainfall variability and its effectiveness to fulfil plant water requirement. It presents the results of applying a water stress coefficient to rainfall data of 2009 and 2010. This serves the purpose of showing that water stress is a real and regular occurrence even in an environment with annual average rainfall of 3824 mm.
Chapters 5 to 8 are collectively the experimental and analytical body of the thesis. Chapter 5 focuses on objectives A and B, while Chapter 6 is objective C, Chapter 7 is objective D and finally Chapter 8 achieves objective E.
Chapter 5, focusing on the physiology and yield of mature tea stands, is the largest section of the thesis. It presents in logical (but not temporal) sequence the results of the 3 experiments (Experiments 1-3) as follows. The hypotheses of each experiment are presented in italics. Exp 1: Physiological and yield performance of two contrasting tea cultivars in response to irrigation H1 – There is a cultivar difference in physiological and yield response to irrigation H2 - Air temperature is the main environmental factor determining yield Exp 2: Water use of tea under rain-fed and irrigated conditions H3 - Transpiration is closely related to the plant productivity and air temperature is the key environmental factor controlling transpiration Exp 3: Physiological response of new cultivars to water stress H4 – Cultivar selection is, by itself, an inadequate strategy to cope with water stress.
21 Chapter 6 focuses on irrigation technology.
Exp 4: Evaluation of irrigation technology where the performance of tea physiology and yield are evaluated under drip and sprinkler irrigation H5 - Different micro-irrigation methods differ in their effect on tea physiology and productivity
Chapter 7 comprises three experiments involving young tea plants where the short dry spell effect on young tea growth is evaluated in field and glass house conditions. Exp 5: Effect of water stress duration on young tea plant growth H6 - For young tea, even short duration water stress retard the plant growth Exp 6: Effect of partial irrigation on young tea growth H7 – Optimal growth of young tea can be maintained under partial irrigation Exp 7: Effect of irrigation and raised bed on young tea growth H8 - Effect of irrigation on plant growth can be enhanced by lowering the soil compaction in growth bed.
Chapter 8 is a financial evaluation of drip irrigation, using the yield response of two contrasting tea cultivars over a ten year period. The question being asked is: Under what financial conditions is the irrigation of two contrasting tea cultivars feasible in the low elevation growing areas of Sri Lanka?
Another way of expressing the logical connections between the hypotheses and question listed above is as follows.
Short-term water stress will have a negative impact on production related key physiological parameters such as photosynthesis, stomatal conductance, transpiration and instantaneous water use efficiency and differences will be observed between different cultivars to their ability to withstand drought (H1). However, completely drought tolerant cultivars are not available (H4). Not only long term drought causes significant yield and plant losses, but even very short duration rain-less periods can cause significant growth retardation in early growth stages (H6). Even partial irrigation can maintain good plant growth of young establishing tea (H7)
Ambient temperature is the main driver behind transpiration which determines dry matter production and increased dry matter production in irrigated tea is related to increased transpiration (H3). Increase in air temperature is also the main negative environmental factor for tea yield, though irrigation has the ability to increase dry season tea yield (H2).
In terms of growth response, irrigation effect can be enhanced by removing other constraints like soil compaction (H8). Type of irrigation technology used (i.e. sprinkler 22 vs drip) also will affect the yield response by nature of different physiological performance (H5). The important practical question, which is not presented as a hypothesis, is whether establishment of irrigated tea in low-grown areas of Sri Lanka is worth the investment by land holders.
Chapter 9 draws all the conclusions from the experimental chapters together and discusses them in the light of the overall research aim and objectives.
Chapter 10 summarizes the main results and conclusions from the whole thesis.
There are two appendices describing two parallel experiments. They are relevant to understanding the climate and tea physiology, especially in relation to the environment of low elevation tea growing areas of Sri Lanka. However, since they were not directly related to the research objectives they are not included in the body of the thesis.
23
24 Chapter 2
Sri Lanka Tea Industry – History, Production and Scope for Irrigated Cultivation
2.1 Introduction
The scope of this thesis is to evaluate the response of tea cultivation to irrigation in low elevation tea growing areas of Sri Lanka. As the cultivation and industry structure of tea varies greatly among the tea producing countries of the world, it is important to understand the specific nature of tea production in this country. This chapter briefly outlines the history of tea cultivation, cultivation method, recent shifts in production areas and the drought faced by the low elevation tea growers and their efforts and wishes to test micro-irrigation.
2.2 The Emergence of “Ceylon” Tea
The export of perennial crops like cinnamon from Sri Lanka began with the Portuguese invaders in 1505. The earlier introduction of perennial cultivations spread mainly around the coastal areas of the country. After the British took control of the entire country then called “Ceylon” in 1815 and crops like coffee and rubber were introduced to the inland mountain areas. Tea plants were first planted in Sri Lanka at the Royal Botanical Garden, Peradeniya in 1839 with seeds were imported from India. James Taylor, who was a Scotch planter and considered to be the pioneer of Ceylon tea, planted 19 acres (8 ha) at Loolcondera Estate, Hewaheta. Then curator of Royal Botanical Garden, H.K.Thwaites supplied the Assam origin seeds. The spread of Coffee Rust (a fungal infection caused by Hemileia vastatrix), identified in 1869 at a coffee estate in Gampola, paved the way for replacing coffee estates with tea. The first central tea factory was commenced at Fairyland (Siles, Rey et al.) Estate, Nuwara Eliya. Tea cultivation exceeded coffee cultivation in 1888 and growing to nearly 166,000 ha in 1899. From the small beginning of 8 ha, the present tea cultivation in the country is 221,969 ha in 2009 (Forbes & Walker 2010).
2.2 Cultivation of Tea
From the inception of seedling cultivation in 1869, tea is cultivated as a rain-fed cultivation. Seedling planting was later replaced with vegetative propagated (VP) cuttings raised in a nursery. For a successful cultivation suitable land selection, preparation and correct agronomic practices, including harvesting are very important.
25 2.2.1 Land suitability
Tea is a shade loving plant which requires an annual minimum rainfall of at least 1200mm. Under Sri Lankan conditions uniform distribution of rainfall throughout the year is also important for successful cultivation. Due to the necessity of year round precipitation, tea is cultivated in the central highlands of Sri Lanka and to the wetter western slopes of the country. Tea plant prefers slight acid soil of pH 4.5-5.5. Well drained soils are preferred for planting tea. Since the cultivation is entirely ranging in mountainous or at least hilly areas, land preparation and other cultural operations are mainly done by manual labor.
2.2.2 Land preparation and field planting
As it is not easy to use machinery for the land preparation for planting, the usual method for land preparation is the cultivation of a grass crop as a soil rehabilitation crop for 1½ to 2 year period. Either Mana (Cymbopogon confertiflorus) or Guatemala (Tripsacum laxum) grass is cultivated during the soil rehabilitation period. The grass is periodically cut and lopping is added to soil as mulch. The usual practice is to plant grasses with onset of the monsoon. Prior to planting grasses, land is prepared by constructing contour and main drains, and stone terraces where it is necessary, to prevent the soil erosion.
Field planting of the young plants is done using vegetative propagated (VP) plants, raised in a nursery for 9-10 months, after the soil rehabilitation period. Nursery plants can be raised either from a seed or single node cutting. Earlier practice was the planting of tea seedlings or seeds in the field, but now VP plants are the preferred option. Since 1934, VP plants are used extensively as they produce higher yield and more uniform growth in the field. The standard spacing for the planting is 60cm within row and 120cm in between rows. Field planting is done in accordance with the onset of monsoon rainfall in the country. The majority of tea growing areas receive the south west monsoon during April and July of the year. Generally, field planting is started during May after the soil receives enough moisture. However, in the eastern slopes of hill country areas like Badulla and Passara, field planting is done during October, where the north east monsoon prevails. After 3 months in the field the plant is pruned to a height of 30cm to encourage the side branches to grow into a bush.
2.2.2 Harvesting
Harvest is usually commenced after 3 years in field planting. In cooler environments harvesting may take more than 3 years of field planting because of the slower growth rate. In Sri Lanka, manual harvesting, known as plucking, is practiced entirely. The main reason for manual harvesting is the better selection of fresh young tea shoots leaving mature leaves and
26 immature buds in the plant. This ensures the high quality for which Sri Lankan tea is known. The usual harvesting period of the plant is 7 days.
The standard harvesting practice is to pluck the shoot with two leaves and the unfurled leaf (bud). However in practice the harvest can consist of a mixture of one leaf plus bud, two leaves plus bud or three leaves plus bud. Depending on the weather condition and location of the shoot in the plant (either in top surface or inside the bush), either single leaf plus bud or 3 leaf plus bud could be harvested. During the high rain months like May, three leaves and shoots are harvested. Similarly during the dry period, the harvest may be limited to single leaf and bud largely. During the harvest care is taken so that fresh leaves are at the correct maturity, proportions and composition. Hard fibrous material develops in tea leaves with maturity. Hence if the shoots are overgrown, it will decrease the quality of made tea.
2.2.3 Other agronomic operations
Tea is a plant that grows into medium to tall shrub when it is grown freely. However in tea plantations, the plant is pruned to form a bush. Plants are maintained at a height of around 1m height. There are three objectives for pruning: (1) To stimulate vegetative growth phase of the plant to produce more active shoots; (2) To maintain an easy plant height for easy harvesting and (Li, Yang et al.) To clean infect branches (e.g. shot hole borer infection) (Danthanarayana 1966).
The pruning period or cycle differs according to the geographic location. In low elevation tea growing areas, and in many other areas of Sri Lanka, the tea plant is pruned usually every three years time. But in the very cool climates of high altitudes, the pruning cycle may be extended, up to 5 years.
Like the field planting, the time of the pruning coincides with the onset of monsoonal rains. Soil moisture stress causes poor recovery after pruning, sometimes causing death of the plant as well. Prior to pruning, plants are rested for 4-6 weeks, without plucking as a mean of preparing the plant for the pruning. During the rest period, dry matter partitioning to roots is encouraged. Care is taken to protect the pruned plant from high solar radiation by covering it with pruned branches. Some farmers in low elevation growing areas also apply water to the recovering pruned plants.
During pruning, it is recommended to fork the soil to reduce the compaction and to increase rainwater infiltration. Pruned lopping is buried in soil to enhance the soil nutrient and also to ease the penetration of rain water to the soil. But the practice is largely not followed as it require high amount of labor.
27 The cultivation of shade trees is widely practiced in Sri Lanka tea fields. Apart from shade these trees provide some additional advantages such as organic matter addition and soil moisture conservation during dry periods. Shade trees can be classified as either medium shade or high shade trees, depending on the canopy height of the shade plant. Albizia (Falcataria moluccana formerly known as Albicia molucana (ILDIS 2005)) and silver oak (Grevillea robusta) are two common high shade species, while gliricidia (Gliricidia maculata) and dadap (Erythrina variegata (syn. E.indica)) are commonly used as medium shade trees.
2.3 Tea Growing Areas of Sri Lanka and Productivity Constraints
2.3.1 General climate of Sri Lanka
Sri Lanka climate varies strongly over space and time particularly with respect to rainfall. This is largely due to Sri Lanka’s, location near the equator and the influence of monsoonal circulation over south Asia (Punyawardena 2004). Among the various climatic regions, south western quadrant of Sri Lanka has a sharp climatic contrast between rest of the country due to the presence of central mountainous region, with a peak elevation of 2524m. This is the area where most of the low elevation tea is located, in the main administrative districts of Ratnapura, Galle Matara and Kalutara (Figure 2.1).
Sri Lanka has been classified into three climatic zones based on the rainfall distribution (Figure 2.1). The south western part including central highlands is the Wet Zone. The Dry Zone predominately covers the northern and eastern parts of the island. The Intermediate Zone, skirting the central hills, except south and the west, separates the Wet Zone and Dry Zone. The annual rainfall of the Wet Zone is over 2500mm, while the Dry Zone receives a mean annual rainfall of less than 1750mm with a distinct dry season from May to September (Punyawardena 2004). The Intermediate Zone receives a mean annual rainfall between 1750 to 2500mm with a short and less dry season.
28 A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 2.1 Agro-climatic zones of Sri Lanka with district boundaries. Main low elevation tea growing districts are Galle, Ratnapura and Matara (Punyawardena 2004)
Temperature regimes in Sri Lanka are also characterized by significant decreases in the temperature with altitude. There is vertical lapse of temperature, approximately around 5-70C for every 100m rise in elevation (Punyawardena 2004). There is a considerable variation in temperature in the Wet Zone. As low temperature is an important factor affecting plant growth in Wet and Intermediate Zones another classification has been created based on elevation to classify agro climatic zones within each climatic zone (Figure 2.1). In this climatic classification, Low country is demarcated as the land below 300m of mean sea level
29 (msl), Mid-country is the area with elevation between, 300-900m, and Up-country is the area above 900m msl elevation.
2.3.2 Tea growing area classification
Tea growing areas of Sri Lanka are also classified according to the elevation above mean sea level (amsl). This differs slightly from the agro-climate classification outlined in Figure 2.1. Tea fields up to 600m amsl are classified as low elevation/country tea. Tea fields in the elevation range between 600 – 1200m amsl are called mid elevation/country tea. The rest of the tea fields above 1200m amsl are classified as up country tea. The demarcation of the tea growing areas by the elevation is not only related to the geographic location, but and for management purposes as these areas based on elevation creates distinct features of the made tea quality. Also as the climatic parameters vary with the elevation, so do the growth rate and the level of environmental stress vary with the growing elevation. In particular relevance to the low elevation tea areas, it experiences severe drought stress during some part of the year.
When tea was being established as an industry in days of Ceylon the focus was in the mid and high elevation tea growing areas and to a much lesser extent in low elevation tea growing areas. Because of this history these larger mid and high elevation estates belong to either companies or wealthy families and the resident managers of the properties not the owners. Since the latter part of the 20th century, more tea growing areas emerged in low elevation tea growing areas of Sri Lanka. In 1992 there was a significant decline in total tea area in Sri Lanka as a part of a program of privatization and registration of tea fields (Figure 2.2). This period also witnessed a significant increase in the area of low-grown tea. In 2010, 60% of total tea production was produced from low elevation tea fields as shown in Figure 2.2 (Anon. 2011).
An important feature of the low elevation tea fields is the higher percentage of land belonging to small holder tea farmers. Small holder tea farmers are land owners who own less than 20 ha according to Land Reform Act of 1972 (ADB 2000). There has been a steady increase in the number of small holders and the cultivation extent since 1983. Table 2.1 shows the increase in land extent and ownership in three main tea growing districts of low elevation area. With the expansion of the area and ownership, small holder farmers have become the main producers of the national tea production. For example, in first six months of 2008 small holder farmers produced 74% of Sri Lanka’s tea (MPISL 2008).
30 A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 2.2 Total, high , medium and low elevation tea extent from 1959 - 2000 (there was a drastic reduction in high and medium sector, while increase in low sector after privatization of tea estates in 1992) (Holsinger 2002)
Table 2.1 Number of and extent tea small holdings over three census periods in the Wet Zone of Sri Lanka (FAO 2010)
1983 Census 1994 Census 2005 Census District Extent Extent Extent Number Number Number (ha) (ha) (ha) Galle 36,479 13,603 56,547 17,855 90,524 25,325
Matara 27,964 13,342 44,051 16,886 67,613 22,971
Ratnapura 17,713 9,818 49,161 17,789 97,984 28,232
The total tea production of Sri Lanka is largely supplied by low elevation tea growing areas, throughout the year. As shown in monthly tea production data of 2010 (Figure 2.3), total production largely followed the production fluctuation of low elevation sector in most months. Monthly largest tea production in May 2010, was however contributed by the peak monthly production of mid elevation and high elevation tea growing areas.
31 high 40000 medium low 35000 Total
30000
25000
20000
15000
Production (made tea '000kg) tea (made Production 10000
5000
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 2.3 Total tea production in Sri Lanka classified according to elevation during 2010 (SLTB 2011)
Though there has been this increase in total tea production in low-grown tea areas, yields have not been consistent with other areas. A comparison of yields between 2004 and 2008 indicates that production is stagnant at Galle district and there was a 15% reduction of productivity in small holder tea fields in Ratnapura (SLTB 2010). The reason for loss of the production in low elevation tea growing areas can be attributed to the age of tea fields (Wijeratne and Shyamalie 2009), high soil erosion in low elevation tea fields (Ananda and Herath 2001) and high damage to low elevation tea fields during drought periods (Fuchs 1989).
Raising productivity of small holder fields in low elevation tea growing areas, can be accomplished by introducing new technology, including improved cultivar and better management practices. However, there is a limit to increase the productivity through improved cultivar introduction, since tea inherently has a limitation to increase productivity, because of its nature of photosynthesis mechanism (Raghavendra 2003). Irrigation technology can be considered as an alternative to cultivar selection for drought mitigation in the mean time.
2.3.3 Adapting technology to mitigate drought
The technology dissemination of the small holding sector is satisfactory, yet the adaptation of present soil and moisture conservation practices (without irrigation as a component) is at
32 around 60% adoption level (Jayamanne, Wijeratne et al. 2002). However, it was observed during the recent short term droughts that considerable number of farmers practiced irrigation as a mean to prevent drought damage and to increase the productivity (Gopal, Shilpakar et al. 2010). Figure 2.4 indicates the resourcefulness and effort of some farmers to adapt available technology in an attempt to irrigate tea under stress.
Figure 2.4 Farmer using small lawn sprinkler to irrigate mature tea field during a dry spell (February, 2010) in Urubokka, Sri Lanka
Also surveys indicate a strong willingness among farmer groups to consider irrigation. Table 2.2 shows the answers of participants at an advisory workshop during a drought awareness program conducted for tea advisory officers and representatives of small holder tea growing societies in the low elevation tea growing area (Mahindapala and Bandara 2005). In this survey 80% of the participants believe that drought affects tea fields and there is a need for further study about irrigation. Another revelation of this survey was that among the 9% of crops (mainly young fields) under severe drought stress most farmers used traditional surface irrigation by directly pouring water to fields using rubber hoses. Only one respondent replied with using sprinkler and another mentioned about soil injectors.
33 Table 2.2 Results of drought awareness survey. The survey was conducted among tea growers and advisory officers (n=41) (Bandara and Mahindapala 2009)
A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
2.4 Conclusion
This chapter has outlined the historical, geographical and agronomic specifics of the Sri Lankan tea industry. It shows that the production frontier of the Sri Lankan tea industry has shifted from high and mid elevation tea growing areas to low elevation areas. Also the industry in these areas is dominated by small holder production sytems. Productivity constraints in this area are strongly related to short seasonal droughts and with expected climate change these problems will increase, as shown in Figure 2.3. Efforts are needed do address the main constraints faced by the tea growers in those areas. Already farmers are adapting irrigation at a small scale to combat drought. However considerable research is needed to inform the best ways for farmers to use such technology, if indeed it is worth the effort. This thesis evaluates the feasibility of introducing irrigation to low elevation tea growing areas by evaluating the physiological and financial performance of tea. The following chapter lays out the literature that underpins this research.
34 Chapter 3
The Botany and Physiology of Tea and its Water Relations
3.1 Introduction
Tea growers in all tea growing countries face different types of plant biotic and abiotic stresses while keeping the cultivation in an economically sustainable level. The plant experiences several such stresses either concurrently or at different times through growing season that frequently limit the growth and productivity (Tester and Bacic 2005). Common abiotic stresses include decreased availability of water, extremes in temperature, decreased availability of soil nutrients, extreme light or hardness of soil that restrict root growth (Verslues, Agarwal et al. 2006). It is estimated that abiotic stresses represent serious limitations to agriculture, more than halving average yields for major crops (DaMatta and Ramalho 2006). Among them drought is the most important abiotic stress that affects field grown crops. The tea plant is also subjected to drought under various growing conditions in different growing environments. Now, drought has become a factor that determine the total tea production in Sri Lanka as well the long term prevailing of the tea in some areas of the country (Gunasekara 2010).
Sri Lankan tea cultivation is predominantly rain dependant. In contrast some competitive countries in Asia and East African countries, to some extent have converted their tea lands into irrigated cultivations to mitigate drought and to enhance productivity. Up to now drought mitigation of Sri Lankan cultivation was largely dependent on screening of drought tolerant cultivars, improvement and implementation of soil conservation measures, proper shade management and expansion of multi cropping system (Wijeratne 1996; Eriyagama, Smakhtin et al. 2010). Sri Lanka has invested relatively little into water management and irrigation compared with east African tea growing areas and some Central Asian tea growing countries (Carr 2010). This is largely because the Sri Lankan industry has until the last decade been dominated by mid and high elevation large estates in relatively cooler climes. Now the industry has a significant small holder sector in low elevation areas where a hot humid climate prevails with high bimodal rainfall distribution associated with hot short duration dry spells.
The aim of this chapter is to review the irrigation research in the international arena, particularly regarding tea. It will identify key issues on which irrigation research in the low elevation tea sector of Sri Lanka should focus.
35 The opening section summarizes the botany of tea plant including the origin and distribution, different varieties and growth pattern. The second section will review the physiology of tea plant particularly relating to the important physiological parameters and their relationship with water stress. The third section of the chapter will then review the climatic requirements of the plant. It will summarize the key environmental parameters that affect the productivity and survival of the plant. The fourth section will review the drought experienced in different tea growing regions of the world and it will shed light on the unique characters among the drought occurrence in low elevation tea growing areas of Sri Lanka and also impeding drought that could occur as a result to global climate change in this region. The final section will review different research done on tea irrigation and related irrigation on some horticultural crops and future directions for irrigation studies in the study area.
3.2 Botany of Tea Plant
3.21 Origin and distribution
The tea plant belongs to genus Camellia, family Theaceae and tribe Gordonieae. After several revisions, the name of the tea plant has now become Camellia sinensis (L.) O. Kuntze (Paul, Wachira et al. 1997; Hajra 2001). The evergreen plant naturally grows to a height of 10-15m. Tea plant was originated in south east Asia, specifically around the intersection of latitude 290N and longitude 980E, the point of confluence of the lands of northeast India, north Burma and south west China and Tibet (Kingdon Ward 1950; Mondal 2007). The climate of this area is a monsoon type with a warm, wet summer and cool dry winter (Carr and Stephens 1992). Most tea growing lands are associated with high precipitation regimes. Global tea cultivation now extends from Mediterranean type climates to hot humid tropics in 31 countries (Hajra 2001). The geographical area is from 490N, from the Outer Carpathians mountain region in former Soviet Russia to Natal, South Africa, 330 S (Shoubo 1989). Altitudes of tea cultivations can vary from sea level in Japan to over 2700m in Kenya and Rwanda (Ng'etich and Stephens 2001; Owuor, Obanda et al. 2008) . Due to worldwide demand, tea cultivation is widely spread, while the major tea producing countries in the world are India, Kenya, China, Japan and Sri Lanka (FAOSTAT 2010).
The Chinese had dominated the art of tea cultivation for many centuries. Tea was then introduced to Japan from China in the early part of eighth century. From Japan, tea cultivation was spread to Indonesia in the 17th century. Meanwhile in India, commercial tea cultivation was started in early 19th century. Commercial tea cultivation started in Sri Lanka in late 19th century. Tea cultivation started in the USSR in the end of 19th century and in East African countries during early 20th century.
36 3.2.2 Varietal difference
There are mainly three types of tea (also known as jats in South Asia). They are called China (Camellia sinensis var. sinensis), Assam (Camellia sinensis var. assamica) and Cambod (Camellia sinensis var. cambod) types. The China type is a shrub (1-3m tall), with many stems arising from the base. The relatively small, thick and leathery leaves have stomata sunken in the lamina. Short and stout petioles gives leaves an erect pose and are usually 3-7 in number. It grows in open spaces. The Cambod type plant is an upright tree (6-10m tall). It has several almost developed branches and more or less erect, glossy light green to coppery-yellow or pinkish red leaves. Leaf size is intermediate. The Assam type is a 10-15m high tree with a trunk and robust branching system, with large, light-green to yellowish leaves, thin glossy leaves, grown under the canopy of large trees in natural habitats (Mondal 2009; Nair 2010). Cross pollination of tea gives vigorous better quality progeny than parents. Bio types cross freely with each other.
In commercial tea plantations, the crop consist of mixed hybridized populations of China, Assam and Cambod types. Tea cultivars found in tea plantations are mostly a mix of China and Assam tea traits. Tolerance of the tea plant to environment stress depends on the variety, from which the particular cultivar was derived. Tea cultivars derived from China type varieties are slow growing but tolerant to drought, with small semi-erect leaves, whereas cultivars derived from Assam type are fast growing with less tolerance to drought. The varietal characters are important in selecting plants for drought tolerance and for irrigation. Localized varieties of tea have been given vernacular names in different tea-growing countries. The commercially available tea cultivars are also widely known as tea clones in the industry. Therefore the term cultivar is used throughout this document and cultivar is exactly to clone in the tea industry nomenclature,
Table 3.1 shows the characters of some tea cultivars released by tea breeders in Sri Lanka. The degree of tolerance to drought varies among cultivars (Nagarajah and Ratnasuriya 1981) and harvesting yield as well. These two characters mainly determine the selection of particular cultivar suitability for potential irrigated cultivation. Among all, cultivar TRI 2023 is the highest yielding tea cultivar in the tea breeding yield books in Sri Lanka (Piyasundara 2009). However, the particular cultivar had to be withdrawn from the suitability list for low elevation tea growing areas, due to high infestation of stem canker disease, during drought. The cultivar may have an ability to respond with high yields under irrigation during drought periods. Hence it is necessary to evaluate different cultivar to irrigation and their ability to withstand drought.
37 Table 3.1 Characters of some popular tea cultivars cultivated in Sri Lanka (Piyasundara 2008)
A It held This is to included by comply figure/table/image the University with in the NOTE: copyright print of has copy Adelaide been regulations. of removed the Library. thesis
38 3.2.3 Growth pattern
Under natural conditions tea plants are medium sized trees, but under cultivation the tea plant is grown as a bush of 1.2-1.5m height for ease of harvest and to increase productivity. To maintain the appropriate height the plant is pruned at regular intervals of 3-5 years, removing some leaf bearing branches of the plant (Nissanka, Anandacoomaraswamy et al. 2004). In tea, the part harvested to produce green and black tea is a short, vegetative shoot consisting of a bud with two or three immature leaves (Figure 3. 1). This is different from many other tree crops where reproductive parts, like flowers and seeds, are harvested. Hence, keeping the plant in the vegetative phase during its life span is important for profitability.
The growth of tea shoots also important for high productivity. The natural growth pattern of the tea shoot, which contain a tea bud, has alternative dormant and active phases (Tanton 1981). Four to seven foliages leaves grow alternatively, above two scale leaves and then become dormant. When the plant is under stress from low ambient temperature of water stress, dormancy occurs after production of only two or three leaves. In a tea bush, there are always active and dormant (banji) shoots, both of which are included in the harvest. Inclusion of a higher proportion of dormant buds in the harvest is disadvantageous for producing good quality tea, also lower the productivity, because dormant buds usually weigh less and have coarse leaves. The proportion of plants containing dormant buds at the harvest depends on the age of the bush, stage of the pruning cycle and water stress. Plants tend to produce more dormant buds when water stressed (Stephens and Carr 1991; Wijeratne and Fordham 1996). Maintaining a favourable soil water balance is important to keep the plants in the vegetative phase.
The harvesting interval of tea, i.e. the duration between two successive harvests, depends on the time taken for a young bud to come to the harvesting stage of 10-15cm. This duration is known as the shoot replacement cycle, which may vary from 30 days to 490 days (Carr & Stephen, 1992), but the harvesting interval usually varies from 7-25 days (Stephens and Carr 1994; Wijeratne and Fordham 1996). The main factor determining the harvest interval is temperature (Carr and Stephens 1992). At any point in time there are shoots of different stages of development on a bush. Tea is harvested at seven-day intervals in Sri Lanka. The yield, at each weekly plucking reflects the level of environmental stress during shoot development period. Regular tea yield (weekly, monthly or seasonally) is a good parameter to reflect seasonal environmental conditions.
39 A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 3. 1 Development of a tea shoot (Burgess & Carr, 1998)
3.3 Physiology of Tea Plant
Early identification of the physiological and morphological characters suitable for irrigated tea cultivation reduces the potential risk of investing for a cultivar that respond poorly to the irrigation (Squire 1985; Smith, Burgess et al. 1994). In a recent review, physiological activity of tea plant and their relationship with environmental parameters has been discussed by De Costa et al (2007).
3.3.1 Photosynthesis
Photosynthesis is the process which produce carbon assimilates to yield formation and growth of maintenance foliage, stem and branch structure and the roots of the tea plant (De Costa, Mohotti et al. 2007). Tea yield is determined by the photosynthetic rate of the maintenance foliage and the shoot extension rate (Manivel and Hussain 1982; Okano, Matsuo et al. 1996).
The tea plant exhibits C3 type photosynthesis pathway (Roberts and Keys 1978). While the major organs of the tea plant photosynthesis are leaves, mature brown stems also assimilate
CO2 but with low efficiency (Sivapalan 1975). In pruned tea, newly emerging shoots assimilate CO2 through brown stems. Tea leaves shows significant photorespiration. Under normal atmospheric conditions, photorespiration accounts for 19% of net photosynthesis (De Costa, Mohotti et al. 2007).
40 In comparison to other crops, little work has been done on the study of the tea photosynthesis
(Mohotti and Lawlor 2002). C3 photosynthesis has three main phases (Raghavendra 2003), viz: (i) During the first phase of carboxylation, carbon dioxide is accepted by ribulose-1, 5- biphosphate (RuBP) to give two molecules to 3-phosphoglycerate (PGA); (ii) next phase is called reduction where PGA is reduced to triose phosphate (triose-P) using adenosine 5 triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH); (iii) last phase is the regeneration of the primary acceptor of CO2, RuBP from triose phosphate through a series of reactions.
Low rates of net photosynthesis and stomatal conductance have been reported for tea (Smith, Stephens et al. 1993; Mohotti and Lawlor 2002). In Sri Lankan conditions a maximum -2 -1 stomatal conductance value was reported as 0.8mol H2O m s under field conditions -2 -1 (Mohotti and Lawlor 2002). Much lower values of 0.015-0.098mol H2O m s have been reported in South India (Joshi and Palni 1998). In mature tea when the stomatal conductance lowers during morning hours and photosynthesis increases, it reduces the intercellular carbon dioxide concentration. Intercellular carbon dioxide concentration regulates the photosynthesis process in the tea plant (De Costa, Mohotti et al. 2007).
Photosynthesis is controlled by many external and internal factors which determine the final productivity of the plant. The internal factors controlling photosynthesis are stomatal conductance (Smith, Stephens et al. 1993), leaf age and position in the plant (Okano and Matsuo 1994; Raj Kumar, Manivel et al. 1998), varietal difference (Smith, Burgess et al. 1994; Joshi and Palni 1998) and pest and disease occurrences (Ponmurugan, Baby et al. 2007). The external factors controlling the photosynthesis are light intensity and shade (Mohotti and Lawlor 2002; Karunaratne, Mohotti et al. 2003), temperature (Joshi and Palni
1998; Netto, Jayaram et al. 2005), CO2 concentration (Kumar, Venkatesalu et al. 1993), mineral nutrition (Smith, Stephens et al. 1993) and water deficit (Smith, Burgess et al. 1994; Luo and Pan 1996; Marimuthu and Kumar 1998). Irrigation has the ability to control many external factors affecting the photosynthesis of tea. Irrigation also increases the stomatal conductance, photosynthetic rate and ratio between stomatal conductance and photosynthesis. It also maintains favourable leaf temperature level optimum for photosynthesis and reduced photo-inhibition at high luminance (Smith, Burgess et al. 1994). However, there is a factor associated with the age of the tea plant, where old tea plants do not increase the photosynthesis rate of tea as compared with young tea as a response to irrigation (Smith, Burgess et al. 1994). The cultivar effect of tea is also visible in the response to photosynthetic rate to the irrigation effect. Photosynthesis has been selected as a tool to investigate drought effects primarily because of its role in determining productivity of
41 tea (Bannerjee 1993), sensitivity to drought (Berry and Bjo¨rkman 1980) and its reflection of the temperature effect (Joshi and Palni 1998) which is critical during drought at low elevation.
As the present planting recommendation include many cultivars for low elevation tea growing areas (Table 3.1), it would be advisable to evaluate their physiological response to irrigation to minimize the time consumption for field selection procedure. Otherwise, unlike for an annual crop, the financial cost of installing a irrigation system in a new planting field and harvesting lower yield after 2 or 3 years would be a high economic cost on the farmer.
3.3.2 Plant water relations
Tea crop water use can be expressed as its transpiration, since completely grown tea canopy covers ground surface, there is usually minimal soil evaporation. Nevertheless there are only few studies that directly measure transpiration in tea crops viz: Kigalu, 2007 and Anandacoomaraswamy et al 2000. Average daily transpiration from 1.9 to 5.5mmday-1 were reported from Mufindi, Tanzania (Kigalu 2007) from well watered soils. Water use per unit leaf area for young tea varied with cultivar and crop density. Higher transpiration rates were observed from the plants with lower plant densities. In Sri Lanka, transpiration rates ranging from 1.6-3.3mm per day were reported under low temperature conditions (Ananthacumaraswamy, De Costa et al. 2000). High transpiration in tea plants cause significant water loss from the soil suppressing shoot growth (Stephens and Carr 1993). Stephen and Carr (1993) found this interaction while conducting experiment at relatively a lower air temperature value of 15-200C in Tanzania. So the studies related to tea transpiration has been done entirely in low elevation regimes as compared to the high temperature (>300C in dry months) prevail in Ratnapura, Sri Lanka. So the transpiration of tea in such environment is affected by the behaviour of stomata, as influenced by soil water stress as well as the high temperature.
Tea plant canopies have high transpiration rates which causes soil water deficits leading to decreased leaf expansion rates (Squire 1990; Stephens and Carr 1993). The reduction of leaf expansion rate is directly inverse to the green leaf yield. Excessive transpiration occurs even under wet conditions, when soil contains adequate soil moisture, due to high level of irradiance and saturation deficit (Smith, Burgess et al. 1994). Temporary water deficits occur within the plant as a result. Productivity is directly affected by the water deficits of the plant. Apart from irradiance and saturation deficit, there is a chance for high ambient temperature to increase transpiration mostly in rainless periods in hot humid environment, which is not apparent in the studies of cool dry environments like East African tea growing regions.
42 Tea plant water use has previously been estimated from the grass reference evapotranspiration and multiplied with a crop coefficient. The crop coefficient adjusts the difference between the selected crop species (tea) and the grass, by considering the species physiology, age and method of training (Allen, Pereira et al. 1998). However, reference crop cannot be evaluated as representing a tea crop in estimating the water use (Dragoni, Lakso et al. 2006). Water use of tea has to be evaluated separately specially in the humid conditions (Annandale and Stockle 1994). Unlike in short grass conditions, in tree crops transpiration is more controlled by bulk air conditions than net radiation (McNaughton and Jarvis 1991).
Stomata close partially during the day even when the soil is wet (Williams 1971; Carr 1977; Squire and Callander 1981). Stomatal closure is a response to an internal water deficit in the shoot. Due to possible specific characteristics in the absorbing region of the root system and/ or the xylem vessels, the rate of root water absorption and subsequent transfer through xylem is not very efficient (De Costa, Mohotti et al. 2007). Xylem water potential is more sensitive to air temperature and vapour pressure deficit when soil is wet, than when soil is dry. Low stomatal conductance is associated with drought tolerance, and it is a useful selection parameter for selecting the tea cultivar for drought tolerance (Saikia and Dey 1984). When there are drought conditions in Ratnapura, plants experience not only soil water stress but high temperature stress as well. The behaviour of the different cultivars under drought condition predicts the better quality cultivar selection for irrigated tea cultivation.
3.4 Climatic Requirement
3.4.1 Air temperature
Ambient temperature is one of the major climatic factors which determines the shoot extension rate and shoot weight of tea, and hence its geographical distribution (Squire and Callander 1981; Wijeratne and Fordham 1996). The temperature at which shoot extension ceases is known as the base temperature. Several authors have reported various values for the base temperature, according to cultivar and location. A base temperature value of 12.50C has been reported for cultivar SFS 204 in Malawi (Tanton 1982), which was validated by Carr (1992) and used to calculate the shoot replacement cycle in many locations. However, in Mufindi, Tanzania, base temperature values ranging from 8.9 to 11.30C have been reported for four different tea cultivars (Burgess and Carr 1997). Though growth ceases in tea shoots during cool winter periods, the plant survives to start shoot development when the warmer season arrives. The response of tea to irrigation during cool dry months is less than in warmer months due to the lower air temperature that restricts shoot growth.
43 The upper, or ceiling, temperature above which shoot growth is restricted has not been determined. Wijeratne and Fordham (1996) maintain that it is higher vapour pressure deficit rather than higher ambient temperature that restricts growth (Wijeratne and Fordham 1996). Nevertheless, temperature above 300C is generally considered not suitable for tea growth (Das and Barua 1987). Wijeratne (1996) found that the extension rate of tea shoots was reduced at mean temperatures above 260C. Under controlled environment conditions, it has been found that the optimal temperature for leaf photosynthesis is 350C and it decreases rapidly above 370C (Hadfield, 1975 cited in Hajra, 2001). Where ambient temperature in tea areas exceeds 300C cultivation of shade trees like Falcataria mollucana or Gliricidia maculata is advised (Das and Barua 1987).
Prevailing ambient air temperature, above a base temperature of 12.50C, can be used to calculate the time taken for a bud to reach a harvestable stage of 10-15cm long shoot (Carr and Stephens 1992). Shoot replacement cycles vary from 160 days in the winter months for cooler tea growing areas to 30 days in warmer regions during the hot season when other factors are not limiting the growth rate. Under field conditions at low elevations in Sri Lanka, it takes 42-49 days in the wet season and 56-64 days in the dry season for the shoot replacement cycle (Wijeratne and Fordham 1996). This can be considered as a base to evaluate the low response even during wet season for high ambient temperature prevailing in the area, when soil moisture is not limiting.
3.4.2 Soil temperature
Soil temperature influences growth and yield of tea and also has a great influence on plant survival (Hajra 2001). The growth rate of tea shoots decreases at soil temperatures above 250C at 0.3m (Fordham 1971). The lower limit of soil temperature for tea plant growth is considered to be as 200C (Carr and Stephens 1992). Measurement of lowest soil temperature for tea plant growth is difficult as isolation between low soil moisture and high air temperature is not practicable.
Application of mulches and planting of shade trees are practised in tea plantations to reduce the adverse effects of high soil temperature during dry seasons. There are two types of mulches that are used in tea fields frequently. Organic mulches derived from plant materials and synthetic mulches derived from plastic, polythene or other suitable material. Organic mulches such as grass reduces the soil moisture level, whereas mulching with polythene increases the soil temperature (Othieno, Stephens et al. 1992). Cultivation of shade trees is important in controlling soil moisture levels in dry periods as well as providing mulching materials. Further experiments are necessary to study the effect of high soil temperature without soil moisture stress conditions. 44 3.4.3 Vapor pressure deficit
Vapour pressure deficit (VPD) is mostly related with high ambient temperature. Squire (1979) was the first to report an inverse linear relationship between VPD and shoot growth rate of tea between 0.8-3.2kPa. Tanton (1982) reported that when VPD of air exceeded 2.3 kPa, it depressed the growth of tea shoots. This value is widely used as the critical level of VPD for predicting tea yield. In contrast, a critical value of 1.2kPa has been reported at low elevations of Sri Lanka (Wijeratne and Fordham 1996). This is significant as the relative humidity of low-grown tea areas do not fall to very low levels of 40-50%. This result suggests the possible effect of high air temperature on shoot growth. In controlled conditions with 280C constant day temperature value, Balasuriya (1997) found a reduction in shoot extension rate as VPD increased from 1.6 to 2.1kPa. All these differences in response of shoot growth rate can be due to different air temperature levels that prevailed in experimental periods. However, high saturation deficit of even more than 2.0kP lasting for short durations, for few hours in a day, does not have any effect on plant yield (Rahman and Nath 1994).
The negative effect of saturation deficit is due to inductance of high leaf water potential, which is a result of the high rate of transpiration that occurs under dry conditions. The effect of saturation deficit was observed in irrigation trials of Malawi and Tanzania. The yield response to irrigation during dry seasons in Malawi was lower than that in Tanzania, because of the higher saturation deficit that prevails in Malawi, during the dry season, as compared to Tanzania (Tanton 1982). In an irrigation experiment by Carr et al (1987), it was found that tea yield was limited by the high VPD (2-4kPa) during the month of October and November after removal of major limiting factors of air temperature and soil water deficit
Although it is not practical in tea cultivations, application of mist irrigation was successful to control the adverse effects of high VPD (Tanton 1982). Response to the two main irrigation types, sprinkler and drip irrigation, will be important under low elevation conditions in Sri Lanka, because, sprinkler irrigation has the ability to modify the microclimate of the tea bush.
3.4.4 Rainfall
Tea is grown as a rain-fed crop in Sri Lanka. Even distribution of annual rainfall is very important in determining the geographical distribution of the tea plant, especially in hot climates. Tea is grown in areas receiving a rainfall of 700mm (Chipinga, Zimbabwe) to 5000mm (Sri Lanka Tea Board) per annum. Supplementary irrigation is considered necessary for areas receiving less than 1150mm (Carr and Stephens 1992). But this widely claimed rainfall requirement has been calculated for eastern African countries, where the air temperature does not increase 300C often and drought period is known as winter drought,
45 where low temperature prevails. Under low-grown Sri Lanka conditions, total monthly rainfall and its distribution within months are also important factors, because total monthly rainfall value does not reflect the climatic factors of the month (Wadasinghe and Peiris 1987).
In Low elevation tea growing areas of Sri Lanka, ambient air temperature generally increases with the onset of consecutive rainless days. Hence regular rainfall throughout the month or year is important for a uniform production in the year. (Wijeratne 2010) In contrast, in many tea growing areas in Africa, distribution of rainfall is not very important for the survival of plants, mainly because of the low temperature prevailing in the dry season. Due to the low temperature water loss from plants is lower compared to warm climate. For example, in Kenya, 90% of the 2100 mm annual rainfall is received between mid March and mid November (Hajra 2001). Due to relatively uniform distribution of monthly ambient air temperature, except spikes in dry spells, yield of tea plantations in the low- grown region of Sri Lanka is highly correlated with rainfall.
3.5 Drought in Tea Cultivations
In meteorological terms drought refers to a period in which rainfall falls below potential evapotranspiration (Smakhtin and Hughes 2004). However, regarding agriculture drought particularly in Tropics, drought has to be considered as a multidimensional stress, since drought is aggravated by solar radiation and high temperature (DaMatta and Ramalho 2006). Since the tea plant is sensitive to temperature stress, and dryness of air, drought occurrence in tea cultivations is sometimes masked, if analysed in meteorological terms. Many studies concerning drought effects on tea plant physiology has been done on container grown plants in controlled or semi-controlled environments. DeMatta and Rmalho (2006) suggest some limitations in such experiments to compare with field conditions, such as: (i) root growth is particularly restricted in pots; (ii) soil substrates in pot experiments usually creates short, local water deficit in roots; (iii) transpiration is decoupling, since air surrounding the plant is isolated from external atmosphere; and (iv) if humidity and temperature is not controlled, evaporative demand may rise. Thus, it arises the need for field studies of drought related physiology.
3.5.1 Occurrence of the drought
Drought is the main abiotic stress that reduces productivity and occur in all tea growing regions of the world (Mondal 2007). Barua (1989) reported that 60-65% of the total world tea area is subjected to periods of drought varying from 3 to 20 or more weeks. It is a phenomenon that occurs in any temperature or precipitation regime (Fuchs 1989). Even though tea is grown in regions where the annual rainfall can be high, many tea plantations are
46 subjected to dry spell, which may ranges from only a few weeks to up to six months, due to the annual cycle of tropical weather (Squire and Callander 1981).
The drought season often consists with combination of dry soil, dry air and high temperature (Squire and Callander 1981). The severity of drought and its effect on the growth of the tea bush varies with air temperature and humidity. For example, in Mufindi, Tanzania, there is a six-month dry period with low air temperature. Low temperature does not allow the saturation deficit to reach critical values and the plants do not undergo much stress. In contrast, in Mulanje, Malawi, the dry season is shorter than Mufindi, Tanzania but the high saturation deficit, associated with high air temperature, results in more severe water stress (Squire and Callander 1981). Table 3.2 shows the characters of drought season in different tea growing areas of the world. Except Ratnapura, Sri Lanka, all other regions have low rainfall and maximum temperature is less than 300C during dry season. Even though the average monthly rainfall during the dry season is also more than 150mm, distribution of the rainfall within the month is not uniform. The number of rainy days is limited to a few, leaving large number of consecutive rainless days.
3.5.2 Drought in Sri Lankan tea plantations
In Sri Lanka, periods of drought are considered to be typical, although irregular and not wide spread in monsoon climate (Domros 1977). In an analysis of rainfall and tea yields in Sri Lanka, Fuchs (1989) reported that any month having less than 50 mm of rainfall causes reduction in yield and growth retardation of young plants. Even within the high rainfall months, 15 consecutive rainless days can induce losses in production from drought (Wadasinghe and Peiris 1987).
The climate of Sri Lanka has distinct wet and dry seasons, which are associated with the monsoons. There are two monsoons, the southwest and northeast monsoons, both of which bring rains to lowland tea areas of the island. The southwest monsoon occurs between April- July and the northeast monsoon occurs between October and December. The period following the southeast monsoon is the dry spell for low-grown tea region (Wadasinghe and Peiris 1987), but the inter monsoon period of July-September also creates a relatively dry environment, lasting for 4 to 5 weeks. In addition to the annual dry spells, recurrent droughts which usually have 10 year intervals mainly result in widespread plant death, as happened in 1983 and 1992 (Fuchs 1989; Wijeratne 1996). The greatest losses were reported in low-grown tea areas during the1989 drought spell, though the duration was comparatively shorter (Fuchs 1989).
47 Table 3.2 Characteristics of drought period in some tea areas (Squire and Callander 1981; Wijeratne and Fordham 1996)
ET0 Altitude Main Dry Monthly RF VPD Tmax Site Location 0 (mm/day (m) Period (mm) (kPa) ( C) ) Mufindi 8033’S 1890 May-Oct <10mm 1-2 18-24 3-5 (Tanzania) 35010’E Kericho 0022’S Dec- 2178 Variable 2.5-3.5 23-27 4-6 (Kenya) 35021’E mid Mar Mulanje 16005’S mid Aug- 650 15-30mm 3-4 26-33 5-7 (Malawi) 35037’E mid Nov Tocklai 26017’N 80 Dec-Feb 10-30mm 1.5 23-26 5 (India) 94012’E Annamali, 100N 1060 Jan-March <10mm 1.5-3.0 25-30 - (India) 770E
Krasnodar 450N No rain 2-6 100 Jun-Aug 2.0-2.5 30 - (former USSR) 390E weeks
Ratnapura, Variable 6040’N (Sri Lanka Tea 29 Jan-Mar Avg 2.5 31-33 3-5 80025’E Board) >135mm
The drought season in January –March of each year usually associated with a maximum temperature of >340C. The plants are subjected to heat stress in addition to drought stress. Continuous exposure of coffee plants to temperature as high as 300C, has resulted in decreased growth, leaf abnormalities and growth of tumours at the base of the stem (DaMatta and Ramalho 2006). It was found that both canopy photosynthetic rate and photo chemical efficiency was reduced when the plants are subjected to both drought and heat stress together. The canopy photosynthetic rate fell nearly zero, when subjected to heat and drought stress together as compared to 20 days in drought alone and 34 days of heat stress in Kentucky blue grass (Jiang and Huang 2000).
The usual weather pattern in the southwest of Sri Lanka is to have maximum temperature around 1400hrs and followed by rain showers in the evening. This prevents increasing the leaf temperature, and opens stomata, which closes during the mid-day. During the drought period of January-March, incidence of solar radiation, sunshine hours and maximum temperature increases respectively up to 19.1 MJ/m2/d, 6.8hrs and 32.80C (Table 3.3). This results in having multiple stress factors on plant growth.
There is an interaction hence between ambient air, light variation and depletion of plant available water in the drought. During a day, leaf temperature increases relative to the air 48 above the leaf. When leaf temperature increases, internal vapour pressure increases in comparison to that in the air, causing vapour pressure gradient to increase (Nilsen and Orcutt 1996). In response, transpiration increases and induces stomatal closure and wilting at the end of the day.
Table 3.3 Characteristics of dry season (Jan-March) and wet season (Apr-Dec) in Ratnapura, Sri Lanka (FAO, 2005) (ET0 – Potential evapotranspiration)
N Rn VPD Tmax Season -2 -1 0 (hours) (MJm day ) (kPa) ( C)
Dry 5.4 17.1 1.0 34.1
Wet 4.4 15.5 0.9 32.6
3.5.3 Tea plant response to drought
Visible indicators of the effects of drought conditions on tea plants are leaf wilting, shedding of leaves and eventual die back (Fuchs 1989) and yield reduction (Burgess and Carr 1996). In Tanzania, after a 16-week drought treatment, it was found that means light interception was reduced by 25% and mean radiation efficiency was reduced by 78%. Further drought reduced the dry matter partition to leaves, stems and shoots by 80-95 % (Burgess and Carr 1996). Bud break and shoot growth is inhibited by moderate water deficits (Borchert 1994). There was a reduction in relative extension and development rates of tea shoots due to drought (Burgess and Carr 1997). It has been found in other crops also that shoot growth is very sensitive to water deficit induced by drought (Chartzoulakis, Noitsakis et al. 1993). In a thorough analysis of drought conditions in low-grown tea areas of Sri Lanka, Wijeratne (1996) reported a reduction of shoot population density, shoot extension rates and weight of tea shoots in two drought tolerant and drought susceptible cultivars.
Jeyaramaja et al (2005) observed a 50% reduction of photosynthesis under a relatively moderate water deficit of 1.5 KPa. When soil water potential reaches 2.0 KPa, or severe water stress, there was a complete reduction in photosynthesis (Jeyaramraja, Meenakshi et al. 2005). The time taken for soil to shift from mild to severe stress depends on the rate of water loss from the plant. Hence in high temperature zones like Ratnapura, plants can undergo severe stress even during short dry spells
Fully grown tea canopy covers the ground completely allowing little solar radiation to penetrate down to the soil surface. Hence evapotranspiration is almost equal to transpiration in tea (De Costa, Mohotti et al. 2007). Tea transpiration rate is sensitive to soil water
49 availability. The transpiration reduction is attributed to gradual stomatal closure and consequent reduction in stomatal conductance (De Costa, Mohotti et al. 2007)
While lack of available water is the major cause of drought stress, disease may exacerbate the effects of water stress. For example, stem canker disease of tea is associated with drought conditions (Carr and Stephens 1992). Heat stress causes cracks in the stem and this reduces the ability of the plant to transport water to the shoot. The water supply from soil to canopy has an effect on the daily trunk shrinkage (Ortuno, Alarcon et al. 2005), which may have an effect on stem cracks. The cultivar TRI 2023, while being a high dry matter producer, is highly susceptible to stem canker and hence is not considered suitable for cultivation. It will be important to study the swelling and shrinkage of the stem to evaluate the relationship between drought and stem canker disease occurrence. Further, the high incidence of stem canker in fields with gravel and stone could be associated with high soil temperature and low water holding capacity during dry periods.
3.5.4 Drought mitigation
There are various methods applied in tea plantations to mitigate the effects of drought, including the selection of drought tolerant cultivars, application of plant growth regulators (Wei 2004), grafting (Luo and Liu 2000) application of soil mulches (Othieno 1980), maintenance of shade plants (Wijeratne and Ekanayake 1990) and supplementary irrigation (Carr, Dale et al. 1987). The main reasons for poor adoption of drought mitigation strategies in the field are their additional cost and that many of them, apart from irrigation, have a history of failure. Mulching is practiced commonly to mitigate drought, using either grass or plastic mulch. Plastic mulch however reduces the soil water infiltration during the rainy season and effect of grass mulch on soil moisture conservation depend on nature and/or rate of decay (Othieno 1980). Root stock selection for grafting tea to mitigate drought stress is very laborious (Prakash, Sood et al. 1999). Chemical application for drought mitigation has to be carried our prior to ceasing the rainfall and which is mostly very difficult to predict with nature of rainfall.
When selecting drought mitigation strategies, growers are mostly concerned with the investment cost and the return on investment with increased effect on yields. Selection of drought tolerant cultivars appears to be an attractive option, but most of the drought tolerant cultivars have low yields (Wijeratne 1996). Widespread use of vegetatively propagated plants has resulted in a lower ability to withstand drought in tea plantations as compared to seedling tea. Seedling tea usually have a deeper root system, sometimes more than 5.0m (Carr, Dale et al. 1987). During dry season seedling tea maintained a higher xylem water potential and higher degree of stomatal opening, showing a lower drought stress than vegetative propagated 50 tea (Carr 1977). It is advised to include drought tolerant tea cultivars in the planting program. However, when selecting new plant varieties though seedlings are not normally included (Wijeratne and Ekanayake 1990). Though application of plant growth regulators is an easy and popular method in some tea growing countries, it is not recommended under Sri Lanka conditions, as application of chemicals may have a negative effect on consumer demand.
High yielding cultivar scions grafted either to seedling or drought tolerant vegetative propagated tea stock is termed a composite plant. Use of composite plants is a cost effective solution to convert low yielding, drought resistant tea plantations into high yielding plantations, with scions selected from high yielding tea cultivars. However, this technique is not popular except in China and some African countries because it is a complex process and some root stock x scion combinations are not giving promising results (Luo, Qian et al. 1999; Mizambwa 2002).
3.5.5 Impact of climate change in low elevation tea sector
There are growing indications that the effect of climate change has already been experienced in Sri Lanka. Scientists attribute that the country is experiencing both a global change effect and local heat island effect, caused by rapid urbanization (Emmanuel 2001; Basnayake, Fernando et al. 2002; Fernando and Basnayake 2002). Annual temperature is on an increasing trend and rainfall is on a decreasing trend. During the 1961-1990 period, mean air temperatures of the country increased by 0.0160C per year. Mean annual precipitation decreased by 134mm (7%) compared to 1931-1960 (Chandrapala 1996). Verifying this claim, Herath and Ratnayake (2004) found a clear sign of decreasing rainfall in tea growing regions and reduction in rainy days, after analysing 60 observation point rainfall data. And further to this, Madduma Bandara and Wickramagamage (2004) observed that the decline in rainfall in western slopes of Central Highlands is particularly significant because it is the catchment area for low elevation tea growing areas. The significance of this for Sri Lankan tea production is that we know for every 100mm reduction in annual rainfall we can expect a reduction of 30- 80 kg of made tea per hectare (Wijeratna, Ananthacoomaraswamy et al. 2007).
Climate models predict that mean annual temperature of Sri Lanka will increased by 0.9- 4.00C over the base line 1960-1990, by the year 2100 (Eriyagama, Smakhtin et al. 2010). While the effect of such changes is effective in all over the island, there are some climate hot spots where effects are more significant than other areas. Also there are some crops that are more vulnerable to the climate change. Low elevation grown tea is more vulnerable to changes in climate changes than high elevation grown tea (Wijeratna, Ananthacoomaraswamy et al. 2007). The above findings suggest that studying the drought related stress in tea cultivation is vital for long term survival of the industry. 51 3.6 Irrigation in Tea Plantations
Tea is most commonly grown as a rain-fed crop. The earliest investigations into tea irrigation in the world were made in Sri Lanka in the 1950s (Rogers 1959). The irrigation idea did not take root in Ceylon (as Sri Lanka was called before 1972) but was practiced in many tea growing countries, like Iran (Salardini 1978), Azerbaijan (Rekvava 1986; Kuliev 1988), Malavi (Willatt 1970), Tanzania (Carr 1974), Kenya (Othieno 1978) and India (Hajra 2001) . The main types of the irrigation practised were sprinkler irrigation largely but surface irrigation is also practiced in some tea growing regions (Tkebuchava 1988; Dabral and Rao 1997). Recently there has been much interest on drip/trickle irrigation in Tanzania and Malawi (Möller and Weatherhead 2006; Kigalu, Kimamboa et al. 2008) as well. Indeed, nearly 20% of Tanzania’s tea plantations are under irrigation (Möller and Weatherhead 2006). When compared to other major crops research on tea irrigation has been limited. Many experimental details were based on the research done on East African tea growing countries.
Adaptation of the irrigation in the Asian region has been limited to some Indian plantations, both in northern India and southern India, Iran and some former Soviet Union countries. Even though Sri Lanka is one world’s leading tea exporters, there has been little interest in irrigation technology until recently. The lack of interest is mainly due to lack of water sources, cost of irrigation systems, significant profit even under rain-fed cultivation and lack of knowledge about the irrigation among growers.
Response of the tea plant to supplemental irrigation has been analysed on yield, physiological, growth and financial basis. The response largely depends on climate of the study period and temperature and associated saturation deficit of the dry period being the most significant co factors (Carr, Dale et al. 1987). Irrigation has been mostly successful in the tea growing regions where there is only one rainy season and a prolonged dry season that can be lasted for 4 to 6 months (Carr 2010). For other tea growing regions, where bimodal rainfall pattern prevails, drought mitigation methods are generally advised. However, when the short term drought is coupled with high temperature stress as well, it may need more robust drought mitigation methods (Fuchs 1989). One of the main factors facilitating the application of irrigation and subsequent research on irrigation is facilitated by the close proximity of the cultivation to water sources. In many tea growing locations there are no water sources for irrigation. When such drought occurrences happen growers either shift the cultivation to some other crop (e.g. rubber is such an alternative crop, used by growers in low elevation tea growing areas of Sri Lanka) that resist drought or start a new planting program.
52 3.6.1 Yield response to irrigation
The early work in Ceylon of Rogers (1959) reported yield increases of more than 50% in six year old tea plants under overhead sprinkler irrigation. In Malawi yield increases from 1000 to 2000 kgha-1, were reported from six year old heterogeneous seedling tea during the period 1967-1970 (Carr 1974). Part of that annual yield increase was associated with reduced yield fluctuations across the seasons. The water productivity (which will be referred to as ‘water use efficiency’ in this thesis) of that irrigation treatment was reported as1.4 kg(ha.mm)-1. However seasonal yield elevation was not observed when the ambient temperature fell below base temperature of 12.50 C required for plant shoot growth (Carr 1971). But the yield increase once the limiting factor is eliminated in the subsequent raining months. Also when the air temperature reached above 300C for 30 consecutive days and dry air (saturation deficits >2.0kPa) conditions experienced, the tea for the irrigation treatment was reduced to 0.3kg(ha.mm)-1 of water applied (Carr, Dale et al. 1987). This is equal to less than one third in a normal year. Modifications of the microclimate have been recommended in such instances to control air temperature and humidity. This has been observed in field trials done in Malawi and Tanzania. Application of mist irrigation at short time intervals increased tea yield during the dry season, as compared to sprinkler irrigation in Malawi (Tanton 1982).
In Malawi, vegetatively propagated tea yielded twice more than heterogeneous seedling tea under sprinkler irrigation, Some varieties gave a very high response to irrigation water applied, for cultivar, 6/8, gave a water productivity of 1.9-2.9 kg(ha.mm)-1 (Stephens and Carr 1991). However in Tanzania seedling tea gave a comparable yield increase with vegetatively propagated tea, when the saturation deficit was controlled by micro jet mist irrigation, in addition to sprinkler irrigation (Clowes and Starch 1988).
The response of different tea cultivars to irrigation is not the same. Some cultivars produce very high response, while some other cultivars produce low response (Burgess and Carr 1996), and this difference is attributed to the dry matter partitioning of different cultivars (Burgess and Carr 1996). Also in the areas where drought prevails during winter periods, the difference in the shoot basal temperature for different cultivars has an effect on tea yield (Burgess and Carr 1997; Carr 2010). In Kenya, Smith et al (1993) found no interaction between cultivar and irrigation. So when implementing an irrigation system on tea plantations, it is imperative to assess the available tea varieties to make the best of the irrigation investment.
53 3.6.2 Effect on tea physiology
Studies have been undertaken to measure the irrigation effects on physiology of tea plants. The relative significance of photosynthesis on yield is sometimes confusing in tea. Some researchers suggest that since tea have a low harvesting index, photosynthesis does not have effect on tea yield (Squire and Callander 1981). However, others have shown that photosynthesis and yield are not independent (Stephens and Carr 1991; Smith, Stephens et al. 1993). Even if there are low yields from irrigated plants due to climatic limitations other than moisture stress, an increase in photosynthesis has an effect on the later yield. So the enhanced photosynthesis of the plant due to irrigation during low yielding months, either very hot or cool, can accumulate carbohydrate reserves and mobilized to promote emerging shoots (Hakamata and Sakai 1980).
Water stress reduces photosynthesis through direct influence of metabolic or photochemical process in the leaf, or indirect influence on stomatal closure and cessation of leaf growth which results in decreased leaf surface (Dejong 1996). When the leaf water potential reached -2.0MPa under growth-room experiment, there was a nearly complete inhibition of photosynthesis (Jeyaramraja, Meenakshi et al. 2005). Irrigation increases the photosynthesis by increasing photosynthetic rate per unit leaf area and by increasing the proportion of light intercepted by photosynthetically efficient leaves. Increase in photosynthetic rate could be accounted for by increases in stomatal conductance and associated reduction in leaf temperature (Carr 2010). Irrigation also prevents the photosynthetic rate decreasing under high luminance (Smith, Burgess et al. 1994). Otherwise tea photosynthetic rate decreases when the photon flux density increases beyond 1000 µmolm-2s-1 (Squire 1977).
3.6.3 Varietal difference in response to irrigation
Irrigation increased the photosynthesis rate and stomatal conductance while reducing leaf temperature of cultivar 6/8 in Tanzania (Smith, Stephens et al. 1993). In the same experiment, it was shown that there was no cultivar interaction with irrigation treatment. However in later studies done on young tea plants Burgess (1996) found that there was a cultivar difference in response to irrigation.
The botanical basis of yield increase in tea is based on changes in basal shoot population, advanced peak population density with onset of rains and an increase in shoot weight (Stephens and Carr 1994). As most of the results are based on sprinkler irrigation trials it will be important to evaluate the yield increment of drip irrigation, in dry season as compared to wet season.
54 3.6.4 Growth response to irrigation
Difference in dry matter production in different cultivars as response to irrigation was visible when analysing the pruning weight of irrigated and drought affected plants. Dry weight of foliage removed at pruning were greatest for cultivar TN 1403(7 years of age), is 34 t/ha (well-irrigated) and 22 t/ha (droughted). For the same age cultivar, PC81, well irrigated treatment produced 27t/ha (Carr 2010). In comparison to other popular drought mitigation method of grass mulch, irrigation has resulted in higher root depth and distribution (Willatt 1970). As against the popular belief that irrigation results in lower root volume and distribution, there are some evidence that irrigation has increased the root development (Carr 2010). But there was no difference in canopy root:shoot ratio among irrigation and rain-fed plants (Nixon and Sanga 1995). The above findings are however based on experiments conducted at Kenya and Tanzania, where soils are deep and root depth of more than 5m have been reported. In those sites, winter climate is the mostly limiting factor for root growth. But in contrast, low elevation tea fields in Sri Lanka, soils are shallower (Panabokke 1996) and favourable temperature prevail throughout the year for root growth.
3.6.5 Economic evaluation of tea irrigation
There are mainly two types of irrigation systems, drip irrigation and sprinkler irrigation. On a commercial level in India, sprinkler irrigation of tea produces higher yields than drip irrigation (Hudson 1991). But in other crops, orange in humid climates and cashew in Australia, drip irrigation has been able to produce similar yields as with sprinkler irrigation (Myers and Harrison 1978; Blaikie, Chacko et al. 2001). The importance of maintaining vegetative growth in contrast to the balance between vegetative and reproductive phase in most tree crops, can be a decisive factor between selection of drip and sprinkler irrigation. Renewed interest has been placed in African tea growing countries due to the pressure applied on the existing surface water sources (Möller and Weatherhead 2006; Kigalu, Kimamboa et al. 2008). Benefits of drip irrigation include better plant survival, greater yields, more efficient distribution of nutrients and less plant stress (Çetin, Yazgan et al. 2004; Kigalu, Kimamboa et al. 2008). Drip irrigation would be an ideal tool for the tea producing region like low elevation tea growing areas of Sri Lanka, where water sources are scarce for irrigation during the drought periods. The recent introduction of in-line drip tubes for closed orchard cultivation has been an alternative for the high cost associated with in-line drip emitter installation for a high density crop like tea. Industrial communications and some unpublished data also suggest high yield with drip irrigation in Sri Lanka (Ananthacumarswamy, Herath et al. 1985).
55 The conventional drought mitigation method of using drought-tolerant tea cultivars is associated with lower productivity and the success of the methods mostly depends on the severity of the drought, unlike irrigation. The characteristics that confer drought-tolerance, and consequently lower productivity, are smaller leaves, thicker cuticle, lower transpiration and photosynthesis rates. Application of irrigation with other inputs like fertilizer and pesticides could increase the productivity and income (Carruthers and Clark 1983). Irrigation system selection is affected by water source, land to be irrigated, plant and soil type (Çetin, Yazgan et al. 2004). An early and isolated tea irrigation experiment in Sri Lanka from the 1950s, before the introduction of drip systems, illustrated the relative profitability of sprinkler irrigation compared to rain-fed cultivation (Rogers 1959). Trials with drip irrigation in low temperature, high elevation tea growing area of Sri Lanka showed positive benefit/cost ratio (BCR) of 7.7 for irrigation (Ananthacumarswamy, Herath et al. 1985). These earlier trials from the highlands have encouraged the exploration of drip irrigation in low elevation areas. However the growing environments are very different between highland and lowland. Also the TRISL breeding program has focussed on cultivar performance under rain-fed conditions. So the financial viability of drip irrigation systems in low elevation areas is going to depend not only on the costs of production (pump and line installation, wage rates) and tea leaf price, but very much on the yield response of the chosen cultivar to irrigation.
3.7 Conclusion
This chapter has reviewed the botany and physiology of the tea plant particularly with respect to its water relations. Even though the tea plant originated in a cool humid environment, it is cultivated across a wide range of environments; of particular interest here is its performance in the hot humid low elevation tea growing areas of Sri Lanka. In such an environment the tea plant experiences significant water stress on a seasonal basis. Irrigation has been of international interest in tea for 50 years, but only recently has it been seriously considered in Sri Lanka. Strong cultivar differences in response to irrigation were observed in East Africa (Smith, Burgess et al. 1994) and such differences are likely in Sri Lanka as well. Also in East Africa low ambient temperature effect was found to be the main limiting factor in irrigation trials. This is unlikely to be a problem in the low elevation areas of Sri Lanka. However, high air temperatures are characteristic of this environment and the impact of upper temperatures has not been defined. Therefore this review supports the need for research on the response of tea to irrigation to mitigate water stress under hot humid conditions.
56 Chapter 4
Experimental Site Description and Location
4.1 Introduction
This thesis is concerned with understanding the behaviour of tea plants during hot humid drought in the low elevation tea growing areas of Sri Lanka and their response to irrigation. It includes five field trials and two glass house experiments. The field and glass house experiments were carried out at the St. Joachim Estate, Tea Research Institute, Ratnapura, Sri Lanka. The results of these experiments and investigations are presented in the sequence of Chapters 5 to 7. As the materials and methods of many of the experiments overlap they are presented here for the sake of brevity and clarity. This chapter begins with a brief summary of the parameters of each experiment in the form of a table including objectives, location, duration and measurements (Table 4.1). This is followed by a description of the main field research site, its long term climatic conditions and description of the methods used across the experiments. The characteristics of the different tea cultivars tested and environmental conditions during the experiments will be discussed in the Chapter 5 to 7 separately.
4.2 Experimental Overview
Table 4.1 presents an overview of the key parameters of each experiment, particularly the ‘where’, ‘when’, ‘with what’ and ‘how’ of each experiment. Field experiments were conducted with the object of revealing the performance of the tea plant during hot humid dry period and inferring specially the weather relations. Experiments 1 and 2 in Chapter 5 and experiment 6 in Chapter 7 are based on an in-line drip irrigation trial. Meanwhile experiment 3 of Chapter 5 evaluates the performance of the newly released tea clones during drought in same field conditions, but not under irrigation. Glass house experiments (Chapter 7) were conducted to simulate the conditions that could not be achieved in the field. The maximum dry period observed in recent years at Ratnapura is 32 days in 1992. Because of this it was necessary to simulate drought with two experiments involving young tea plants in a glass house. In total, the experimental work occurred over the period 2007-2009. This table may help to explain why the explication of the experimental work in the results chapters follows a logical, rather than temporal, sequence.
57 Table 4.1 An overview of the experimental parameters
Chapter Experiment Description Location / Period Period Cultivars Measurements section 1) Physiological and yield St.Joachim Yield related physiological Pn, gs, El, Tl 5.2 performance of two contrasting Estate 2007 Jan-Mar TRI 2023 tea cultivars in response to parameters , water potential TRI 3025 irrigation Ratnapura. St.Joachim Transpiration measurement in wet 2008 June- Sap flow measurement 2) Water use of tea under rain- 5.3 Estate TRI 2023 fed and irrigated conditions and dry seasons 2009 Mar dry matter production Ratnapura,
St.Joachim TRI 3014 Physiological response of newly TRI 3025 3) Physiological response of 5.4 Estate 2009 Jan-Mar TRI 4053 Pn, El, Tl new cultivars to water stress released tea cultivars to drought Ratnapura TRI 4047 TRI 4049 Physiological and yield response to St.Joachim 2008 Jan- P , g , E , T 4) Evaluation of irrigation n s l l 6.0 TRI 2023 technology drip and sprinkler irrigation Estate Ratnapura 2009 Mar yield Effect on duration of stress period Glass House growth parameters 5) Effect of water stress duration 7.2 on young plant growth and 2007 Jul-Nov TRI 4042 on young tea plant growth Ratnapura dry matter production physiology Effect of water application rate on Glass House growth parameters 6) Effect of partial irrigation on 7.3 2007 Jul-Aug TRI 4042 young tea growth young tea plant growth and duration Ratnapura dry matter production Growth response of young tea in St.Joachim 2008 May- growth parameters 7) Effect of irrigation and raised 7.4 field to drip irrigation and raised Estate TRI 2023 bed on young tea growth 2009 Mar dry matter production beds Ratnapura St.Joachim Economic analysis of drip irrigation Net Present Value, Internal 8.0 8) Financial evaluation of drip Estate 2001-2009 TRI 2023 irrigation for low elevation tea growing area TRI 3025 Rate of Return Ratnapura
58 4.3 Field Experiment Site and Location
4.3.1 Site and soil description
The field and glasshouse experiments were carried out at Field no 01, St. Joachim Estate, Tea Research Institute, Ratnapura Sri Lanka (latitude: 6040’N; longitude: 80025’E, altitude 29m amsl). St.Joachim Estate is a research estate managed by Tea Research Institute of Sri Lanka. In addition to the main crop of tea, there are some divisions with rubber cultivation in the estate. Tea fields in low elevation areas are scattered, unlike tea estates in up country Sri Lanka (Figure 4.1). The estate landscape is flat to undulating. Field no 01 is generally a flat area and represent the general climate of low elevation tea growing area (Figure 4.2 and 4.3)
In addition to main crops like tea, rubber and rice (in flat alluvial plains), there is perennial vegetation, planted on homesteads or growing naturally. This is a very different landscape from high-grown tea, where there is little perennial vegetation other than trees regularly planted as shade trees. Perennial vegetation improves the environment substantially as a whole (Wickramasinghe 1992). Presence of perennial vegetation provides shade and coolness, lessening the impact of tropical sun (Renault, Hemakumara et al. 2001). Cultivation of shade trees as either high or medium shade is an approved method in the area, however, as the planted perennial trees too consume large portion of water in the area (Renault, Hemakumara et al. 2001), shade trees were not planted in the irrigation research site. In the small holder fields, it is rare to find proper high shade trees, as it is difficult to maintain high shade plants in a small plot of land. The other assumption is that during wet season of the year (Figure 4.4), shade plants could limit the sunshine hours available limiting the productivity of irrigated plants.
The soil group of the site belongs to Red Yellow Podzol (Panabokke 1996), according to local classification. According to the FAO/UNESCO classification, it is classified as Haplic Alisol (Mapa, Somasiri et al. 1999). Surface soil layers are sandy clay loam and the sub surface soil layers are clay loam soil. In agricultural terms, the soil is shallow and weathered rock fragments are redundantly available in soil. The shallow soil depth is responsible for shallow rooting of tea plants in the area. In the research field, there is an impermeable soil layer sometimes as shallow as 1-2m. The volumetric moisture content of the soil at field capacity and permanent wilting point are 27% and 14% (v/v) respectively. The water holding capacity of the soil is 130mm/m. Mature tea plants in the Field no 01 were affected severely in a major drought in 1992. Since then, a replanting program was launched with cultivar TRI 2026. Experimental section of the drip and sprinkler irrigation trials were planted in 1999 and 2000 (Details are given in Chapter 5.0 and 6.0).
59 A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 4.1 Geographical distribution of major plantation crops in Sri Lanka. Ratnaura geographical location 6040’N, 80025’E (Department of Census & Statistics, Colombo)
60 A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 4.2 Aerial view of main research field at St. Joachim Estate, Ratnapura, Sri Lanka (a) Drip irrigation site (Experiments 1 and 2) (b) Sprinkler irrigation site (Experiment 4) (Source : map.google.com accessed on 26.09.2011)
Figure 4.3 View of sprinkler operation at Experiment 4 site. (Rain Bird SW 2000 sidewinder)
61
Figure 4.4 View of drip irrigated and rain-fed TRI 3025 plots in experiment 1. The scattered line shows the boundary between irrigated and rain-fed plants. The rain-fed plants show defoliation during drought in 2009 March. (White plastic was to cover sap flow sensors during heavy rains)
4.3.2 Climate
The climate of the St Joachim Estate, Ratnapura can be described as hot humid climate, where average air temperature is 28.0(±0.1)0C and relative humidity is 77(±0.4)%. Highest maximum air temperature are observed in February (34.5(±0.1)0C) and March (34.8(±0.2)0C). The rest of the months are relatively cooler. The lowest air temperature of 22.1(±0.1)0C are recorded in the months of January and February. The minimum air temperature is always above the basal minimum temperature that supposedly needed to maintain the shoot growth in tea (Burgess and Carr 1997). The initial 3 months of January to March receives highest sunshine hours of more than 5 hours a day and the gloomiest month is October, where average sun-shine hours is only 3.7(±0.2) hrs a day. The availability of solar radiation is 15.9MJm-2s-1 and again the months of January to March receive highest solar radiation. Vapour Pressure deficit (VPD) is an important component in tea drought physiology and VPD above 2.0kPa limits the shoot growth of tea (Squire 1979). The average VPD in the estate is 0.9(±0.02) kPa. Only in the months of February and March is there a 10% increase of the VPD. The daily average plant water requirement according to FAO modified Penman- Monteith calculation (Allen, Pereira et al. 1998) is 2.9(±0.06)mm. Nevertheless, February and March are high water demanding months, where crop water requirement average is 3.2 and 3.3mm/day respectively.
62 4.3.2 Rainfall pattern
The area belongs to WL1 agro ecological region (Wet zone, Low elevation), where 75% expectancy of annual rainfall is 3200mm (Punyawardana 2008). The average annual rainfall is 3824(±41) mm, with a bimodal rainfall pattern with two monsoonal rain peaks. The long term annual rainfall data of more than 140 years of data showed no significant trend either increasing or decreasing, despite concern about global climate change (Figure 4.5). The variation of the annual rainfall is minimal (CV=12.6%). There is a chance of one in five years of the annual rainfall varying either 3392 or 4254mm (Figure 4.6). It means that extreme low or high rainfall years are rare. However, variation within the year was not analysed. The South-West monsoon is very moist and variable, (mid May to September), and North- East monsoon from December to February, which is dry and consistent (Fuchs 1989). In addition, there are two inter-monsoonal periods also bringing rain. The first inter- monsoonal rains occur between March to April and second inter-monsoonal period occurs between October to November (Eriyagama, Smakhtin et al. 2010).
There is a high variation in monthly rainfall, May and October are the highest rain months, with mean monthly rainfall of 477(±18) and 474(±13) mm respectively. Average monthly rain of January and February is lower, each month only receiving 4% of annual rainfall (Figure 4.7). Figure 4.8 shows the monthly tea crop water requirement (FAO modified Penman-Monteith method) and monthly rainfall availability. The average water requirement in a month is 88(±1.7) mm/month. March is the highest water consuming month, with evapotranspiration demand of 100mm. The average total rainfall availability in a month is always above the crop water requirement. However as rainfall is lost from the root zone as run-off or drainage, effective rainfall is lower than total rainfall (Burman, Cuenca et al. 1983; Allen, Pereira et al. 1998). Effective rainfall calculated for each month according to USDA soil conservation method (USDA 1970; Allen, Pereira et al. 1998) and field based method are given in Figure 4.8. The effective rainfall of January and February, according to USDA (1970) method, are 25 and 19% higher than the crop water requirement, which is a marginal increase. The USDA method of evaluating effective rainfall was based on the long-term data analysis. However, according to field based estimation, effective rainfall is lower than plant water requirement in the period from January to March. Field based methods are more accurate as they based on field and soil characters rather than just monthly rainfall.
63 maximum 36 (a) minimum average 34
32 C) 0 30
28
26
Temperature ( 24
22
20 7 (b) 18 sunshine )
radiation -1
16 6 day -2
5 14
Sunshine (hours) Sunshine 4 12 Net Radiation (MJm Radiation Net
3 10 1.2 (c)
1.1
1.0
0.9
0.8 Vapor pressure deificit (kPa) deificit pressure Vapor
0.7 3.4 (d)
3.2
3.0
2.8
2.6
Crop evapotranspiration (mm/day) 2.4 Jan Feb Mar Apr May Jun July Aug Sep Oct Nov Dec Month
Figure 4.5 Monthly climate average of St. Joachim Estate, Ratnapura (a)air temperature (0C) (b)sun shine hours and solar radiation (MJm—2day-1) (c)vapour pressure deficit (kPa) (d)tea crop evapotranspiration (mm/day). (Data shown based on 10 year weekly average, except tea crop evapotranspiration. Error bars show the standard error)
64 A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 4.6 Annual rainfall (mm) Ratnapura, Sri Lanka 1869-2010 (DeCosta 2011). Dashed line shows the mean annual rainfall
A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 4.7 Percentile of annual rainfall distribution 1869-2010 (DeCosta 2011)
65 1200 minimum mean 1000 maximum 25% 75% 800
600
400 Rainfall (mm/month) Rainfall
200
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Figure 4.8 Monthly variation of rainfall Ratnapura, Sri Lanka 1869-2010. Monthly rainfall values of mean, maximum, minimum, 25% and 75% expectant rainfall values are shown 4.3.3 Seasonal aridity
In the field tea plants experience water stress due to short rainless periods. Such short dry spells may not be termed as conventional agricultural drought in countries other than Sri Lanka. Drought is defined as a temporary lack of water, caused by abnormal climate, damaging the environment (Wilk and Hughes 2002). More precisely, such short dry spells can be termed as a seasonal aridity (Kallis 2008), i.e. a recurrent dry period after monsoon rains. Nevertheless, the word “drought” is commonly used to refer to rainless periods greater than 5 days for perennial crops in Wet zone of Sri Lanka (Sumanasena 2008). Even though similar seasonal aridity occurs after the South West monsoon in August–September, and in January – March, after North East monsoon, severity is high in the January–March period, due to low monthly rainfall as well as high ambient temperature. (Figure 4.5 & 4.8).
4.4 Significance of short rain-free periods in creating water stress
Long term climate averages for Ratnapura were presented earlier to indicate the general climate of the area and to identify water stress periods. However, combining the actual weather data and plant water use enables us to identify the level of stress plants actually experience in the field, especially during the establishment period. This section presents the results of a desk-top analysis of long term rainfall data for Ratnapura to:
1. determine the frequency of various rain-free periods, and
66 2. to use a water stress coefficient to two case study seasons to illustrate the distribution of water stress.
700 ET0 total RF 600 effective RF (USDA) effective RF (field)
500
400
300
200 Water use & Rainfall (mm/month) Rainfall & use Water 100
0 Jan Feb Mar Apr May Jun July Aug Sep Oct Nov Dec
Month
Figure 4.9 Monthly tea crop water requirement (FAO modified Penman-Monteith method), monthly total rainfall and effective rainfall according to USDA and field estimated method using long-term (>140 years) monthly rainfall data 4.4.1 Daily rainfall intensity
The average annual rainfall in Ratnapura area for the period 1869-2010 period is 3822(±41) mm. This rain is however not evenly distributed across the year. The high rain is mainly caused by two monsoonal peaks which were described earlier. Based on the daily rainfall data of 1986-2010, the average number of rainless days in a year is 156(±2.1) days (Figure 4.10a). In tea, to consider as a wet day, daily rainfall of at least 3mm should be received (Ananthacumaraswamy and Prematunga 2008). The number of days, receiving rainfall less than 3mm, is 200(±2.4).
Figure 4.10b shows the average duration of different frequency rain-free periods from 5 days to 30 days during the period 1986-2010. The most frequent rain-free periods across the year are of 6 to 7 days duration. On average there are between 3 to 4 such rain-free periods in a year. During the measurement period, there were two major duration drought events of more than 30 days. These were in 1989 and 1992. The 1992 drought lasted for 32 days and average production of tea dropped by 25% due to this period
67 250 (a)
200
150 Days 100
50
0 0>2>3
Rain (mm/day)
4 (b)
3
2 Frequency
1
0 5 6-7 8-10 11-14 15-20 20-30 >30 Drought length (days)
Figure 4.10 (a) Average number of days without rain, >2mm/day or >3mm/day and (b) Frequency of drought duration from 5 days to >30 during 1986-2010 period
4.4.2 Water stress coefficient (Ks)
Figure 4.10 in the above paragraph shows that on average tea plants experience 6-7 day drought at least 3 times a year. Nevertheless, it does not represent that the tea plant is experiencing severe drought stress in each of such occurrence due to (1) age of the plant - mature plants has an ability to withstand drought. (2) soil factors, like water retention (Li, Yang et al.) climatic factors – specially previous rainfall and air temperature during the rainless period. It is a common factor that young tea plants are responsive for even short duration drought. An attempt was made to quantify the drought effect one year old tea plant could experience in low elevation tea growing area.
Water stress coefficient (Ks) for young tea was calculated based on the following formula (Allen, Pereira et al. 1998).
68 ܶܣܹ െ ܦ ܭ ൌ ݎ ݏ ሺͳെሻܶܣܹ
Where, Dr is the root zone depletion (mm), and TAW is the total available soil water in the root zone (mm). Dr is calculated using Instat climate software (Version 3.036), for a root zone depth of 35cm. Dr was calculated based on the effective rainfall in an open tea field, based on run off, drainage, stem fall and through fall (Appendix 1). Root zone depth was based on sampling root length of large number (n=48) of one year old tea plants. A field survey of rooting depths of mature tea bushes in the low country wet zone of Sri Lanka showed that in this environment the root system was confined to a depth of 30-35cm. It was further observed that beyond the above depth the root system moves laterally (Vithana 2003). Water retention of the soil was calculated earlier for rain-fed fields as 130mm/m. p is the fraction of TAW that a crop can extract from the root zone without suffering water stress. For tea, p is given as 0.40 for non-shaded tree for evapotranspiration of 5.0mm/day. Hence for Ratnapura p is calculated as follows (Allen, Pereira et al. 1998)
ൌ ͲǤͶ ͲǤͲͶሺͷ െ ܧܶሻ
Where, ET is the tea evapotranspiration (mm/day).
Tea evapotranspiration was calculated based on the FAO modified Penman-Monteith equation, with a crop coefficient of 0.85 (Allen, Pereira et al. 1998). There were some days, where some climate variables were missing. In such days, ET0 was calculated based on the Class A pan evaporation value and multiplying with crop coefficient of 0.85 (Laycock 1964).
The daily water stress coefficients for 2009 and 2010 are shown as the broken line in Figure 4.11, while the vertical bars are daily rainfall events. This figure shows that young plants are experiencing severe water stress condition due to lack of rainfall specially in the first 4 months of the year. In addition, similar serious water stress conditions were observed in May 2009 and August 2010 a time of year normally considered as the wet season. The maximum rain-free period in 2009 was 22 days and in 2010 it was 16 days. Overall 2009 was a very wet year and it received annual rainfall of 4969mm, which is 30% higher than long term annual average of 3824(±41) mm. It shows that even in very wet year, there is a chance for young tea plants to experience severe water stress in the field.
69 Rain 180 (a) KS 1.0 160
140 0.5 ) S
120 0.0 100
80 -0.5
Rainfall (mm/day) Rainfall 60 -1.0
40 Water stress coefficient (K -1.5 20
0 -2.0 180 (b) 1.0 160
140 0.5 ) S
120 0.0 100
80 -0.5
Rainfall (mm/day) Rainfall 60 -1.0
40 Water stress coefficient (K -1.5 20
0 -2.0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Figure 4.11 Water stress coefficient (Ks) for young tea, based on daily crop water requirement and daily effective rainfall (a) year 2009 (b) year 2010
4.5 Conclusion
The field experiment site at St. Joachim Estate Ratnapura, belongs to the WL1a agro-climate zone and shows a hot humid climate. While the annual rainfall far exceeds the plant water requirement, the temporal distribution of rainfall within a year and lower rainfall availability in January-March period, indicate distinct periods of water stress. These periods also receive the maximum temperatures in the year. This analysis based on daily rainfall, crop water use and soil water retention, supports the claim that plants experience severe water stress even within a year with very high annual rainfall.
70 Chapter 5
Physiology, Water Use and Yield in Low Elevation Tea – Analyzing with Special Reference to Dry Period of the Year
5.1 Introduction
In low elevation tea growing areas of Sri Lanka, drought causes significant yield reductions and sometimes death of tea plants during the comparatively short dry period between January and March. One prominent feature associated with drought in these areas is the high ambient temperature which sometimes reaches more than 350C (Fuchs 1989). The effect of high vapour pressure deficit (VPD) during drought is the main factor in lowering yield in other tea growing regions, such as central and east Africa (Squire and Callander 1981). VPD does not exceed 2.0 kPa in the low country of Sri Lanka and is not so much a problem. However, exposure to high temperature is commonly recognised to adversely affect growth within a few days, subsequently to impact on yield and sometimes the death of the plants.
Like water stress, high temperature stress strongly affects photosynthesis, growth and the survival of plants (Chaves, Pereira et al. 2002). In tea, photosynthesis and related physiological parameters such as stomatal conductance, transpiration rate and transpiration efficiency are used to evaluate the effects of drought (Lin 1998; Marimuthu and Kumar 1998; Ajayakumar, Jayakumar et al. 2001). Among them, photosynthesis is particularly useful as it is a direct measure of productivity (Bannerjee 1993), sensitivity to drought (Berry and Bjo¨rkman 1980) and strongly influenced by temperature (Joshi and Palni 1998).
The influence of drought has been well recorded in the low elevation tea areas of Sri Lanka, where monthly tea yields closely follow the bimodal rainfall pattern (Wijeratne and Fordham 1996). The relative responses of different tea cultivars have also been recorded and one of the main strategies in controlling drought damage is the selection of drought-resistant cultivars (TRISL 2002). Nevertheless, considerable drought-induced yield reduction still occurs even with drought-resistant cultivars. This has prompted the need to find alternative ways to minimize the effect of drought other than cultivar selection. Irrigation is one such method applied in other tea growing regions such as in Tanzania (Möller and Weatherhead 2006; Kigalu, Kimamboa et al. 2008).
There has been only one limited exploration of tea irrigation in Sri Lanka by Ananthacumaraswamy et al, (1985). He applied drip irrigation to a young and mature stand of TRI 2026, a drought-susceptible cultivar, for one season. The degree of the resultant yield increases were such that suggested irrigation could be economically feasible. However, this 71 was conducted at Talawakelle which is a high elevation location. In this part of Sri Lanka the tea estates are large and are resistant to invest in irrigation infrastructure. Also the impact of irrigation in preventing plant death is minimal in this relatively cooler region.
As the dry period in low elevation areas is different to that at Talawakelle, and as the low elevation growing areas are an increasingly significant contributor to Sri Lanka’s national tea production, it is important to study the physiological performance, yield and water use of the plant in response to irrigation in this region.
Even though irrigation has the potential to emerge as an alternative to cultivar selection as a drought mitigation strategy, the existing cultivar breeding program still remains the key mitigating drought strategy for the Sri Lankan tea industry. Progressing from previous TRI 2000 and TRI 3000 series cultivars, now the industry is using TRI 4000 series cultivars released in the late 1990s. Nevertheless, the drought-susceptible cultivar TRI 2023 remains a benchmark because of its high yield potential. Similarly TRI 3025 is considered a benchmark for drought tolerance. So it will be necessary to evaluate some of the later releases against these benchmarks.
This first step in understanding the feasibility of irrigation in low elevation areas of Sri Lanka begins with the aims to:
1. describe the effect of irrigation on the physiology, water use and yield specifically in the dry period of the year, and
2. assess some of the cultivar variation against benchmark cultivars.
It does this with the objectives of
a) understanding the environmental limitations to productivity, and
b) suggesting improved management practices for this environment.
Accordingly, this chapter describes three separate field experiments, conducted during the 2007-2009 period, which evaluate the physiology, productivity, water use performance in low elevation tea growing area as response to drought and irrigation.
Experiment 1 evaluates the physiological and yield response of two benchmark tea cultivars under irrigation.
Experiment 2, examines the water use in the wet and dry period of the most productive cultivar (TRI 2023) in low elevation tea areas.
Experiment 3 evaluates the physiological performance of the present generation new cultivars against the benchmark drought-resistant cultivar used in Experiment 1.
72 Each of these experiments is presented with their own methods and results and interpretation sections. A plenary discussion section will draw together the interpretations from each of the experiments to satisfy the chapter’s research aim and objectives.
73 5.2 Experiment 1: Physiological and Yield Performance of Two Contrasting Tea Cultivars in Response to Irrigation
5.2.1 Introduction
This experiment evaluates the production-related physiology of two tea cultivars, TRI 2023 and TRI 3025 during the drought months of 2007. It also evaluates the ability of supplementary drip irrigation to compensate for the effect of drought on physiological processes. The measurement of physiological parameters was made only over the drought months of January and February when irrigation was applied. However weekly yield measurements were made over the whole year to see if this short irrigation period had a meaningful effect on annual yield.
5.2.2 Method
5.2.2.1 Experimental design and field layout
The trial was conducted in an established drip irrigation plot in Field no 01 of St. Joachim Estate, Ratnapura. Details about location, and average climatic conditions were given in Chapter 4. The experimental design was a split-split plot. The system was laid out as two sections, irrigated and non-irrigated with two cultivar subsections. Statistical analysis was done using SAS statistical software package – version 9.0 (SAS Inc., USA).
Ideally, the irrigated and non-irrigated sections would have been broken up into blocks randomly placed over the site. However, this facility was the only mature drip irrigated tea in the area. There was no space on the estate, and no time, (another 3 years), to establish a mature irrigated system. Given this limitation many measures were taken to limit the field variation. Prior to planting, the field was excavated and all boulders and large stones were removed. Also water proof plastic sheet was inserted to a 1m depth, in between the two treatment plots to prevent sub surface water flow between plots. An aerial view of the research field is shown on Figure 4.2. The main chemical and physical soil parameters that could give rise to variation in yield were analysed. There were no significant discrepancies between the irrigation and non irrigation plots in these parameters (see Table 5.1). One of the main fertility parameter in the soil that determines tea yield is organic carbon content (Wijeratne and Shyamalie 2009). Organic carbon content in the both irrigated and non irrigated plots were similar except for 20cm depth. There were no significant differences among two physical parameters of water holding capacity and bulk density.
74 Table 5.1 Major soil physical and chemical properties of Experiment 1 site at different soil depths Depth Parameter Non Irrigated Irrigated (cm) 10 2.2(±0.05) 2.2(±0.06)
Organic Carbon (%) 20 1.8(±0.06) 2.0(±0.04)
30 1.8(±0.05) 1.8(±0.05)
10 137.7(±12.5) 159.1(±13.3)
Potassium (ppm) 20 105.4(±10.5) 123.1(±9.7)
30 92.4(±9.0) 107.8(±8.2)
10 121.8(±9.4) 136.6(±8.4)
Phosphorous (ppm) 20 81.4(±7.9) 85.8(±8.4)
30 72.7(±8.8) 65.3(±5.9)
10 41.6(±5.7) 43.9(±3.5)
Manganese (ppm) 20 38.6(±4.38) 44.3(±4.5)
30 37.5(4.3) 41.9(±4.8)
Water holding Capacity (mm/m) 130(±7.3) 127(7.4)
Bulk density (g/cm-3) 1.57(±0.02) 1.54(±0.03)
(Organic carbon –(Walkley and Black 1934), K, P & Mn - (Westerman 1990))
5.2.2.2 Tea cultivar, planting and cultural practices
Vegetatively propagated plants of cultivar TRI 2023 and TRI 3025 were used for the experiment. The plants were planted in the field in May 1999 and plucking was commenced in October 2001. The spacing of the planting was according to a double hedgerow system. Two plant rows were grown at 90cm spacing apart in a raised bed of 30cm height. Row spacing was 60cm, where as distance between two raised beds was 150cm. The plant density of the planting system is 12,500 plants/ha, which is the recommended plant density for tea crops in Sri Lanka. Shade trees are conventionally planted in tea plantations. However, in this situation they were not planted to avoid confounding factors like uneven solar radiation and competition for water. The plants were pruned to a height of 60cm on May 2004 and May 2006 respectively. After each pruning, harvesting was commenced following a 4-5 months of recovery.
Both cultivars are of Assam origin. TRI 2023 is a broad leaf variety which has a higher susceptibility to drought. However, this cultivar has vigorous growth and high yielding ability. The cultivar is the highest producing cultivar in the region but its productivity is very
75 vulnerable to drought (Piyasundara 2009). Commercial cultivation of this cultivar is not advised in the region under normal rain-fed systems. TRI 3025 cultivar is also classified as a high yielding variety, but it has an ability to withstand drought as well (TRISL 2002).
5.2.2.2 Water application and moisture measurement
Drip irrigation system was installed and commissioned in April 1999. The system consisted of a pump unit, head control system with filtration and fertilizer tank, block valves and laterals. Drip laterals were made up of low density 16 mm diameter polyethylene, laid on the surface. The lateral drip lines were placed on each row of tea. Integral pressure compensated drip lines (RAM 17D, Netafim, Israel) with a design discharge of 1.6L/dripper/h were spaced at 60cm distance along the laterals. Accordingly, each plant had a dripper for irrigation.
Irrigation was specifically practised during the inter-monsoon dry spell between January and March each year. The water requirement was calculated based on the Pan evaporation rate of the previous day. A crop coefficient value of 0.85 has been used to estimate the actual water requirement of tea (Laycock 1964). Pan evaporation based irrigation scheduling was used earlier in other tea irrigation trials (Stephens and Carr 1991). Based on the water holding capacity of the soil and temperature stress (when maximum air temperature exceeds 350C) on non rainy days in the area, irrigation was commenced daily after 5 days of rainless period. In wet zone of Sri Lanka, a 5 day rainless period causes significant drought stress to perennial crops (Sumanasena 2008).
Though the design discharge of the dripper is 1.6L/hr, after a study of measuring the uniformity of application, using 32 catch cans placed evenly for each cultivar 10 plots, it was found that there was a variation in water application uniformity. The water application rates for each cultivar are given in Table 5.2. When operating the system, average water application was assumed to be 1.4L/hr. However, irrigation water received for two cultivars did not differ significantly. As a precautionary measure to prevent plants from dying, all rain- fed plants were manually irrigated with 9L/plant, on 23 February 2007. By this time, there was a very high defoliation of rain-fed plants and high number of dead twigs was visible on canopy. This was equivalent for 13mm/plant irrigation. The maximum potential water deficits of the irrigated and rain-fed plots were calculated as a difference between the rainfall or irrigation and potential evapotranspiration of tea (ET), using Instat Climate Analysis Software (Instat for Windows, 3.036, Statistical Services Centre, University of Reading, UK).
Maximum ET0 was calculated based on FAO modified Penman Monteith calculation. To convert the potential evapotranspiration value for tea, 0.85 was used as the crop coefficient (Allen, Pereira et al. 1998).
76 Table 5.2 Water application rates of two cultivars. (Two application rates are not significantly different as shown by high standard error in parenthesis) Water application Cultivar rate(L/hr) TRI 2023 1.4(±0.2)
TRI 3025 1.3(±0.2)
5.2.2.3 Water potential measurements (Ψ)
Leaf water potential (Ψ) of the plants was measured at pre-dawn - Ψdawn (0530hrs) and mid- day - Ψnoon (1200hrs), after the fifth week. Measurements were taken at the field itself using a Scholander pressure bomb (Soil Moisture Corp.). At least 10 fully grown top-most leaves were selected from each treatment. Selected leaves were detached from the stem using a sharp knife and enclosed in a sealed polythene bag prior to taking to the instrument.
5.2.2.3 Gas exchange measurements
Instantaneous net photosynthetic rate (Pn) Instantaneous transpiration rate (El), leaf temperature (Tl) and stomatal conductance (gs) were measured in the topmost dark green mature leaves, exposed to full sunlight, using a portable infra-red gas analyser (LCA-4, ADC BioScientific, UK). Measurements were made under saturating light intensities when there was no cloud cover, between 1200-1400 hours of the sampling date except in days where we tested the diurnal variation of physiological activities. This was the time of the day when water and heat stresses on plants were expected to be at their maximum. At least five plants were measured in each treatment. The diurnal variation of Pn, El, gs and Tl were measured on 26 February 2007 at two-hour intervals.
The light response of photosynthesis was measured on a 27 March 2007, a cloudless day, between 1100 and 1300 hrs by varying the light intensity incident on top of the leaf using different layers of shade cloth to cover the leaf chamber. Following asymptotic exponential function was used to fit the photosynthetic light response of each treatment (Boote and Loomis 1991).
Ȁሺܲ݉ܽݔ ܴ݀ ሻܫെ ൌܲ݉ܽݔ െ ሺܲ݉ܽݔ ܴ݀ ሻ݁ ݊ܲ
-2 -1 Where Pn is net photosynthetic rate (µmolCO2m s ), I is the light intensity incident on the -2 -1 leaf (µmolm s ), Pmax is the light-saturated maximum Pn and Rd is the dark respiration rate. Φ is the quantum efficiency (μmol(PAR)-1). Parameters were estimated using the PROC NLIN procedure of SAS statistical package.
77 5.2.2.4 Harvesting
Manual harvesting was practised at 7 day intervals. The standard plant part used for plucking is 2 leaves and the young unfurled bud. However, during the rainy period, the third leaf was also harvested if it was found to be in a tender stage. Harvesting was carried out by regular workers (tea pluckers) attached to research fields of the research station. Hence, chances for the variation of yield, due to harvesting errors by the tea harvesters were minimal during all experimental periods. Weight measurements were taken in the field, just after harvesting to minimize the error of weight loss in transportation. Results are expressed in terms of made tea by multiplying the fresh weight by 22.2% (Stephens and Carr 1991).
5.2.2.5 Stem canker infection
The incidence of stem canker, a fungal disease caused by Phomopsis theae, of irrigated and rain-fed plots of either cultivar was assessed in June 2008. The evaluation ranked the incident of stem canker in each treatment by a visual score of the disease. Examination was conducted by two staff members of Plant Pathology Division, Tea Research Institute, Talawakelle, Sri Lanka. Scoring was according to severity of infection, highest being 4 for severely infected and 1 for non-infected plants.
5.2.3 Results
5.2.3.1 Meteorological conditions during the study period
The 10-week dry period, during which the irrigation treatment was applied, received a total of 221 mm of rainfall which is only 6% of the annual rainfall on this site. Moreover, monthly rainfall throughout this period was lower than the minimum of 100 mm month-1 that is necessary for successful tea production (Fuchs 1989). The site received more than 5 hours of sunshine a day, except for the second week, in which high rainfall (76.7mm) was received. Potential evapotranspiration also dropped to 2.6mm/day during the second week. Maximum temperature and minimum temperature rose towards the 10th week. After the third week, maximum temperature remained above 340C. After the fourth week, saturation VPD remained more than 1.0 kPa. However, it did not reached 2.0kPa, which is found to be critical for tea cultivation in the area (Wijeratne 1994).
78 Table 5.3 Meteorological condition during first 10 weeks of 2007(time period when physiological observations were made). Except rainfall others daily average values; ET0- potential evapotranspiration, N- sunshine hours/day, RF-Rainfall, VPD – Vapor pressure deficit Temperature 0 Wind RF VPD ET0 Wk Period ( C) N (kmh-1) (mm) (kPa) (mm) Max Min
1 1-7 Jan 34.1 19.2 0.31 6.9 0.0 0.9 3.6
2 8-14 Jan 31.3 21.6 0.24 3.0 76.7 0.8 2.6
3 15-21 Jan 33.6 21.8 0.33 5.2 4.5 0.9 3.3
4 22-28 Jan 34.0 20.4 0.30 4.9 22.6 0.9 3.2
5 29 Jan-4 Feb 34.4 20.9 0.53 6.6 0.4 1.1 3.8
6 05-11 Feb 34.8 20.8 0.24 7.5 0.0 1.2 4.0
7 12-18 Feb 35.9 21.6 0.05 7.7 0.0 1.3 4.2
8 19-25 Feb 35.9 22.0 0.04 5.6 2.6 1.3 3.7
9 26F-4 Mar 34.6 22.6 0.35 5.0 57.9 1.0 3.6
10 05-11 Mar 35.6 22.2 0.18 6.9 0.0 1.2 3.5
250 rain 25
ET0
200 20
150 15 (mm/week) 100 10 0 ET Rain (mm/week)
50 5
0 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 5.1 Average rainfall (mm/week) and potential evapotranspiration–ET0 (mm/week) during 2007. (Standard error bar indicates the fluctuation of the rain and ET0 in each of the week in a month)
Overall the year 2007 was a typical year for the low elevation tea growing areas showing the bimodal pattern of rainfall. Heavy rains were not available in the first three months period. Also wide spread plant deaths due to drought, such as what happened in1982 or 1992, were 79 not reported. Each month’s average weekly potential evapotranspiration and weekly rainfall are shown in Figure 5.1. Weekly values of rainfall are presented here as it reflects more accurately the effect of rain or drought on tea yield, which is harvested at 7 day intervals. Rainfall was low during the initial 3 month period of the year and during February; the maximum water use per week exceeds weekly rainfall. April and September typically received the highest rains, and rain was lower again in December.
Maximum soil water deficit during the study period, calculated as the difference between maximum potential water use and rainfall, is shown in Figure 5.2. From the second week, there was a reduction in soil moisture continuously until the eighth week. Rainfall of 23mm during fourth week was unable to check the decreasing soil moisture content, but there was a replenishment of the soil moisture after the ninth week when 58mm rain was received.
Week
2 4 6 8 10 12 0
-20
-40
-60
-80 Soil water deficit (mm)
-100 irrigated rainfed -120
Figure 5.2 Maximum potential soil water deficit(mm/m) of irrigated and rain-fed treatments up to 1m depth of soil from January to March, 2007
In summary for this section, it is clear that this was a typical rainfall season and that unirrigated plants were exposed to significant water stress during the excessively hot dry period.
5.2.3.2 Leaf water potential (Ψ)
Pre-dawn and mid-day water potential after week 5 is given in Figure 5.3. Differences in pre- dawn water potential between irrigated and non irrigated TRI 2023 were not significant until the week 6. After that it decreased rapidly for rain-fed plants of TRI 2023. Similarly for TRI 3025 too, it was not significantly different until week 7 for rain-fed and irrigated plants. For both cultivars there was an increase in pre-dawn water potential of the irrigated plants by week 10, but for irrigated TRI 2023, the increase was 54% higher than irrigated TRI 3025. 80 Mid-day water potential of TRI 2023 showed a distinct difference between irrigated and rain- fed plants in many of the observed days. The widest gap was observed at week 10. There was no significant difference in TRI 3025 mid-day water potential due to irrigation till 8th week. But after that, rain-fed TRI 3025 showed the highest mid-day water potential of 10.1bar. Mid-day water potential of the irrigated plots of both cultivars did not increase with the progress of the drought (until week 10). This may be due to the irrigation application of the plants since morning.
0.0 (a) (b)
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0 0 (c) (d)
-2 Water potential (MPa) potential Water -4
-6
-8
-10 TRI 2023 irrigated TRI 3025 irrigated -12 TRI 2023 rainfed TRI 3025 rainfed
-14 4 5 6 7 8 9 10 11 4567891011
Week no
Figure 5.3 Leaf water potential of two irrigated and rain-fed cultivar (a & b) at 0530hrs, (c & d) at 1200hrs
In summary, rain-fed and irrigated TRI 3025 showed a higher Ψdawn than TRI 2023 most th weeks. In TRI 2023, Ψdawn started decrease only after 8 week. There was a difference in th Ψnoon of TRI 2023. Highest Ψnoon was reported in rain-fed TRI 3025 at 10 week.
5.2.3.3 Photosynthesis (Pn)
The time courses of variation of Pn in both cultivars (Figure 5.4) showed significant increases of Pn in the irrigated treatment as compared to the respective rain-fed treatment. The response to irrigation was clearer and greater in TRI 2023 than in TRI 3025. Pn of the rain-fed treatment of both cultivars showed a gradual decline during the study period, probably because of increasing soil water deficits. The decline in the rain-fed TRI 3025 was higher 2 2 (r =0.82, P=0.002) than rain-fed TRI 2023 (r =0.61, P=0.02). Pn of the rain-fed TRI 2023 had 81 increased in week 5, probably in response to the rainfall that occurred during week 4(Table
5.4). In contrast, Pn of TRI 3025 did not respond to this rainfall. Moreover, Pn of both cultivars was not able to respond to the rainfall that had occurred during week 9. Under irrigation, TRI 2023 had a slightly greater Pn than TRI 3025. During the period from week 3 to week 6, irrigated treatments of both cultivars showed a gradual decline in Pn, with the decline in TRI 3025 being more pronounced than that of TRI 2023. However Pn recovered in later weeks for irrigated plants.
16 Rain 100 TRI 2023 irigated 14 TRI 2023 rainfed 80 12
10 60
) 8 -1 s 40 -2 6 m 2 4 20 mol CO mol
2
0 0 16 100
Rain 14 Rainfall (mm/week) TRI 3025 irrigated TRI 3025 rainfed 80
Photosynthesis rate ( 12
10 60
8
40 6
4 20 2
0 0 024681012 Week no Figure 5.4 Mid-day photosynthetic rate of top most mature tea leaves of two cultivars (TRI 2023 above, TRI 3025 below) during drought affected 10 weeks. Vertical bar indicates the rain received during each week.
In summary, the photosynthetic rates of both cultivars were suppressed during the dry period, even when under drip irrigation. Nevertheless, photosynthesis of the irrigated cultivars was 33% and 38% higher for TRI 2023 and TRI 3025 respectively than under the rain-fed treatments. Under irrigation TRI 2023 was 32% stronger photosynthesis than TRI 3025.
82 5.2.3.4 Stomatal conductance (gs)
The instantaneous stomatal conductance (gs) showed a similar pattern between the two water regimes and the cultivars (Figure 5.5). (Measurements were taken only for 7 weeks, because of breakdown of the instruments). Particularly, gs of TRI 3025 did not differ significantly between the two water regimes. Although the gs of TRI 2023 showed significant variation between the two water regimes, the variation was not consistent. For example, the irrigated treatment had significantly greater gs during weeks 1 and 7, whereas the rain-fed treatment had significantly greater gs during week 3. Stomatal conductance of both cultivars and water -2 -1 regimes were within the range of 0.1 – 0.3mmol H2O m s on most days of measurement, with occasional increases up to 0.5mmol m-2 s-1. During week 5, there was a surge in the stomatal conductance of the all treatments, irrespective of water application or cultivar.
0.6 Rain 100 TRI 2023 irrigated TRI 2023 rainfed 0.5 80
0.4 60 ) 1 - 0.3 s 2 - 40 O m 2 0.2
20 0.1
0.0 0 0.6 100 Rain TRI 3025 irrigated
0.5 TRI 3025 rainfed (mm/week) Rainfall Stomatal conductance (mmol H 80
0.4 60
0.3
40 0.2
20 0.1
0.0 0 012345678 Week
Figure 5.5 Mid-day stomatal conductance of top most mature tea leaves of two tea cultivars during drought affected 7 week period. Vertical bar shows the rain received during each week.
83 In summary, both cultivars showed a similar trend line and a surge in the stomatal conductance in week 5. However, gs were 40% and 7% higher under irrigation for TRI 2023 and TRI 3025 respectively, while rain-fed gs for both cultivars showed similar values.
5.2.3.5 Leaf transpiration (El)
In TRI 2023, El under irrigation was significantly greater than that under rain-fed conditions on a majority of measurement days (Figure 5.6). In contrast, there was no such significant variation between El the two treatments in TRI 3025. In TRI 3025, there was a continuous decline in the transpiration over the drought season. Irrigated TRI 3025 plants showed the greatest reduction (r2 = 0.8, P = 0.003). On the other hand, with the exception of week 4 which received rainfall, El of rain-fed TRI 3025 showed a gradual decline during the experimental period. In rain-fed TRI 2023, such a downward decline was evident only after week 5. On average, TRI 3025 showed a lower El than TRI 2023 under both irrigated and rain-fed conditions. Even though gs showed a surge at week 5, El was increased under rain- fed TRI 2023 only. For the irrigated TRI 2023 it was the same rate as the previous week.
In summary, TRI 3025 showed a decline in transpiration in rain-fed and irrigated plants. The average El irrigated plants were 36% higher for TRI 2023 and 6% lower for TRI 3025. Nevertheless under rain-fed conditions, TRI 2023 transpiration rate was 17% higher than TRI 3025.
5.2.3.6 Diurnal variation of photosynthesis, transpiration and leaf temperature
Diurnal variation of Pn showed clear differences between the two cultivars, especially under irrigation (Figure 5.7). The irrigated TRI 2023 maintained higher Pn levels throughout the morning and up to 1430 hours in the afternoon beyond which a significant decline was observed. On the other hand, in TRI 3025, the significant decline of Pn started earlier around 1230 hours.
Furthermore, TRI 2023 achieved a greater maximum Pn than TRI 3025. In both cultivars, Pn of the rain-fed treatment remained significantly lower than the respective values in the irrigated treatment at all times of the day except 1800 hours. Moreover, there was no significant diurnal variation of Pn in the rain-fed crops until after 1600 hours. The Pn of rain- fed TRI 2023 was slightly higher than the corresponding values of TRI 3025.
84 Rain 8 TRI 2023 irrigated 100 TRI 2023 rainfed
80 6
60
4
40 ) -1 s
-2 2 20 O m 2
0 0 8 100
Rain Rainfall (mm) TRI 3025 irrigated 80 TRI 3025 rainfed 6
Transpiration rate (mmol H 60
4
40
2 20
0 0 0 2 4 6 8 10 12 Week
Figure 5.6 Leaf transpiration rate during mid-day of top most mature tea leaves of two tea cultivars during drought affected 10 weeks of 2007. Vertical bar shows the rain received during each week.
The diurnal variation patterns of El revealed clearer differences between the two cultivars and between the two water regimes. El of the irrigated treatment of both cultivars increased during the morning, reached a peak around mid-day and decreased thereafter throughout the afternoon. El of irrigated TRI 3025 reached its peak earlier and also began its afternoon decline earlier than TRI 2023. The peak El was significantly greater in TRI 2023 as compared to TRI 3025. El of the rain-fed TRI 2023 also showed a diurnal pattern of variation which was similar to that of its irrigated treatment. However, the increase of El during the first half of the day was much lower than that of the irrigated treatment. Except at 1800 hours, El of irrigated TRI 2023 was significantly greater than its corresponding rain-fed treatment. In contrast, in TRI 3025, a significant increase of El in the irrigated treatment over the rain-fed was shown only up to 1430 hours.
85 40 1000
800 ) -2 C) 30 0
600 20 400
Air temperatureAir ( 10 air temperature 200 (Wm radiation Solar solar radiation
0 0 16 ) 1 - s 2 - 14 TRI 2023 irrigated m 2 TRI 2023 rainfed 12 TRI 3025 irrigated TRI 3025 rainfed molCO 10 8
6
4
2
Photosynthesis rate ( rate Photosynthesis 0
) 4 1 - s 2 - 3 O m 2
2
1
0 Transpiration rate (mmol H
800 1000 1200 1400 1600 1800 2000 Time (hours)
Figure 5.7 Diurnal variation of air temperature, and solar radiation and physiological parameters (photosynthesis and transpiration) from 800 to 1800 hrs at 29 Feb 2007
In summary whilst under irrigation TRI 3025 cultivar had a decline in both Pn and El rate as a result to the increasing temperature, during day time. Irrigated TRI 2023 maintained significantly higher Pn and El during day time.
The diurnal variation in leaf temperature (Tl) of the irrigated treatment of both cultivars closely followed that of air temperature (Ta) during the morning up to noon (Figure 5.8).
Importantly, Tl of the irrigated crops was significantly lower than Ta throughout the afternoon, with the difference reaching a maximum of around 5oC around 1600 hours. In contrast, rain- fed treatments of both cultivars had Tl values which were significantly greater than Ta o throughout the morning and early afternoon. Tl of both rain-fed cultivars exceeded 40 C
86 during the period by 1200 hours and this represented an increase of about 8oC over the prevailing Ta. There was significant cultivar variation in Tl under both water regimes. In the rain-fed treatments, TRI 3025 had significantly greater Tl than TRI 2023 during the morning, but, this trend was reversed during the afternoon. For rain-fed plants of both cultivars, Tl remained above the critical temperature threshold of 350C during most of the day (from nearly 1000 to 1600 hrs). Also the leaf temperature remained more than 50C above air temperature, for rain-fed plants.
45 TRI 2023 irrigated TRI 2023 rainfed TRI 3025 irrigated 40 TRI 3025 rainfed air temperature C) 0 35
30 Temperature (
25
20 600 800 1000 1200 1400 1600 1800 2000
Time (hours) Figure 5.8 Diurnal variation of leaf temperature of top most mature tea leaves and air temperature from 800 to 1800 hours on 29 Feb 2007
In summary leaf temperature of rain-fed, TRI 3025 rose rapidly up to 1200hrs showing significantly higher values than rain-fed TRI 2023. The difference among rain-fed and irrigated plants were as high as nearly 100C, at around 1400hours when all the treatments showed the maximum leaf temperature increase.
5.2.3.7 Light response
Light response showed clear cultivar variation (Figure 5.9), with TRI 2023 showing significantly greater response to increasing incident light intensity than TRI 3025 -2 -1 -2 -1 -1 -2 -1 (i.e.0.0066µmol CO2 m s [µmol PAR m s ] as compared to 0.0033µmol CO2 m s [µmol PAR m-2s-1]-1). On the other hand, light response did not differ significantly between the two cultivars under rain-fed conditions. Irrigation significantly increased the light response of Pn in both cultivars. The figure shows the light saturated maximum
87 photosynthetic rate (Pmax), dark respiration (Rd) and quantum efficiency (Φ). Pmax increased under the irrigation by 119% and 100% for cultivar TRI 2023 and TRI 3025 respectively.
The cultivar TRI 2023 had a 62% higher Pmax under irrigation than for the cultivar TRI 3025. Higher quantum efficiency (Φ) was observed in the TRI 3025 than TRI 2023 under both irrigated and rain-fed conditions, and the former showed the higher value. For the cultivar TRI 2023, Φ was much smaller comparatively and the highest Φ was observed with rain-fed plants. For the cultivar TRI 2023, dark respiration (Rd) was 52% higher. In contrast for TRI 3025, under irrigated plants, it was 51% higher than rain-fed plants.
In summary, cultivar TRI 2023 showed a higher efficiency in assimilation under irrigated condition and rain-fed conditions as well. Higher photorespiration was observed in TRI 3025 showing reduced photosynthetic efficiency in its leaves.
88 20 TRI 2023 rain-fed (Pmax = 6.8, Rd = 0.47, = 0.003) 15
10
5
0
-5
-10 20 TRI 2023 irrigated (Pmax = 14.9, Rd= 0.31, = 0.002)
15
10 ) -1
s 5 -2 m
2 0
-5 mol CO
-10 20 TRI 3025 rain-fed (Pmax = 4.6, Rd = 0.72, = 0.013)
15
10
5
Photosynthetic rate ( rate Photosynthetic 0
-5
-10 20 TRI 3025 irrigated (Pmax = 9.2, Rd = 1.09, = 0.021) 15
10
5
0 0 500 1000 1500 2000 2500 -5 -2 -10 Solar radiation(Wm )
Figure 5.9 Light response of TRI 2023 and TRI 3025 rain-fed and irrigated plants during the dry spell of January- March 2009. Measurements were taken on 27 Mar 2007. Note:Pmax=light saturated maximum photosynthetic -2 -1 -1 rate(μmol CO2 m s ), Rd=dark respiration, Φ=quantum efficiency(μmol(PAR) )
5.2.3.8 Response to ambient temperature
Figure 5.10 presents the relationship between photosynthetic rate and the daily maximum air temperature of TRI 2023 and TRI 3025 under rain-fed and irrigated conditions during January to March 2007. A more negative relationship was observed in rain-fed plants of both cultivars with TRI 3025 was more sensitive. For each 10C increase in air temperature 0 -2 -1 between 33-36 C, Pn of TRI 3025 was reduced by 2.75 µmol CO2 m s , compared with 2.44 µmolm-2s-1 in TRI 2023. Among the irrigated treatments, TRI 3025 showed a significantly negative relationship (r2=0.57, P=0.03) with increasing ambient temperature. Decline in 89 -2 -1 photosynthesis with rising temperature in irrigated TRI 3025 was 1.83µmol CO2 m s per 10C. Nevertheless, photosynthesis in the irrigated TRI 2023 was not significantly affected to temperature.
14 TRI 2023
12
) 10 -1 s
-2 8 m 2 6
mol CO 4
2 irrigated y = 36.9 - 0.76x (r2=0.11, P=0.4) rain-fed y = 92.2 - 2.44x (r2=0.75, P=0.005) 0 14 TRI 3025 12 Photosynthetic rate ( rate Photosynthetic 10
8
6
4
2 2 irrigated y = 71.8 - 1.83x (r =0.57, P=0.03) rain-fed y=101-2.75x (r2=0.75, P=0.005) 0 33 34 35 36
Maximum air temperature (0C)
Figure 5.10 Relationship between maximum air temperature and photosynthetic rate during January-March, 2007 in low elevation tea growing area. (Air temperature shows the maximum air temperature of each day)
In summary TRI 3025 was more sensitive to the increasing ambient air temperature, even with irrigation in maintaining the mid-day Pn.
5.2.3.9 Annual yield variation
Average weekly yield in made tea kg/ha is given in Figure 5.11. According to the graph, during months of January to April, there was a decline in the tea harvest in both cultivars, even under irrigation. Lowest yields were recorded in months of February and March. For the cultivar TRI 2023 average bi-monthly yield of February and March was 75 and 63% lower than monthly average of the year for 2007, for rain-fed and irrigated plants respectively. For the cultivar TRI 3025, yield reductions for the same moths were 84 and 71% respectively for rain-fed and irrigated plants.
90 200 irrigation
150
100
50
TRI 2023 irrigated TRI 2023 rainfed
0 200 irrigation
150 Made tea yield (kg/ha/week)
100
50 TRI 3025 irrigated TRI 3025 rainfed
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Figure 5.11 Average made tea yield (kg/ha/week) during 2007. Standard error shows the yield fluctuation within month
Even though irrigation was applied only during first three months of the year, as shown in the Figure 5.11, cultivar TRI 2023 produced significantly higher yield under the irrigation treatment in each month except January and June. The rain-fed treatment never produced higher yield than irrigated treatment for cultivar TRI 2023. Irrigated TRI 2023 produced significantly higher yields (P=0.004) than rain-fed TRI 2023 in July, August and September. Only in August, irrigated TRI 3025 produced significantly higher (P=0.03) yield than rain-fed TRI 3025 during wet season. There were some months in the year where rain-fed treatment produced higher yield than irrigated treatment for TRI 3025.
Total made tea yield during for the year 2007 is given in Table 5.4. Cultivar TRI 2023 produced the highest yield of made tea 5650 (±220) kg/ha. Irrigated TRI 2023 yield is 16% higher than irrigated TRI 2023 cultivar. Both cultivar effect and treatment effect are highly 91 significant. However, there is no cultivar and irrigation interaction. The monthly variation of yield within the year was higher for cultivar TRI 2023 than TRI 3025.
Table 5.4 Total tea yield for year 2007. Standard error is given in parenthesis. Yield (Made tea Treatment kg/ha) TRI 2023 rain-fed 4468(±327)
TRI 2023 irrigated 5650(±220)
TRI 3025 rain-fed 4176(±137)
TRI 3025 irrigated 4859(±160)
Significance
Cultivar 0.0009
Irrigation 0.0001
Cultivar X Irrigation 0.24
In summary, while the irrigation treatment did not rise the dry season yields to that of the wet season yields it did significantly increase the annual made tea production. Irrigated TRI 2023 yielded 27% more than rain-fed TRI 2023. TRI 2023 also yielded 16% more than TRI 3025 under irrigation. There was relatively little effect of irrigation on TRI 3025.
5.2.3.10 Yield response to climatic factors:
The climate of 2007 followed a normal pattern for the WL1 agro-ecological zone of Sri Lanka (Punyawardana 2008). The drought period of 2007 is normal for the area, which prevailed during January to March of the year. The choice of 2007 as the reference year was valid because it is the second year of the third pruning cycle and it justifies the presence of average rainfall for the year. A stepwise regression (forward selection) was used to analyse best predictive variable for expressing tea weekly tea yield. Weekly tea yield of 47 harvested weeks and climate parameters of the previous week, temperature (min. and max.), sunshine duration, rainfall, evaporation and vapour pressure deficit was used for the analysis. Table 5.5 presents the relationships between key micrometeorological parameters and made tea yield under the four irrigation-cultivar treatments.
92 Table 5.5 Relationship between micrometeorological parameters and yield of irrigated and rain-fed two tea cultivars during 2007 (Tea yield of 47 weeks were used for the calculation)
TRI 2023 rain-fed TRI 2023 irrigate TRI 3025 rain-fed TRI 3025 irrigate Variable slope P slope. P slope. P slope. P
Tmin 6.1 0.27 13.0 0.02 4.5 0.32 9.6 0.06
Tmax -28.8 <0.001 -26.4 0.002 -27.1 <0.001 -22.9 0.003
N 1.2 0.58 1.0 0.64 -0.09 0.64 -0.9 0.66
RF -0.06 0.64 -0.04 0.78 0.03 0.75 0.03 0.83
Evaporation 1.4 0.42 0.9 0.59 0.15 0.91 -0.09 0.95
VPD 30.0 0.60 53.0 0.36 73.7 0.13 66.7 0.2
This analysis shows that daily maximum air temperature is the best variable to express the made tea yield in low elevation tea growing area for both cultivars, under irrigated or rain-fed condition. Nevertheless, the lowest effect was shown by irrigated TRI 3025 cultivar with the coefficient of -22.9(P=0.003). The irrigation treatment tended to reduce the effect with the greater response being observed in TRI 3025.
5.2.3.11 Stem canker infection
Severity of the stem canker infection of the plant is illustrated in Figure 5.12, according to an assessment on June 2008. Analysis revealed the level of disease infection after exposing the plants to series of dry spells for nearly 9 years in the field. Cultivar TRI 2023 was more prone to the infection even under irrigation. Nevertheless, irrigation reduced the infection significantly among both cultivars. Under the usual practise of rain-fed cultivation, 93% of TRI 2023 was severely infested and almost all the plants were infested to a certain degree in both cultivar. Cultivar difference in resistance to the disease was prominent even under irrigation. The proportion of severely and moderately affected plants was more than double in irrigated TRI 2023 than irrigated TRI 3025.
93 100
severe moderate 80 less infection no infection
60
40 Infected populationInfected (%)
20
0
TRI 2023 rain-fed TRI 2023 irrigated TRI 3025 rain-fed TRI 3025 irrigated
Treatment
Figure 5.12 Incidence of stem canker among tea plants of two tea cultivar as assessed by June 2008.
In summary when grown as a rain-fed crop, all TRI 2023 and TRI 3025 plants were infected with stem canker. Severe infestation level in TRI 2023 was more than double that of rain-fed TRI 3025. Irrigation reduced the sever infestation nearly 80% in both cultivars.
5.2.4 Discussion
The physiological behavior of two drought-susceptible and drought-resistant tea cultivars was evaluated during the dry spell of January to March, 2007. In addition to physiological behavior, made tea yield and the incidence of major drought related fungal infection were also evaluated as these two factors are very important in irrigation adaptation and cultivar selection for low elevation tea growing area. This section discusses mainly the above characters of the two cultivars in relation to irrigation.
Leaf water potential of both cultivars at pre-dawn and mid-day are shown in figure 5.3.
Irrigation effect was observed mainly in TRI 2023. Till the week 10, Ψmidday was equal in both irrigated and rain-fed TRI 3025. But the difference was seen in Ψmidday of irrigated and rain-fed TRI 2023. For both cultivar under rain-fed condition Ψmidday had a close negative 2 relationship with Pn (r =0.82 for TRI 2023 and 0.78 for TRI 3025). But the relationship is poor for irrigated plants.
94 Initial decline in the soil moisture caused the Pn to reduce in the plants. Even for a small reduction of soil moisture (nearly 20%) of the irrigated plants, coupled with high temperature caused further decline of Pn in irrigated plants. The physiological response to irrigation varies with tea cultivar. Similar cultivar difference as response to irrigation was reported under low temperature growing areas as well (Smith, Burgess et al. 1993).
TRI 3025 cultivar showed a continuous decline in the Pn and El even under irrigated conditions. This is a strong adaptation to survive the drought period. Due to the horizontal nature of the TRI 3025 plants, the leaf surface receives more solar radiation and could excite energy to build up heat within short time as shown in Figure 5.5. By 1200hours, rain-fed TRI 3025 leaves were showing 40C above rain-fed TRI 3025. On the other hand TRI 2023 leaves are not horizontal and leaf temperature increase is slower within day time. Even though 3- 40C increase in leaf temperature above the ambient temperature were recorded in other crops (Tőkei and Dunkel 2004), such leaf temperature was not recorded for tea in other places with cool dry periods.
As there was no major reduction in the gs of the tea plants, El reduction could more be attributed to the less absorption of the water by roots. There is a slight difference in the rates of gs for both treatments in both cultivars, however the rate of reduction is not as prominent as it is for Pn and El. Stomatal closure of the leaves is related to increasing VPD (Ishida, Toma et al. 1999) or low-light condition (Larcher 1995) and mainly in tea the response of gs is more associated with high vapour pressure deficit in tea (Hajra and Kumar 1999). VPD however did not exceed 1.3kPa during the trial period, which is much lower than the critical value of 2.0kPa for the tea plant (Squire 1985). Hence in the low elevation tea growing areas hence, vapour pressure deficit is not the most influential factor controlling the stomatal activity.
The factors controlling the effects of temperature on the physiology of the plant are not very clear in tea (Barbora 1994). However, both stomatal and mesophyll factors control the temperature dependence of the photosynthesis of the plant (Joshi and Palni 1998). As there was no significant variation in gs, the control of Pn is more likely attributed to mesophyll factors (Lin 1998) in the low elevation tea growing area. But in other crops, mid-day depression of the Pn is more attributed to the reduction in gs (Winkel and Rambal 1993; Pathre, Sinha et al. 1998).
The two cultivars showed clear difference in their response to light and temperature. The irrigated TRI 2023 was very high in its photosynthetic activity as a response to PAR. Lower quantum efficiency (Φ) of rain-fed plants indicates the photo-inhibition caused by the heat and drought stress (Pallardy 2008). Damage to the photosynthesis system may be the reason
95 for the rain-fed plants to produce lower yield even after the soil moisture condition replenished with onset of rains.
In earlier studies of tea there were suggestions that Pn was more related to plant productivity than other perennial crops as vegetative young shoots are harvested (Roberts and Keys 1978).
But later came the argument that Pn and shoot growth could be uncoupled during drought (Squire and Callander 1981). Nevertheless in early studies, drought effect consisted of soil moisture deficit, high saturation deficit and low temperature, factors that control the shoot growth not only photosynthesis (Smith, Stephens et al. 1993). In the present study, high temperature effect can be termed as the limiting factor of Pn.
Leaf temperature increase up to 80C than ambient temperature was observed among rain-fed
TRI 2023 and TRI 3025. Among the two cultivars showed a higher Tl increase during day time (Figure 5.8). Leaf orientation of the TRI 3025 is horizontal and exposes to more solar radiation. As shown in Figure 5.9, TRI 3025 receive higher PAR than it required for Pmax. Receiving excess light beyond photosynthetic demand causes photo-inhibition of photosystem II (Long, Humphries et al. 1994) and continuous photo-inhibition leads to photo-damage
(Asada 1999). Photo-damage of rain-fed plants then result in lower Pn and subsequently lower yield in wet season months like April and May.
Irrigation increased the yield of TRI 2023 by 27% and TRI 3025 by 16%. Cultivar differences were clearly visible in response to irrigation. Similar differences have been observed in other tea growing environment as well (Stephens and Carr 1991; Kigalu, Kimamboa et al. 2008). Of particular note in the current study the irrigated plants produced higher yield long after ceasing irrigation during wet period of the year. In addition to damages to photosynthetic apparatus of rain-fed plants, the high leaf area usually associated with irrigated plants could be another reason for higher yields. Production dips in irrigated plants in dry months can be related to lower Pn and lower dry matter partition to shoot production. As it is one irrigation regime was tested in this experiment, it is premature to assume that irrigation deficiency caused the yield loss. However, it will be important to test several water application efficiencies, with higher application rate than the tested one.
The incidence of stem canker was very high among rain-fed TRI 2023 (92% severely infested) while cultivar TRI 3025 was more resistant to the disease. Irrigation has the ability to reduce the severely affected percentage by nearly 80% in both cultivars. This experiment confirms Carr’s (2010) proposal that plants grown under irrigation from field planting are healthier and more resistant to drought-related stem canker. This fungal disease caused by Phomopsis theae is encouraged by cavitations in the stem due to deficits in the plant water balance (Kuhns, Garrett et al. 1985). The main reason to abandon the highly productive TRI 96 2023 in the cultivation program is the heavy infestation of stem canker in dry weather. On the other hand TRI 3025 was more resistant to the disease in rain-fed condition and is the main reason for introducing to the growers as drought tolerant cultivar. Under irrigation there is good cause to re-consider TRI 2023’s role in the national cultivation program.
5.2.5 Summary of Results
Difference in Ψ was observed among two cultivars at dawn mostly.
TRI 2023, however, showed difference in Ψnoon according to irrigation treatment. th Rain-fed TRI 2023, showed the lowest Ψnoon by 10 week, closer to permanent wilting point.
There was a decline in Pn rate with drought and it did not recover even after rain occurrence under rain-fed conditions for both cultivars.
For cultivar TRI 3025, even though gs, increased in some weeks amidst drought
condition, due to favourable atmospheric conditions, El did not increase accordingly.
Cultivar TRI 2023 responded favourable to changes in gs. This factor further indicates the increased water requirement for highly productive cultivar and the stomatal activity of TRI 3025 was lower than TRI 2023, even under irrigated conditions.
Diurnal pattern of Tl showed that under rain-fed conditions in low elevation tea 0 growing areas, Tl increased above critical level of 35 C for several hours during the day even within short dry periods. There is a dip in the average weekly made tea production even for irrigated plants during dry periods. The cultivar difference in the production gap is visible under rain-fed and irrigated conditions. There is no interaction between irrigation and cultivar selection Advantage of irrigation in yield is reflected prominently even during wet period for this leafy crop. Incidence of dry weather stem infection of canker was more related to cultivar variation even under irrigation. However, irrigation was effective in controlling the disease.
97 5.3 Experiment 2: Water Use of Tea under Rain-fed and Irrigated Conditions
5.3.1 Introduction
The previous experiment showed that TRI 2023 responds more favourably to irrigation than TRI 3025. It seems that TRI 2023’s advantage lies, at least partially, in its ability to maintain photosynthesis under higher temperature. A short period of 3 months irrigation during the dry season resulted in 27% increased annual yield. The effect of irrigation in the dry period carried over into increased yields during the unirrigated months of the wet season. This ability to produce more even when the irrigation is turned off may lie in the fact that greater branching and plucking points develop over time and also physiological reasons. However, to follow on from our understanding of the influence of temperature and photosynthesis on yield, this next experiment, evaluates the relationship between the water use of the TRI 2023 and dry matter production. It was undertaken in 2008-2009, on the same site as in Experiment 1, when the necessary sap flow measurement equipment became available. Together, Experiments 1 and 2 satisfy the first aim of this chapter.
5.3.2 Materials and Method
Experiment 2 was conducted at Tea Research Institute, Ratnapura, Sri Lanka, at the same site where experiment 1 was conducted. A site description is provided in Chapter 4.0.
5.3.2.1 General experiment details
Mature (8 years old) tea plants of cultivar TRI 2023 were used for the experiment. The plants were established in the field in 1999 at a spacing of 60cm X 90cm X 150cm (double hedge row planting). Shade trees were not planted in the field to avoid possible use of irrigation water by the shade trees and to prevent uneven solar radiation availability to the field. The plants were in the third pruning cycle and last pruning was done in 2006 to a height of 90cm.
Data collection was conducted from August 2008 to March 2009. This duration had a wet period and a short inter-monsoon dry spell. Sensors were removed during prolonged rainy days, as the stem flow and through fall was very high causing possible moisture leak to the sensors. Netafim Ram17D integral drip lines were used to irrigate the plants. The spacing between two drippers was each 60cm and each drip delivered 1.6L/hour. This spacing of the drippers was selected to ensure irrigation water for each plant from a dripper. To measure the dry matter production during non drought period and environmental effects, transpiration and shoot development of rain-fed tea was monitored continuously during August–November, 2008 for nearly 3 months. This season is a wet period with a short inter-monsoon dry spell. To monitor the water use of irrigated and rain-fed plants for a continuous dry period,
98 transpiration and shoot development of irrigated and rain-fed TRI 2023 plants were measured during January–March 2009. This period was a dry season, and supplementary irrigation was applied throughout the period as described earlier. To measure dry matter production, all new shoots were removed at 7 day intervals. The shoots were then oven dried at a temperature of 1050C to reach a constant weight after 12 hours. Total dry matter production of the plant per week was estimated using a harvest index of 0.10 (Magambo and Cannell 1981)
Figure 5.13 Sap flow sensor fixed to a drip irrigated TRI 2023 tea plant. Sensor is covered with insulating material to prevent heat absorption. (Note: Data logger shown is Campbell CR10X datalogger) 5.3.2.2 Instrumentation
Though there are several methods available to estimate the transpiration, sap flow estimation is the most popular method (Dragoni, Lakso et al. 2005) and this method provide direct readings on the water use of plants. When compared to lysimeter technique, this is a less expensive method as well. Sap flow is measured by heat pulse as a tracer (Cohen, Fuchs et al. 1981). The principle of the measurement is based on measuring the time required for a heat pulse applied at a given point of the stem to be transferred and measured at a specific location above the point where the heat pulse was generated (Ananthacumaraswamy, De Costa et al. 2000). The time taken is a measurement of the rate of sap flow in the xylem. By analysing cross sectional area, flow geometry and flow velocity, flow density of sap can be calculated (Swanson 1994).
Measurements were taken with East 30 sap flow sensors (www.east30sensors.com). It consists of a pair of 35mm long stainless steel needles, 6mm apart. One needle contains a heater and other needle contains three precision thermistor sensors evenly placed at 5, 17.5 and 30mm distance from base. Before insertion, two holes were drilled precisely using a drill 99 guide template. The holes were drilled in such a manner that facilitated smooth insertion and full contact of the two rods to the stem. The needles were inserted into the stem after removing the bark slightly. A wax layer was applied to the two rods for easy penetration. The heater was placed below the thermocouple. After installation, the sensor was covered with a sponge material by wrapping over the stem and then it was covered with insulating material to prevent radial heat loss and preventing unnecessary heat build up during very dry day (Figure 5.13). The sensor was then fitted to a CR21X Campbell data logger (Campbell Scientific Inc.,USA). The heater was powered by a 12V rechargeable battery. Data logger control ports switch on/off the heat pulse every second. A current is applied for 8 seconds to generate heat. Duration of the heat pulse was controlled by a programmed counter (Ischida, Campbell et al. 1991). The rise in temperature of the thermocouple was recorded every second and data were averaged and stored at 60 minute intervals. The time taken by pulse peaks to reach the thermistor was related to sap flow velocity. Three thermistors gave the sap flow at three different depths and by multiplying with sap wood area, transpiration rate was calculated (Thermallogic 2002).
5.3.2.3 Meteorological data
Meteorological data were collected using an Automatic Weather Station (Measurement Engineering Australia) established in the same field. Data were collected to Starlog- Prologger (www.unidata.com.au) data logger at 15 minute interval. Temperature, solar radiation, relative humidity and wind speed were recorded in the data logger. Daily rainfall and sunshine hours of each day were recorded at a manual operated weather station, located in the nearby tea estate (<200m), which supply weather data to Department of Meteorology, Sri Lanka.
5.3.2.4 Soil moisture
Soil moisture content was measured gravimetrically using carefully driven undisturbed core samples up to 60cm depth at weekly intervals. The fresh weight of each sample was recorded soon after obtaining the samples in the field. Then the samples were oven dried for 24 hr period at 1050C to reach a constant dry weight. Weight of oven dried samples was recorded to calculate the moisture weight of the core sample.
5.3.3 Results
5.3.3.1 Transpiration (E) of rain-fed plants during wet season
The study period of August to November, 2008 was generally a wet period of the year, where rainfall of more than 400mm was received. The average rainfall availability during the study
100 period was more than 100mm per month. Vapour pressure deficit remained below 1.2kPa. Also maximum and minimum temperature did not show large increases.
Figure 5.14 presents a combination of daily rainfall events (mm/d), weekly gravimetric soil moisture measurements at 3 depths, daily potential evapotranspiration (ET0) as calculated by modified Penman-Monteith and daily transpiration (E) (mm/d) as measured by sapflow. This can be used to indicate the boundaries of transpiration under non-irrigated conditions.
30 70 Rain 20cm 60 25 40cm 60cm 50 20
40 15 30
10 (mm/day) Rainfall 20
Volumetric moisture content (v/v) content moisture Volumetric 5 10
0 0 6 E
ET0 5
4
3
Water use (mm/day) 2
1
0 4/8/08 18/8/08 1/9/08 15/9/08 29/9/08 13/10/08 27/10/08 10/11/08 Date
Figure 5.14 Daily rainfall and weekly volumetric soil moisture level of top 60cm soil (a) and daily plant transpiration E and daily potential evapotranspiration-ET0 (b) from August to November, 2008
Initially, the moisture content of 60cm depth was approximately 10%, which is very low and reaching the permanent wilting point. Low rainfall during the early part of the month may have caused the reduction of the soil moisture in deeper layers. Soil moisture levels replenished with the onset of rain in early September 2008. Soil moisture dropped again with the cessation of rain in the later part of the month. With the start of rain again in October, soil 101 moisture at both 20 and 40cm depth fluctuated, but at 40cm depth it increased with the rainy season.
The average level of transpiration was 2.3(±0.3)mm/day. Although this was the wet season, soil moisture was low and the transpiration was low. The general trend of the transpiration pattern is an upward pattern, which was associated with the increase in rains receiving at the end of the season. Between 19 September 2008 and 21 October 2008, E was over 3mm/day.
E was less than ET0 for the most of the period. The average crop coefficient was 0.848(±0.1). This value is equal to the crop coefficient given in the FAO guideline for tea (Allen, Pereira et al. 1998; Kigalu 2007). However, this is not necessarily a universal value as the Kenyan cultivar AHP S15/10 showed a higher crop coefficient value of 0.98 (Kigalu 2007).
The transpiration exceeded the ET0 value when the solar radiation level became lower (data not shown in figure) during period 16-27 October 2008. Though, ET0 showed a high daily variation, actual plant transpiration was more consistent.
During the cloudless periods when it was not raining (e.g.15 Sept-10 Oct) and soil moistures was low (<20% v/v), transpiration remained relatively high due to relatively increased temperature and sunshine. The maximum temperature level during the study period did not reach the critical level of 350C, unlike the prominent dry periods of January- March each year. The plant was not under significant soil moisture stress during latter part of the study period, nor did temperature reach the critical upper threshold, transpiration remained high.
In summary, transpiration can remain high even without recent rain as critical temperatures are not reached as well as soil moisture is adequate. However, even during the wet season soil moisture deficits in deeper soil layers can impact transpiration processes.
5.3.3.1.1 Dry matter production in the wet season
The average dry matter production of TRI 2023 during this 3 month wet season was 73.6(±8.5)g/plant/week (Figure 5.15). The minimum dry matter production per week was 29.3(±3.4)g/plant and the highest dry matter was recorded in the 110.5(±17.1)g/plant. Generally dry matter production followed the transpiration pattern. The drop off in dry matter production in the first two weeks of September is probably related to the heavy rainfall (see Figure 5.14) and low light levels during this period. This will explain a similar drop off in November. This is also reflected in a slowdown in cumulative transpiration during these events.
102 4 140 (a) E dry matter 120
3 100
2 80
60
Plant water use (mm/day) use water Plant 1 40 Total matter plant production dry (g/week)
0 20 200 (b) 1200
180 1000 160
140 800 120
100 600
80 400 60
Cumulative plant water use (mm) use water plant Cumulative 40 200
20 Cumulative matter plant production dry (g)
0 0 4/8/08 18/8/08 1/9/08 15/9/08 29/9/08 13/10/08 27/10/08 10/11/08 Date
Figure 5.15 Daily transpiration-E and weekly average total dry matter production (a) and cumulative transpiration and dry matter production from August to November, 2008 (b)
5.3.3.1.2 Response to environmental variables:
Figure 5.16 shows the transpiration response to different environmental variables. Transpiration showed strong positive relationships with solar radiation, vapour pressure deficit, maximum temperature and vapour pressure deficit.
Among above parameters, maximum temperature level of the day had a strongest relationship (r2=0.62) with transpiration. According to the forward regression analysis of the parameters, both solar radiation and maximum temperature could be used to predict the transpiration of the tea plant (P=0.001) during wet season. However, maximum temperature level had the highest correlation coefficient. This appears to be the characteristic limiting environmental parameter for tea grown in low elevation areas. Earlier studies on the tea plant transpiration
103 found that transpiration is mainly controlled by solar radiation (Ananthacumaraswamy, De Costa et al. 2000) in low temperature highland areas of Sri Lanka.
4 4 r2 = 0.44, P = <0.0001 r2 = 0.52, P = <0.0001
3 3
2 2
1 1
0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 8 101214161820222426
Vapor pressure deficit (kPa) Solar radiation (MJ/m-2)
4 4 r2 = 0.49, P = <0.0001 r2 = 0.62, P = <0.0001 Transpiration (mm/day) Transpiration
3 3
2 2
1 1
0 0 30 32 34 36 0123456 Maximum temperature (0C) Potential evapotranspiration (mm/day)
Figure 5.16 Relationships between daily transpiration and VPD, solar radiation, maximum temperature and potential evapotranspiration of TRI 2023, during August - November, 2008 period
In summary, during the wet season transpiration is positively related to increasing VPD, solar 2 radiation, air temperature and ET0. However the closest relationship (r =0.62, P=<0.0001) was observed with increasing temperature.
5.3.3.2 Transpiration of Rain-fed and Irrigated Tea Plants during Dry Season
5.3.3.2.1 Climate during the study period
Starting from 12 Feb 2009, the climate of the study area was dry. During the initial period there was no rain up to 22 Feb 2009. A rainfall event of 13mm occurred in day 23 February ending the dry spell (Figure 5.17). There was a rainfall of 76mm in 06 Mar 2009. Solar radiation level was above 20MJm-2day-1, during the initial 3 day period and lowest solar radiation level was observed on 08 Mar 2009, amounting to 8.8MJm-2day-1. Vapour pressure deficit remained around 1.0kPa. Volumetric moisture content of the rain-fed area felt to
104 12.9% during the second week of the experiment. However, later the soil volume moisture content increased with the availability of rain. A gradual decline in the soil moisture content of the irrigated field can be observed with the study period. In overall, the moisture content of the irrigated plots was above 20%.
35 100 Rain moisture (irrigated) moisture (rainfed) 30 80
25 60
20 40 Rainfall (mm/day) Rainfall Soil moisture (v/v) moisture Soil
15 20
10 0 8 E(irrigated) E(rainfed) ET 7 0
6
5
4
3 Water use (mm/day) 2
1
0 09/02/09 16/02/09 23/02/09 02/03/09 09/03/09 16/03/09 Date
Figure 5.17 Daily rainfall and weekly volumetric moisture content of irrigated and rain-fed plots up to top 60cm (above) and transpiration of irrigated and rain-fed plants and potential evapotranspiration from February 09 to March 16, 2009 (below)
5.3.3.2.2 Transpiration rate of irrigated and rain-fed tea
The average estimated ET0 of the period was 3.5(±0.3)mm/day. The potential evapotranspiration level fluctuated and was low at the end of study period. Reduction in 2 VPD, (Table 5.6), mainly caused the reduction of estimated ET0 (r =0.87, P <0.0001). The average E of rain-fed and irrigated plants, remained at 1.3(±0.2) and 2.7(±0.5)mm/day respectively for the season. Difference in E, between irrigated and rain-fed plant was
105 significant (P=<0.001). After the occurrence of high rainfall (06 March 2009), E of the both treatments increased more than 100%..
Table 5.6 Climate, soil moisture content, transpiration and crop coefficient of irrigated and rain-fed TRI 2023 cultivar during February to March, 2009 Soil E Tmax VPD Kc moisture(V/V) Rn 0 ET0 (mm/day) Period -2 ( C) (kPa) (MJm ) (mm/day) Rain-fed Irrigated Rain-fed Irrigated Rain-fed Irrigated
12-18 Feb 12.9 27.7 20.9 36.4 1.6 5.4 0.76 1.54 0.14 0.29
20-26 Feb 15.2 18.7 16.4 35.5 0.8 2.8 0.59 1.11 0.23 0.43
27Feb-5Mar 14.9 22.5 20.4 36.5 1.2 4.2 0.56 1.34 0.13 0.31
6 -12 Mar 16.4 21.3 15.1 34.9 0.7 2.3 1.8 3.6 0.84 1.6
In summary, irrigated plants showed a 113% higher transpiration rate than rain-fed plants.
Until the major rain event occurred on 6 March 2009, Kc remained below 1.0 for both treatments. With commencement of the rains, Kc value increased in both treatments. During the entire experiment period, Kc value of the irrigated plants was around 100% higher than that of rain-fed tea. For the irrigated tea after the rain event, Kc value increased more than 1, showing much higher water use than that of the ET0, estimated by the Penman Monteith equation.
5.3.3.2.3 Dry matter production during dry season
Plant dry matter production of rain-fed and irrigated plants differed significantly (P=0.08) during the dry spell. The average dry matter production per plant during this period of the rain-fed plant was 21 (±6.1)g/wk, whereas irrigated plants production was 37(±5.3)g/week. Dry matter production during the dry period is significantly lower than average dry matter production during wet period. Dry matter production of both cultivars increased with the commencement of rain.
5.3.3.2.4 Diurnal variation in transpiration
The patterns of hourly transpiration of tea plants in two days representing wet season and dry season are shown in Figure 5.18. In the wet season, the transpiration rate of measured two plants showed almost identical diurnal pattern. Air temperature reached never exceeded 350C between 1400-1500 hours. There was a sharp drop in solar radiation after 1200 hrs from 778 to 538 W/m-2. Both plants showed highest E between 1100-1200 hours.
In the dry season day, air temperature reached above 340C by 1400 hours, and by 1500 hours it reached 350C. Plants experienced above 300C, temperature for nearly 4 hours from 1300 106 hours. Solar radiation was higher than during the wet season. It reached 811 Wm-2 by 1100 hours and remained above 800 Wm-2 up to 1300 hours. Both rain-fed and irrigated plants showed lower peak transpiration rate than dry season day. However in both plants, peak E remained stable from 1100 to 1400 hours. Rain-fed plants showed a higher transpiration rate from 900 to 1500 hours. During rest of the day, irrigated plants showed a higher transpiration rate. During night time also, irrigated plants showed a significant transpiration, unlike rain- fed plants and the cumulative E was 34% higher in irrigated plant than rain-fed plant in the day.
1.0 Wet Season (a) irrigated Dry Season (b) plant 2 0.8 rain-fed plant 1
0.6
0.4
0.2 Transpiration (mm/hr) Transpiration
0.0 1000 (c) (d) 36 solar radiation solar radiation temperature ) 34 -2 800 temperature 32 C) 0 600 30 28 400 26
24 ( Temperature 200 Solar radiation (Wm radiation Solar 22 0 20 0 500 1000 1500 2000 0 500 1000 1500 2000
Hours Hours
Figure 5.18 Diurnal variation of tea plant transpiration as changed with air temperature and solar radiation during wet period and dry period in Ratnapura Sri Lanka (a) diurnal transpiration in wet season in rain-fed plants (b) diurnal transpiration on dry season under rain-fed (continuous line) and irrigated (dotted line) plants. (c & d) diurnal variation of solar radiation (dash line) and air temperature (continuous line) 5.3.4 Discussion
Transpiration of the tea in hot humid low elevation area of Sri Lanka was measured in wet and dry seasons. To establish the relationship between E and dry matter production, dry matter production was also monitored during the period. Factors determining transpiration and the relationship between the estimated potential evapotranspiration and plant transpiration were established for the dry season.
107 5.3.4.1 Transpiration and dry matter production
Based on the dry and wet season daily transpiration and dry matter production, transpiration efficiency (TE) for cultivar TRI 2023 is given in Table 5.7. During dry season, tea plant transpiration ratio was >50% less than wet season. The difference between irrigated and rain- fed plants was also only 4%. Even though irrigated tea produced higher dry matter production than rain-fed, transpiration efficiency remained almost the same. During the dry period, transpiration was not mainly meant for dry matter production, but for minimizing the drought/heat stress through higher transpiration.
Table 5.7 Transpiration efficiency (TE) of rain-fed and irrigated TRI 2023, as the ration between daily dry matter production and transpiration per plant. TE Season (g/mm) Wet season 6.0(±0.9)
Dry season, rain-fed 2.9(±0.6)
Dry season, irrigated 3.0(±0.7)
The experiment revealed transpiration of the plants is controlled by not only soil moisture but by other environmental factors like air temperature, solar radiation and vapour pressure deficit. With the provision of soil moisture during dry periods, E occurs at a higher rate than rain-fed plants. However, irrigated plants too reduced its E in dry season than wet season. Decrease in E of irrigated plants can be due to increased canopy resistance or hydraulic resistance (Passioura 1996). It will be productive to find ways to minimize the canopy resistance of irrigated plants, e.g. such as methods like increased water application or high shade planting (minimize ambient temperature). Due to lower soil moisture in lower soil depths, transpiration was lower when the measurements were commenced in 2008. This finding is important in designing irrigation systems, ensuring the irrigation for entire root system. . According to other findings, partial drying of deep root layers may have resulted in the limiting plant transpiration (Sinclair, Holbrook et al. 2005; Duursma, Kolari et al. 2007). Partial drying of the roots is not favourable for the tea plant which produce a leafy product as the final yield, unlike a fruit crop where harvested yield is either fruit or nut (Zhao-JunYing, Wang-LiJun et al. 2005)
Estimated potential evapotranspiration value showed a reflection of the water use of the plant with reference to climatic variables. It is a physical model based or surface and aerodynamic resistance (Kabir, Alam et al. 2007). The tea plant transpiration has however controlled mainly not only by surface and aerodynamic factors, but by plant and soil factors itself 108 according to the experiment. The transpiration of the water mainly controlled by the environmental factors as can be seen in the reduction of transpiration of both irrigated and rain-fed tea. Despite the seasonal changes, transpiration rate of the both plants have been increased, while ET0 decreased at the end of dry season. This suggests that transpiration of the plant is not only governed by the seasonal factors. In a similar experiment it has been found that transpiration from well-irrigated sugar cane has been found to be independent of diurnal variation of stomatal conductance (Meinzer, Goldstein et al. 1993).
The crop coefficient(Kc) of tea is 0.85 (Allen, Pereira et al. 1998). However, variation of Kc was found evident within dry period specially (Table 5.6). With the resumption of rainfall after dry spell, Kc increased >1.0. Not only in tea (Kigalu 2007), but in other crops like grapevine (Dragoni, Lakso et al. 2006) and Chrysothamnus sp (an arid zone species called rabbit brush) (Steinwand, Harrington et al. 2001), there were instances where Kc >1.0 was reported, mainly in hot humid conditions. However, as with irrigated tea in low elevation tea growing areas, increase in Kc is seasonal and depends on cultivar performance as well within a selected crop. For the seasonal increase in tea Kc value, there could be broadly two reasons. One is related to plant performance factors and other related to the calculation procedure of potential Et0.
For the tea plant, during drought under humid conditions, soil moisture stress and temperature effect have negative effect on plant physiological activity. With the onset of rainfall season, maximum air temperature level drops, as well the duration of tea plant exposing to high temperature reduces as a result of afternoon showers. During the drought period, even though tea is irrigated with drip, dry and hot weather reduces transpiration by partially closing stomata (Dragoni, Lakso et al. 2006). (In the glass house experiments, Experiments 4 and 5, transpiration was increased with increased air temperature, as there was no high radiation effect, unlike the humid field condition in Sri Lanka). The start of the rain again rejuvenates the physiological activities. The irrigated plants had a high leaf area index and a higher shoot rate than unirrigated plants. During the drought period also, it maintained twice higher transpiration rate than unirrigated plants. As the transpiration process is more with new leaves than old leaves, irrigated plants which contain more fresh leaves even during drought season, as reflected by higher yields, achieve higher transpiration rates. With the onset of rainy season, is the time when tea plants start producing more leaves and subsequently increased the transpiration rate of both irrigated and non irrigated plants.
109 5.3.5 Summary of Results
Plant transpiration (E) largely followed the calculated ET0 during wet season. However, daily changes in evaporative demand are not reflected immediately on daily E. However the relationship was poor in dry season. Dry matter production during wet season is closely followed the transpiration pattern of the plants. During wet season of the yearly (nearly 9 months), increase in ambient temperature mainly drives the transpiration demand. During the dry period, irrigated plants transpire more water, as high as twice the water use of rain-fed plants. Still with irrigated plants, there is a reduction in water use in dry periods compared with the wet season, perhaps due to stress from atmospheric conditions.
110 5.4 Experiment 3: Physiological Response of New Cultivars to Drought
5.4.1 Introduction
The first experiment of this chapter showed the higher response to irrigation by the bench mark high yielding cultivar TRI 2023 through high assimilation rates and tea yield. In contrast cultivar TRI 3025 showed physiological response of lower stomatal conductance and reduced transpiration. These characters are helpful for the plant to survive the adverse drought period in the field. But the lower yields of the cultivar made it unattractive for the tea growers.
In the second experiment in this chapter, the water use of high yielding cultivar TRI 2023 was evaluated under wet and dry season. In the dry season, average transpiration of the irrigated plants was more than double than that of rain-fed plants, even with lower water productivity. Irrigating the entire plantations in many tea fields at the rate of nearly 2mm/plant/day is a difficult task with the given high density of plant population (12,500 plants/ha). Suitable water sources are not available especially in mountainous areas. For the majority of the tea fields, cultivar selection has to be considered as the alternative to irrigation for drought mitigation. In contrast to TRI 2023, TRI 3025 had shown drought mitigation effects like lower Pn and El . TRI 3025 is resistant to stem canker infection, which is related to drought in low elevation area. As a result, TRI 3025 is advised to be used as a drought tolerant cultivar. Yet with this cultivar, there are some characters that affect productivity and sometimes survival in major drought event, such as high temperature increase in the leaf. Also if the Pn is increased without significant effect to survival, it would be a welcome change by the growers in new cultivar during drought. Therefore an investigation was carried out to evaluate the physiology under rain-fed conditions during dry season together with benchmark drought-resistant TRI 3025 cultivar.
5.4.2 Method
The experiment was conducted at the Field no 04, St. Joachim Estate, Ratnapura. Details of the soil and climate of the area are given in a Chapter 4.
The trial was planted in 2004 at a spacing of 60cm X 120cm for evaluating new cultivar performance and to supply vegetative cuttings to plant nurseries. The experimental design was a complete randomized design, with three plots for each cultivar. There were no shade plants in the area.
Gas exchange measurements were made using an ADC LCA4 (ADC Bio Scientific, UK) during the period 13 February to 13 March 2009 at weekly intervals. All measurements were
111 collected between 1200 – 1300 hrs in each day. This was the time, that plants receive maximum water stress in the field. The top-most mature tea leaves, exposed to fully sunlight, were selected for measurement. At least three plants were selected for measurement in each plot. Volumetric soil moisture measurements were taken using core samples driven to 30cm and 60cm depths at weekly interval. After obtaining samples, the fresh weight of the soil samples were taken in the field. Samples were then oven dried at 1050C for 24 hours to measure the dry weight of the soil. When transporting the soil samples, additional precautions were taken to prevent any soil loss.
All weather measurements were recorded at the automatic weather station at Field no 01 of the estate (described in experiment 1 of the Chapter 5). Rainfall measurements were recorded from the manual rain gauge of the estate weather observatory. Plant potential evapotranspiration was calculated based on FAO modified Penman-Monteith calculation (Allen, Pereira et al. 1998), using Instat climate analysis software, version 3.036 (Statistical Service Centre, University of Reading, UK)
5.4.3 Results
5.4.3.1 Weather during the study period
The experiment field experienced a usual dry period during January-March period. During the study period, dry period was observed in January and February months. The experiment was started after the plants experiencing a dry month of January (20.1mm rainfall, over 6 days). The climate during the study period is shown in Table 5.8. Maximum temperature reached 350C in almost all weeks, except the second day of the measurements (20th Feb). There was no rain during the first two weeks; however rain started from week 3 onwards. The vapour pressure deficit was higher in the first two weeks, however dropped it below 1.0kPa with the onset of rain.
5.4.3.2 Soil moisture
Volumetric moisture content of the soil was around 12% at the beginning of the experiment (Figure 5.19). Initially, the top 30cm layer had slightly higher moisture content. In the first week, soil moisture content fell to 11% as there was no rain. However, the soil moisture content was recharged and had reached the level of around 20% during the latter stage of the experiment. Even though soil moisture level reaches to saturated level, the maximum temperature levels reached more than 350C and ET0 level reached more than 3.0 mm a day.
112 Table 5.8 Weather during February – March, 2009. Except rainfall, others are daily values. Rainfall is given as weekly total
Date 13 Feb 20 Feb 27 Feb 06 Mar 13 Mar
Solar radiation(MJm-2) 21.1 15.5 20.5 17.4 17.2
Min. temperature(0C) 18.7 19.9 22.5 22.9 22.5
Max. temperature(0C) 35.7 34.0 35.7 35.3 35.1
Sunshine hours 11.4 10.8 11.5 10.0 10.8
Vapour pressure deficit (kPa) 1.3 1.26 0.9 0.9 0.7
Potential evapotranspiration (mm) 3.8 2.8 3.8 3.7 3.3
Precipitation (last 7 day) 0 0 19 79 24
In summary, even though this was a dry season, with the resumption of the rain in the end of experiment period, soil moisture content rose to 20%.
22 100
Rain 20 0-30cm 30-60cm 80
18
60 16
14 40 Rainfall (mm) Rainfall Soil moisture (V/V) moisture Soil 12
20 10
8 0 2/9 2/16 2/23 3/2 3/9 3/16
Date Figure 5.19 Volumetric moisture content and total weekly rainfall from February 09 to March 16, 2009
5.4.3.3 Leaf Transpiration (El)
Figure 5.20 shows the El of tea leaves during the study period. With the depletion of the soil moisture, El decreased for many plants in the second week, except cultivar TRI 4049. The -2 -1 lowest transpiration rate of 0.54(±0.1)mmol H2O m s was recorded from the variety TRI 4047 during the second week of observation, under the lowest soil moisture condition.
However, before the onset of rains, there was no significant difference in the El in all varieties and generally it was lower. With the start of the rains, in the third week of assessment, El 113 increased in all plants. The cultivar TRI 3025 was the highest El after the initial rains on 27 th February. In all varieties El fell in 6 March (week 4 of assessing, even though there were rains and soil moisture level was improved.
In summary, with the increment of rain and soil moisture El increased for all plants.
5
TRI 3014 TRI 3025 TRI 4053 ) 4 1 -
s TRI 4047 2 - TRI 4049 O m 2 3
2
Transpiration (mmol H 1
0 2/9 2/16 2/23 3/2 3/9 3/16 Date
Figure 5.20 Transpiration rate of top most mature tea leaves exposed to fully sunlight in mid-day during drought months of 2009
5.4.3.4 Photosynthesis (Pn)
The plant photosynthesis rate (Pn), which can be considered as one of the most sensitive physiological activity for environmental variations, showed a greater difference among the varieties, except the second week of experiment (Figure 5.21). Interestingly, the highest Pn was observed in all varieties in the second week (20 February) of the assessment. Pn increased in all varieties during this particular day and there was no significant difference among different cultivars. This increase in Pn was so significant that it even exceeded the level during the favourable soil moisture periods of later measurement days. Overall, cultivar
TRI 4047, showed a lower Pn during the measurement period, except at week 4. Even though, -2 -1 not significantly different, Pn of cultivar TRI 4047 was only 3.37(±1.9) μmol CO2 m s .
114 14 TRI 3014 TRI 3025
) 12 1 - TRI 4053 s 2 - TRI 4047 m 2 10 TRI 4049
8
6
4
Photosynthetic rate (mmol CO (mmol rate Photosynthetic 2
0 2/16 2/23 3/2 3/9 3/16 Date
Figure 5.21 Photosynthetic rate of topmost mature tea leaves exposed to fully sunlight at mid-day during drought months of 2009
In summary, cultivar TRI 4049 showed higher Pn (18%) and El (4%) higher than TRI 3025 during the period.
5.4.3.5 Leaf temperature (Tl)
Leaf temperature levels (Table 5.9) reached above 430C, during the first observation day. There was no significant difference in cultivars when reaching the highest temperature levels. 0 Highest recorded Tl of 45.9(±0.5) C was recorded on cultivar TRI 4053 in the second week. There was a significant difference in the leaf temperature at the 20th February and 6th March.
In summary all the cultivar showed a higher leaf temperature during the dry days, average leaf temperature for all cultivars was 39.5(±0.3)0C, a high value, potentially damaging the photosynthetic mechanism of leaves.
115 Table 5.9 Leaf temperature at mid-day. (Top most mature tea leaves exposed to direct sunlight were measured, standard error in parenthesis)
Cultivar 13 Feb 20 Feb 06 Mar
TRI 3014 43.8(±0.6) 36.1(±0.2) 36.8(±0.2)
TRI 3025 43.2(±1.2) 37.3(±0.4) 38.4(±0.2)
TRI 4053 45.9(±0.5) 39.4(±0.9) 36.1(±0.3)
TRI 4047 44.9(±0.06) 38.9(±0.4) 34.7(±0.05)
TRI 4049 44.6(±0.4) 38.6(±0.3) 34.4(±0.3)
P< ns 0.002 0.0001
5.4.3.6 Water use efficiency (Wi)
The instantaneous water use efficiency (Wi) of each cultivar was calculated as the ratio between transpiration and photosynthesis (Field, Merino et al. 1983; Jones 2004). Daily Wi and seasonal average is shown (Figure 5.22). Second day of observation showed a larger Wi from two varieties. However, when calculating the average this date was omitted as it was a -2 0 gloomy (solar radiation=15.5MJm ) and cool day (Tmax = 34 C). In average, TRI 3014 and
TRI 4049 showed Wi of above 2.0 during dry season.
Among the tested cultivars, TRI 3014 and TRI 4053 showed significant negative relationship with maximum temperature (Table 5.10). Among the above two cultivars, TRI 4053 showed the most sensitivity (r2=0.88, P=0.02) than TRI 3014. Nevertheless, one positive note regarding the new cultivars is that 3 out of 5 tested cultivars showed no significant negative relationship with maximum air temperature.
116 16 (a) TRI 3014 14 TRI 3025 TRI 4053 12 TRI 4047 TRI 4049 10
8
6 ) i 4
2
0
2/12 2/19 2/26 3/4 3/11 3/18 Date 3.0 (b)
2.5
2.0 Instantaneous water use efficiency (W
1.5
1.0
0.5
0.0 TRI 3014 TRI 3025 TRI 4053 TRI 4047 TRI 4049 Cultivar
Figure 5.22 Instantaneous water use of efficiency (Wi) of tea cultivars (a) during the measuring period (b) average Wi for the season
Table 5.10 Relationship between maximum temperature and photosynthetic rate
Cultivar y-intercept slope r2 P<
TRI 3014 137 -3.7 0.73 0.06
TRI 3025 85 -2.3 0.51 ns
TRI 4053 118 -3.2 0.88 0.02
TRI 4047 34 -0.9 0.17 ns
TRI 4049 109 -2.9 0.53 ns
117 5.4.4 Discussion
The experiment tested four new cultivars with the benchmark TRI 3025 for the physiological performance in the dry season. Three of the tested cultivars showed significant increases in assimilation rates as compared to the benchmark cultivar. However, only TRI 4049, showed a higher Pn and El rate. High Pn and El were physiological traits found in the productive cultivar TRI 2023 in Experiment 1. Anecdotal evidence from the early growers who have already experienced yield performance of these cultivar in the fields, too identify TRI 4049 as a high yielding cultivar. The characters shown by the cultivars like lower Pn and El rate could be important for the drought mitigation (Monclus, Dreyer et al. 2006). However their productivity remained. In the experiment 1, high productive TRI 2023 cultivar, under rain- fed, showed a 38% higher Pn rate than rain-fed TRI 3025.
In terms of net carbon assimilation rate also TRI 4049 showed 27% higher Wi than TRI 3025 and it is also insensitive in carbon assimilation to increasing ambient temperature (Table 5.10). Nevertheless, critical temperature increase, observed in all cultivars tested during dry and hot period is a matter of concern. This shows the vulnerability of the cultivar selection as a drought mitigation method in hot water stress periods. Water deficit, coupled with high temperature stress, cause photochemical damages to C-3 type plant leaves (Joshi and Palni 1998; van Bel, Offler et al. 2003). As the air temperature increase is a one key result of the climate change (Sokona 2009), looking for other drought mitigation options, like growing high shade plants in tea fields is important in the long run. High shade plants control the penetration of solar radiation and prevent increase in the air temperature in tea plant micro climate.
5.4.4 Summary of Results
TRI 4049 showed 17% higher Pn and 4% higher El than TRI 3025.
TRI 3014 and TRI 4049 showed significantly higher Wi than TRI 3025..
TRI 4049 showed higher Wi and Pn was resilient to increasing temperature. Maintenance of favourable leaf temperature during dry days is not significantly different among cultivars
5.5 Conclusion
The aim of this chapter was to describe the effect of irrigation on the physiology, water use and yield of tea, and to assess the performance of some new tea cultivars under rain-fed conditions against a bench mark drought resistant cultivar. Experiment 1 examined the yield
118 and physiological aspects, and Experiment 2 evaluated the water use of tea. In Experiment 3, physiological performance of new tea cultivars was evaluated under rain-fed condition.
In summary, it can be concluded that different tea cultivars respond quite differently to irrigation. Temperature has two contrasting effects on plant production in this environment. During the short dry season (3 months) high ambient temperature (>350C) decreases photosynthesis and yield; meanwhile, during the 9 month wet season moderately high ambient temperature drives transpiration, which increases dry matter production and yield. As high ambient temperature is a critical factor associated with water stress in low elevation growing areas, cultivar selection as a means of drought mitigation seems to be not successful, unless some additional measures like high shade planting are carried out.
119
120 Chapter 6
Experiment 4: Evaluation of Irrigation Technology
6.1 Introduction
The preceding chapters concerned the various aspects of the effect of drip irrigation on the growth and yield of two contrasting tea cultivars. Drip irrigation is considered to be the most appropriate irrigation technology for tea cultivation in Sri Lanka because of the concern for low water availability for irrigation (Eriyagama, Smakhtin et al. 2010) and energy demand. In irrigation system design wise too, drip irrigation is preferred over sprinkler irrigation in hill terrain areas, where wind can cause low uniformity of application. In tea and other crops, there are attempts now to replace the sprinkler irrigated fields with drip irrigation (Möller and Weatherhead 2006; Comis 2011). Yet for the ease of installation, low capital cost and maintenance, some growers may prefer to adopt sprinkler instead of drip irrigation technology. For this reason a parallel, but smaller evaluation trial was established on the St. Joachim Estate.
As the second step in the process of understanding the feasibility of irrigation application in low elevation areas of Sri Lanka, this chapter fulfils the following aims: 1. to describe the effect of irrigation method on key physiological parameters affecting yield, final yield and total plant growth; and
2. to assess the ability of different irrigation methods to alleviate the productivity related environmental limitations during drought.
This evaluation will lead to the following objectives of: a) understanding the best irrigation system according to the location which mainly defines the drought severity; and
b) suggesting technological improvements for drip and sprinkler irrigation.
In summary, this chapter presents the results of a trial which compares the performance the drought-susceptible cultivar but highly productive TRI 2023 under drip and sprinkler irrigation as against the rain-fed cultivation.
6.2 Materials and Method
The experiment was conducted at Field No 01, St. Joachim Estate, Tea Research Institute, Ratnapura, Sri Lanka, during January 2008 to March 2009. A detailed description of the site is available in Chapter 4. High yielding TRI 2023 cultivar was planted in May 2000. When
121 the experiment was commenced in January 2009 plants were in the second year of their third pruning cycle (i.e. the plants were 8 years old). Planting, fertilization, harvesting and other cultural practices were according to the standards specified for low elevation tea fields by Tea Research Institute of Sri Lanka (Zoysa 2008). Plants belonging to the irrigation treatment were irrigated during the January- March drought period in each year by the field staff of St. Joachim Estate. The following treatments were included in the experiment: 1. Rain-fed cultivation
2. Drip irrigation
3. Sprinkler irrigation
The experimental design was a complete randomized block design, with three blocks and each treatment having two replicates in a block. Each plot consisted of 30 tea plants (6 X 5 rows). Two additional plant rows were kept as guard rows at either side of drip and rain-fed plots and 5 additional rows at either side of sprinkler plots to prevent possible moisture seepage. There was a drain of 30cm deep and 30cm width in between two blocks. There were two tea rows were planted as guard rows surrounding the blocks.
6.2.1 Irrigation application
Drip irrigation was provided with Netafim RAM17D integral pressure compensated drip lines which has a dripper discharge of 1.6L/hr. Sprinkler irrigation was provided with Rainbird SW 2000 Plastic impact sprinklers with a water application rate of 4.4mm/hr (Stamps and McColley 1996) Sprinklers were placed at 5m apart, creating a more uniform water application. The trajectory angle of sprinkler nozzles was placed at lowest angle (15-200) to have a minimum operating radius of approximately 5-6m. Irrigation scheduling was based on the previous day pan evaporation value and based on crop coefficient value of 0.85 (Laycock 1964). When operating the irrigation systems, the drip system was assumed to have 0.8 system efficiency and for the sprinkler system it was 0.7 system efficiency (Burman, Cuenca et al. 1983).
During the experiment, irrigation was applied during January – February 2008 and January – March 2009 period. To apply the irrigation in each dry season, it was waiting for 5 consecutive rainless days. Five consecutive rainless days are considered to create water stress in the wet zone of Sri Lanka for perennial crops (Sumanasena 2008). Irrigation was applied continuously, unless it received at least 10mm rain in a day. The number of irrigation days and amount of water applied for each treatment are shown in Table 6.1.
122 Volumetric moisture content was measured at weekly interval, by carefully taken soil cores (~137cm3) to depths of 20, 40 and 60cm. Soil moisture samples were taken at a distance of 15-30cm from plant base. After obtaining the samples, holes were filled with soil.
Table 6.1 Number of irrigation days, amount of water applied and rainfall in Jan-Feb, 2008 and Jan-Mar, 2009
Year 2008 2009
Rain (mm) 99 310
Irrigation (days) 21 58
Drip (mm) 85 264
Sprinkler (mm) 96 302
6.2.2 Harvesting and yield factors
Green leaf was harvested at 7 day interval using manual pluckers. The standard of the harvesting was 2 leaves and a bud. However, single leaf plus bud and third leaf was also harvested depending on the tenderness of leaves and season of the year. Total harvest is usually a collection of the above three shoot types with the majority being 2 leaves plus bud type. Yield weight was recorded at the field for separate plots. To convert the green leaf weight into made tea equivalent, a standard conversion ratio of 22.2% was used (Stephens and Carr 1991).
In the first week of February 2008, after harvesting shoots were separated to three categories of 1 leaf + bud, 2 leaves + bud and 3 leaves + bud. Average fresh weight of each shoot type was weighed then. Shoot extension rate of the plants were measured in between January and February, 2008 after commencement of the irrigation treatments. To measure shoot extension rate, very young shoot from a plant was tagged and initial shoot length was measured using a vernier calliper. Then the growth of the same shoot was measured over two week period at a weekly interval. Harvestable shoot count per plants was measured selecting a plant from each plot during the months of January and February 2008. Measurements were taken prior to harvesting.
6.2.3 Physiological measurements
Physiological measurements, moisture measurement and climate measurements were as according to the description in Chapter 5.0. Soil moisture measurements were taken at weekly interval using carefully obtained gravimetric soil core samples. Samples were measured at the field and then oven dried at 1050C to obtain dry weight of the soil. Average volumetric moisture content of each month is shown in Figure 6.3. 123 6.2.4 Growth measurements
After the experimental period, in April 2009, one plant from each plot was excavated carefully and separated the root and stem parts. When excavating, care was taken to remove all roots of the plants including fine roots. After removing the soil by washing, root samples were air dried for one day. Then root and stem samples were oven dried at 850C till they reached a constant weight. Dry weight of the total root and stem weight was then measured.
6.2.5 Root measurements
To measure the root density, metal cores with a volume of 136cm3 was inserted to depths of 20, 40 and 60cm at each plots. Fine (<2mm) and coarse/structural (>2mm) root samples were separated after washing by using set of sieves. Air dried separated samples were then oven dried at 850C till they reached a constant weight to get the dry weight of the samples. To measure the root density distribution in the soil profile, soil cores were obtained at 20, 40 and 60cm depth using a brass core (height – 6cm, inner diameter 5.4cm). One plant was sampled from each plot for root distribution analysis. Root samples were washed and separated for fine (<2mm) and coarse (>2mm) samples before oven dried at 850C.
6.3 Results
6.3.1 Weather during trial period
The year of 2008 was very wet year, receiving 4257mm of annual rainfall. This value is approximately 10% more than the 10 year average rainfall, in what is already a very high rainfall area. Initial one month period is low rainfall weeks in generally and in the year 2008, rainfall during initial 5 week period, fell below average. Also week 3 and 4 were recorded as completely dry weeks, with no rainfall. High annual rainfall is mainly caused by very high rainfall received during April (week 17), May/June (week 22) and July (week 29) weeks exceeding 300mm. Nevertheless the rainfall pattern still followed the normal bimodal rainfall pattern of the two monsoons (Figure 6.1).
In 2009, the initial months were dry compared to an average year, till the rain start in early March 2009. Monthly rainfall for January and February 2009 was 21.2 and 23.2mm respectively. Irrigation was applied from January to first week of March, 2009. During this period, there were several weeks where maximum temperature exceeded 350C.
124 250 SW monsoon NE monsoon
200 rain average
150 irrigation irrigation
100 Rain (mm/wk)
50
0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr
Month (Jan 2008 - Mar 2009) Figure 6.1 Weekly rain during January, 2008 to March, 2009. 10 year average weekly rainfall is given in dotted line (SW – South West, NE – North East). Error bars =standard error
During the physiological measurement period of January – February, 2008, there was a rainless period from 16 to 28 January (Figure 6.2). Again major rain events started in 4th February and continued the rain for rest of the month. Potential evapotranspiration varied from 0.85 to 4.1mm/day. Rainy days showed a less water demand, average being 3(±0.16)mm/day. The average maximum temperature during the period was 34(±0.3)0C. Average solar radiation during the period was 977(±30)W/m-2. However, there are some days which recorded high solar radiation of more than 1300 W/m-2.
125 40 4.5 Rain
ET0 4.0
30 3.5
3.0
20 2.5 (mm/day) 0 ET
Rain (mm/day) 2.0
10 1.5
1.0
0 0.5 7/01/08 14/01/08 21/01/08 28/01/08 4/02/08 11/02/08
Date
Figure 6.2 Daily rainfall and potential evapotranspiration (ET0) during Janury 12 – February12, 2008. This period was used for evaluation in dry weather physiology
6.3.2 Soil moisture
Monthly volumetric soil moisture variation of the 20, 40 and 60cm depth is displayed on Figure 6.3. Field capacity and volumetric moisture content of the soil is 27% and 14% for the soil in this field. The point at which the tea plant begin to experience water stress was considered to be 50% of available soil moisture (Anandacumaraswamy 2008). Hence, below 20% moisture content can be considered as a water stress level for tea plant. During January and February 2008 moisture content of the sprinkler plots was 2-5% higher than drip plots. Moisture content at the 60cm layer depth was similar or sometimes lower than rain-fed plots. It is only in May 2008 where soil moisture levels became equal in all three treatments. Moisture content was higher in sprinkler plots even during some rainy months (e.g. April and August 2008). However, during the rainy months, where irrigation was not practiced, moisture content in all plots remained above 50% of the available moisture content.
126 Jan 08 Feb 08 Mar 08 Volumetric moisture (%) Volumetric moisture (%) Volumetric moisture (%) 15 20 25 30 15 20 25 30 15 20 25 30 -10
-20
-30
-40
Depth (cm) Depth -50
-60 rainfed drip -70 sprinkler PWP 50% FC PWP 50% FC PWP 50% FC Apr 08 May 08 Jun 08 -10
-20
-30
-40
Depth (cm) Depth -50
-60
-70
Jul 08 Aug 08 Sep08 -10
-20
-30
-40
Depth (cm) Depth -50
-60
-70
Oct 08 Nov 08 Dec 08 -10
-20
-30
-40
Depth (cm) Depth -50
-60
-70
Jan 09 Feb 09 Mar 09 -10
-20
-30
-40
Depth (cm) Depth -50
-60
-70 PWP 50% FC PWP 50% FC PWP 50% FC Figure 6.3 Average monthly volumetric soil moisture content up to 70cm depth. Continuous line shows the rain- fed plots, dotted line shows drip irrigated plots and sprinkler plots are shown by dash line. Vertical bar represent permanent wilting point (PWP), 50% of available moisture and field capacity (UNFCCC)
6.3.3 Photosynthesis (Pn)
Pn of the mature, top-most leaves, measured during January and February 2008, are shown in
Figure 6.4. During the measurement period, mid-day Pn of sprinkler irrigated plots showed 127 the highest activity and lowest by the rain-fed plants. Low solar radiation of 809 and 948W/m-2 caused a reduction in all three treatments on 28th and 30th January. This type of low light conditions is not common usually in the inter-monsoonal dry spell of January-March.
The average Pn rates for the rain-fed, drip irrigated and sprinkler irrigated plants were 7.7(±0.8), 11.1(±0.7) and 12.8(±0/9) µmolm-2s-1 respectively.
16 ) -1 s 14 -2 m 2
12 mol CO mol 10
8
6 rainfed
Photosynthesis rate ( rate Photosynthesis drip sprinkler 4 22/01/08 25/01/08 28/01/08 31/01/08 3/02/08 6/02/08 Date
Figure 6.4 Photosynthetic rate of topmost mature tea leaves in 3 irrigation treatments during mid-day in January - February 2008
In summary Drip irrigated plants had a 46% higher Pn rate than rain-fed plants. Sprinkler irrigated plants Pn rate was 15% higher than drip irrigated plants.
6.3.4 Stomatal conductance (gs)
Stomatal conductance of the rain-fed plants was the lowest values for study period (Figure 6.5). Initially the difference among irrigated and rain-fed plants was lower. The difference widened with the progress of the drought, perhaps related to depletion of soil moisture as -2 -1 well. Drip irrigated plants showed slightly higher gs value of 0.18(±0.01)mol H2O m s as -2 -1 compared to 0.17(±0.01) mol H2O m s of sprinkler. Rain-fed plants showed a significantly -2 -1 lower value (0.14(±0.008) mol H2O m s ), than other two treatments.
128 0.20 ) -1 s
-2 0.15
0.10
0.05 control drip Stomatala conductance (molm conductance Stomatala sprinkler 0.00 22/1/2008 25/1/2008 28/1/2008 31/1/2008 3/2/2008 6/2/2008 Date Figure 6.5 Stomatal conductance of mature tea leaves during mid-day for three irrigation treatments
In summary rain-fed plants showed the lowest gs and for irrigated plants it was 26-30% higher than rain-fed plants.
6.3.5 Transpiration (El)
On average, both drip irrigated plants showed a higher El than other two treatments (Figure rd rd 6.6), except for the 23 day. On the 23 day, highest El was shown by control or rain-fed plants, but without significant difference among three treatments. On the 28th day, drip -2 -1 irrigated plants showed a significantly higher El value of 5.1(±0.3)mmol H2O m s . On the th 28 day, sprinkler and rain-fed plot showed a lower El than drip irrigated plants, which was a cloudy day. Transpiration increased for both irrigated treatments towards the last day of observation, while the rain-fed plants remained at the same level. Lower soil moisture condition in the rain-fed plots could be the reason for lower transpiration rate.
6.0 ) 1 - s 2 - 5.5 O m 2 5.0
4.5
4.0 control 3.5 drip sprinkler Transpiration rate (mmol H (mmol rate Transpiration 3.0 22/01/08 25/01/08 28/01/08 31/01/08 3/02/08 6/02/08 Date Figure 6.6 Instantaneous leaf transpiration of top most mature tea leaves during mid-day
129
In summary drip irrigated plants had a 20% higher transpiration rate than sprinkler irrigated plants.
6.3.6 Diurnal variation of leaf physiology
Diurnal variation of Pn, gs, El and Tleaf was measured on 30 January 2008, from 0800 to 1700 hours as shown in Figure 6.7. Also the air temperature (Tair) and the incidence of photosynthetically active radiation (PAR) were recorded at 15 minute interval. This day was comparatively cooler (maximum temperature <350C) and gloomy (PAR <900Wm-2) day. 0 Nevertheless, within day time, Tair remained above 25 C, observed lowest being at 0800hrs and increasing gradually at an average rate of 10C/hr. It rose to 340C by 1400 and then increased to 350C, in between 1400 and 1500 hrs. Highest PAR level was also recorded at 1400hrs and it declined steeply after.
130 36 800 air temperature (a) solar radiation 34 ) -2
C) 600 0 32
30 400
28 200 Air temperature ( Air temperature 26 radiation (Wm Solar
24 0 )
-1 14 s (b) -2
m 12 2
10 mol CO mol
8
6
4 rainfed drip 2 sprinkler
0 Photosynthetic rate ( rate Photosynthetic )
-1 0.4 s (c) -2
O m 0.3 2
0.2
0.1
0.0 Stomatal conductance (mol H (mol conductance Stomatal 6 (d) ) -1 s 5 -2 O m
2 4
3
2
1 Transpiration (mol H Transpiration (mol 0 600 800 1000 1200 1400 1600 1800 Time (hours)
Figure 6.7 Diurnal variation of: (a) air temperature and solar radiation; (b) photosynthesis rate; (c) stomatal conductance; and (d) transpiration of tea leaves during day time 30 January 2008
-2 -1 Rain-fed plants showed the highest Pn rate of of 8.1 (±0.6) µmol CO2 m s at 0800hrs, which is 30% higher than both irrigated treatments. At 1000hrs, both rain-fed and sprinkler irrigated -2 -1 plants had a Pn rate of over 12µmol CO2 m s , At the time Pn of drip plants only rose 9.8µmolm-2s-1 and remained >9.0 for up to 1400hrs. Rain-fed plants then showed a rapid decline in Pn rate. Sprinkler irrigated plants too showed a decline after 1000 hours, but at a -2 -1 gradual pace. By 1600 hrs, Pn was between 4.5 -5.0µmolm s for the all treatments and it showed no significant difference among treatments, and reached towards 1.0-1.5µmolm-2s-1
131 by 1700hrs. The steep reduction in the PAR could be the reason for the decline of Pn in the rain-fed plants as well.
Morning 0800hours was the time highest gs shown by rain-fed and sprinkler irrigated plants. -2 -1 Drip irrigated plants had a 17% lower gs rate of 0.25mol H2O m s . Stomatal conductance of rain-fed plants decrease rapidly than sprinkler and drip irrigated plants. By 1600hrs, all three treatments showed similar gs values
Variation of El was more symmetrical throughout the day than gs. Highest El was observed -2 -1 from drip plants throughout the day. It had a peak value of 4.7mmol H2O m s , which is more than 15% higher than rain-fed and sprinkler plants. Nevertheless, all three showed the highest El at 1200 hours. Rain-fed plants showed a steep decline in El after 1200 hours than -2 -1 other two treatments. By 1500hrs, El was below 1mmol H2O m s for all three treatments, without significant difference among each other.
In summary Pn of the sprinkler irrigated plants were 10-25% higher than drip irrigated plants, while El drip irrigated plants were 16-45% higher than sprinkler irrigated plants in mid-day.
Nevertheless gs of irrigated treatments remained almost equal. .
45
40 C) 0 35
30 Leaf temperature ( temperature Leaf rainfed 25 drip sprinkler air temperature
20 600 800 1000 1200 1400 1600 1800
Time (hours)
Figure 6.8 Diurnal variation of leaf temperature and air temperature in Experiment 4 (30 January 2008)
On this day, drip irrigated plants showed almost similar temperature levels as ran-fed plants apart from morning 1000hrs (Figure 6.8). But sprinkler irrigated plants showed a 2-40C lower leaf temperature level than the other two treatments between 1000-1400 hours. However,
132 form 0800hrs to 1400 hrs, sprinkler plants too showed a higher leaf temperature than air temperature. At 1200hrs, difference was as high as 70C. During that time, difference between rain-fed/drip irrigated plants and air temperature was more than 100C. Even though, air temperature rose to 340C by 1400hrs, leaf temperature fell in all three treatments.
0 In summary, sprinkler irrigated plants Tleaf was 2-4 C lower than drip treatment in mid-day. 0 In 1200-1400 hours, Tleaf of drip and rain-fed plants were 6-10 C higher than ambient temperature.
6.3.7 Irrigation effect on shoot weight
Size of the harvestable tea shoots during dry period of January 2008 is given in Figure 6.9. Shoot sizes were selected as one leaf + bud, two leaves + bud and three leaves + bud. Shoot weight increased with the size of the shoot. On average, the largest portion of harvested tea consists of two leaves and a bud category. Such shoots showed no distinct difference in shoot weight among the three treatments. The single leaf bud category showed a difference among three treatments, sprinkler showing highest weight. Highest shoot weight of 0.47(±0.1)g/shoot, was recorded by sprinkler treatment, which is 87% higher than rain-fed single leaf+bud shoot weight. In the sprinkler treatment, shoots with a leaf + bud and 2 leaves + bud showed similar weight. But there was a high variation in the weight of leaf + bud shoot. The largest shoot weight was observed from 3 leaves+bud category in all three treatments. However, both irrigated treatments showed a significantly higher shoot weight in this category. Drip irrigated plants produced an average shoot weight of 0.85(±0.06)g/shoot.
In summary, Both single leaf+bud and 3 leaves+bud shoots showed increased weight with drip irrigation. There was a significant difference in the 2 leaves+bud category, with drip irrigated plants showing 18% increase in shoot weight than rain-fed plants.
133 1.0 Control Drip 0.8 Sprinkler
0.6
0.4 Shoot weight(g)
0.2
0.0 leaf+bud 2lvs+bud 3lvs+bud Shoot size
Figure 6.9 Fresh weight of different shoots according to three treatments during the dry period of January 2008
6.3.8 Shoot extension rate and shoot count
TRI 2023 cultivar shoot extension rate in January/February 2008 under 3 different irrigation treatments are shown in Figure 6.10. Sprinkler irrigation showed a higher shoot extension rate of 4.35(±0.5)mm/day than drip irrigation, showing 31% increase among irrigation treatments. Though statistically not significant, drip irrigation too had 11% average shoot growth than rain-fed cultivation.
Figure 5.5.10 illustrates the variation of harvestable total shoot count per plant as measured in January, and February, 2008 (). In January, both sprinkler irrigated plots and rain-fed plots produced similar shoot counts of 58.0(±5.2) and 57.3(±8.9) shoots per plant. Even though there was no significant increase in the number of shoots in the drip irrigation treatment in February, in the sprinkler irrigation plots it rose to 78.2(±7.0) shoots per plant, 57% higher than rain-fed plants. Meanwhile for the rain-fed plots, harvestable shoot count reduced by 13% to 50(±7) shoots per plant.
134 6 (a)
5
4
3
2 Shoot extension (mm/day)
1
0 rain fed drip sprinkler 100 (b) Control Drip 80 Sprinkler
60
40 Harvestable shoots (no/plant) 20
0 January February Month
Figure 6.10 Yield factors in January-February, 2008. Average shoot extension rate according to irrigation treatment (a) and harvestable shoot count per plant (b)
In summary, sprinkler irrigation increased the shoot extension rate by 31% than drip irrigation. Effect of sprinkler irrigation was more significant in increasing harvestable shoot count by 57% in February than control plants.
6.3.9 Tea yield
The average weekly yield of TRI 2023 cultivar from January 2008 to March 2009 is shown in Figure 6.11. Unlike an average year, in 2008 average weekly tea yield has shown a less variation since March, probably due to high rainfall received throughout 2008. In January 2008 sprinkler plots gave a significantly higher yield than both drip and control plots. Sprinkler treatment produced made tea equivalent of 176(±16) kg/ha/wk as against the
135 140(±16) and 136(±14) respectively from drip and sprinkler plots. But in January, 2009 difference between drip irrigation and control treatments were much less.
In February 2008, yield of all treatments reduced compared to previous month, though there was a high rain from week 5 onwards. Sprinkler irrigated plots produced made tea at 100(±14)kg/ha/wk. For drip and control plots, yield results were 72(±8) and 53(±6)kg/ha/wk respectively. In the month of February 2008, there was a significant difference among three treatments. Since March 2008, tea yield of all three treatments were increased. Also the difference among three treatments became less significant, with a slight higher yield from irrigated treatments. Again in July 2008, sprinkler irrigation plots produced a significantly higher yield of 127(±10)kg/ha/wk. In this month, rain-fed treatment produced marginally higher yield increase of (6%) than drip irrigation treatment. In the months of September, October and December, 2008 also sprinkler treatment gave significantly higher yield than other two.
220 rainfed 200 drip sprinkler 180
160
140
120
100
Made tea (kg/ha/wk) 80
60
40 irrigation irrigation 20 Jul 08 Jan 09 Jan Jan 08 Jan Jun 08 Oct 08 Feb 09 Feb 08 Apr 08 Sep 08 Dec 08 Mar 09 Mar Mar 08 Mar Aug 08 08 Nov May 08 May Month
Figure 6.11 Average weekly made tea during each month according to irrigation treatment. Continuous line shows the average rainfall received in each week.
With the onset of the usual dry season in 2009, there was a drastic reduction in the productivity of all three treatments. During the January 2009, there was a less significant difference between sprinkler and drip treatment, but in February 2009, there was a significant difference among all three treatments. Sprinkler irrigated plants produced 20% and 57%
136 higher yield than drip and rain-fed plots. In March 2009, with the onset of rains, a yield increase of 24% was observed only from rain-fed plants.
Table 6.2 Total made tea production according to irrigation treatment from January 2008 to March 2009
Treatment Yield (made tea kg/ha)
Rain-fed 5119(±174)
Drip 5341(±226)
Sprinkler 5688(±336)
LSD (5%) 865.6
Table 6.2 shows the total made tea yield in kilogram per hectare obtained from each irrigation treatment during January 2008 to March 2009 period (total of 15 months). Though treatments are significantly not different from each other in yield terms, higher yields were obtained by both drip and sprinkler irrigated treatments. As compared to rain-fed cultivation, sprinkler and drip irrigation result in 11% and 4% increase respectively. Large plot variation in yield was observed in both sprinkler and drip irrigated fields as compared to rain-fed treatment.
In summary, throughout the study period sprinkler irrigation provided the highest yield in dry and wet season of the year. The final yield increase by sprinkler irrigation is 6 and 11% higher than drip and rain-fed plants.
6.3.10 Water use productivity (WUP)
Irrigation water use productivity was calculated as the made tea yield per 1mm water applied as irrigation or received through dry season for all three treatments (Table 6.3). In 2008 only tea yield of January and February months was considered for calculation and for 2009, January to March monthly yield was considered. In the wet year of 2008, all three treatments showed a nearly 100% higher productivity due to a short dry spell. But in both years, sprinkler treatments water use productivity was 7-9% higher than drip irrigation, being highest in the dry year of 2009. Even though water currently is considered to be freely available in Sri Lankan conditions, this information would be helpful in the event if there is any price allocated for water.
In summary, WUP of rain-fed plants were 57% higher than sprinkler irrigated plants in 2008, but it decreased to 38% in 2009, with longer drought.
137 Table 6.3 Water use productivity (made tea kg per water(mm) applied or received as rain)as a reflection on the yield during the irrigated months of 2008 (Jan-Feb) and 2009 (Jan-Mar)
Year 2008 2009
Rain-fed 3.51 1.74
Drip 2.09 1.16
Sprinkler 2.23 1.26
6.3.11 Irrigation water use efficiency (IWUE)
Irrigation water use efficiency of the drip and sprinkler irrigated plots were calculated as the ratio between increased yield due to irrigation and the amount of water applied as irrigation.
ሺܻ െܻሻ ܫܹܷܧ ൌ ݅ ݎ ܫܴܴ݅
Where, Yi is the irrigated made tea yield; Yr is the rain-fed made tea yield during dry season. IRRi is the amount of water applied as either drip or sprinkler irrigation. Table 6. 4 shows the IWUE of drip and sprinkler irrigated plants for 2008 and 2009 dry season. IWUE was higher in both years for the sprinkler irrigation. But when the extended dry period was active in 2009, IWUE dropped by 16%. However, for drip irrigation it increased by 12%.
Table 6. 4 Irrigation water use efficiency (as made tea kg/ha per water (mm) applied as irrigation) during irrigated months 2008(Jan-Feb) and 2009(Jan-Mar)
Year 2008 2009
Drip 0.43 0.48
Sprinkler 0.91 0.76
In summary, IWUE was 112% higher for sprinkler irrigated plants in 2008 and only 58% in 2009, showing a decrease with increase of drought duration.
6.3.12 Total plant growth
The total growth of the main stem sections and roots are given in Figure 6.12. Sprinkler irrigation canopy growth was 2880(±269)g/plant, which is 43% higher than the drip irrigated plants. Drip irrigated plants only had a 6% increase in canopy growth compared with rain-fed plants. But the variation among drip irrigated plant was much smaller than other two treatments. In the root development, highest root growth per plant was observed from sprinkler irrigated plants, with an average value of 708(±96)g/plant. This is respectively 17 and 34% higher than root development observed in drip and rain-fed plants.
138 In summary, both root and canopy growth was significantly higher in sprinkler irrigated plots.
3500 (a)
3000
2500
2000
1500
Stem weight (g/plant) weight Stem 1000
500
0 2500 (b)
2000
1500
1000 Root (g/plant) weight
500
0 rainfed drip sprinkler Irrigation Treatment
Figure 6.12 Dry mass weight of plants (a) stem weight, (b) root weight, after 9 years growth in the field according to irrigation treatment. (Error bar shows the standard error)
6.3.14 Root Density
Root depth distributions according to soil depth are given in Figure 6.13. Fine root density in top 20cm soil layer was 24% higher in rain-fed plants than sprinkler plants. In the 40cm soil depth also lowest fine root density was observed from drip irrigated plants. But in the bottom 60cm depth there was an increase in root density (65% of drip and 47% of sprinkler) of irrigated plants. As similar to fine root density, coarse root (>2mm) density was highest in top 20cm layer of soil for all treatments, without significant treatment differences. But in bottom 40cm layer, irrigated treatments had a significantly higher root density than rain-fed
139 plants and similar observation was observed in 40cm layer as well. Unlike with fine roots, there was no increase in density in 60cm layer soil, as against 40cm layer.
Density (kg/m-3)
012345 (a)
-20
-40 Depth (cm) Depth
rainfed -60 drip sprinkler
0 1020304050 (b)
-20
-40 Depth (cm) Depth
-60
Figure 6.13 Root density of rain-fed, drip and sprinkler irrigated plants according to soil depth up to 60cm depth. (a) fine (<2mm) roots (b) coarse (>2mm) roots, (Note weight scales are not equal in both graphs) (Error bar represent the standard error)
In summary, fine root density is 24% higher in rain-fed plants than sprinkler irrigated plants only in top 20cm soil layer. In the structural root development, irrigated treatments, outperformed the rain-fed plants in all depths.
6.4 Discussion
Irrigation of mature tea plants by drip and sprinkler irrigation was evaluated in this chapter. Cultivar TRI 2023, a drought susceptible, but highly responsive to drip irrigation, (Chapter 5.0), was used. The main findings which are discussed in details in this section are:
140 (1) physiological activities of the tea plant respond more favourably to sprinkler than drip irrigation;
(2) similar effects were found in the yield and plant growth as well;
(3) however, drip irrigation too had advantages over rain-fed cultivation in the productivity;
(4) moisture variation in the treatment plots over the assessment period had some important revelations about monthly moisture movement which would be important for identifying correct moisture management practices.
Sprinkler irrigation was the most efficient in stimulating Pn during drought. During the drought Pn rate of the sprinkler irrigated plants were 15% higher than drip irrigation. Not only higher Pn rate, but also sprinkler irrigated plants were able to maintain higher Pn rate during most of the day time (Figure 6.7). The increased Pn rate of sprinkler treatment over drip can be caused by either low soil moisture in the drip treatments (Figure 6.3) or increased leaf temperature (Figure 6.8). But in this instance main the reason can be considered as the increased leaf temperature. In this instance, gs was not significantly different among sprinkler and drip treatments (Figure 6.5). Stomatal conductance has a close association with photosynthesis with tea under irrigation (Smith, Stephens et al. 1993). In contrast leaf temperature was 2-40C, higher in drip irrigated plants during day time. However in other crops, mainly with C-4 plants, similar Pn rates were obtained with drip and sprinkler irrigation (Antony and Singandhupe 2004).
Even though stomatal conductance was similar for drip and sprinkler irrigated plants, the transpiration rate of the drip irrigated tea leaves were 20% higher than sprinkler irrigated plants (Figure 6.6). More transpiration sustained the lower leaf temperatures and for increased photosynthesis than rain-fed plants (Remorini and Massai 2003). However, lowering leaf temperature was not achieved in drip irrigation plants as shown in Figure 6.8, where leaf temperature is almost similar to rain-fed plants during day time. Perhaps less water availability in drip irrigated plots than sprinkler irrigated plots could be one reason for lower transpiration rate. One notable feature in the diurnal leaf temperature pattern of this experiment is the leaf temperature levels were 8-120C higher in all treatments than ambient temperature, unlike in the drip irrigation experiment described in Chapter 5.0. Lower ambient temperature during the observation period could be assumed as the reason for this.
Sprinkler irrigated plants had a 43% higher canopy growth than drip irrigated plant. In a densely populated crop of tea, less canopy structure means leaves and could get exposed to more sunlight thereby increasing the leaf temperature (Annandale and Stockle 1994) as well
141 the soil evaporation (Lane, Morris et al. 2004). Increased canopy and root growth in sprinkler irrigation is more related to better wetting pattern of the sprinkler irrigation than drip irrigation, mainly during young stages of the growth (Sivanappan 1987).
Shoot development and the shoot count in the plant were affected with irrigation method. Both newly developed (single leaf+bud) and the oldest (3 leaves+bud) shoot weight was enhanced with irrigation. Low rain and soil moisture in December and January has resulted in loss of shoot weight. The fresh mass of individual shoot linearly increases with the shoot number in shoot (Carr 2010). But under sprinkler irrigated plants, the weight difference between single leaf+bud and 2 leaves+bud was only 15%. In contrast for the rain-fed plants, the difference was 108%. Higher Pn rate in sprinkler irrigated plants has caused more shoot development.
6.4.1 Relationship between physiology and irrigation method
The measurement period for physiological activities was not a prolonged drought period because of the commencement of unexpected rains. Yet the period showed clear drought stress characters on the tea plant with a few rainless weeks and with the high ambient temperature. During the measured two week period, there are days where evapotranspiration varied significantly. Physiological performance of the plants too followed as a response to environmental factors. The high Pn rate observed in the 23 January in all treatments fell when measured in 28 January. Continuous rainless days caused soil moisture depletion and temperature stress can be considered as the causes for reduction. There were rainfall events on 4th and 5th February at a magnitude of 9 and 10mm respectively. Since the rainfall event of 10mm occurred after the measurement in mid-day and it cannot be considered as having effect on moisture replenishment of the field. But due to the previous day rainfall and reduction in atmospheric stress, Pn rate increased in all treatments. Sprinkler irrigation was able to maintain a higher Pn rate than drip irrigation, due to the fact that it maintains a lower leaf temperature level, optimal for Pn. As a result, as shown in Figure 6.14, Sprinkler irrigated plants were able to maintain a higher productivity through maintaining a higher Pn rate during hot humid-days.
Water use of the plant as shown by instantaneous leaf transpiration rate showed a higher water use for drip than sprinkler for three days out of four measurement days. The reason for higher water use by the drip plant can be due to more water use for maintaining a favourable leaf temperature level during the day. For the sprinkler irrigated plants, plant leaves are cooled by irrigation itself. The sudden drop in the leaf temperature level in the 30th January can be considered as a reduction caused by sudden cloudiness over the measuring period, rather than a drought related effect. 142 )
-1 16 s rain fed -2 drip
m 14
2 sprinkler
12 mol CO mol 10
8
6
4 Photosynthetic rate ( rate Photosynthetic 32 33 34 35 36 1.6 2.0 2.4 2.8 3.2 3.6 Maximum temperature (0C) Potential evapotranspiration (mm/day)
Figure 6.14 Relationship between photosynthesis rate and maximum temperature and Potential evapotranspiration for period 23-30 January, 2008
In considering the productivity in utilizing the water for irrigation, it can be noted that there is a higher advantage of using sprinkler for irrigation specially during short dry periods, like in 2008. But with the severity of drought increases, drip irrigation increases the IWUE while decreasing for sprinkler treatment.
6.4.2 Yield response to environmental variables during dry season 2009
The relationship between treatment yield and environmental variables of maximum temperature, solar radiation, vapour pressure deficit and potential evapotranspiration was analysed for the period January – March 2009 (Figure 6.15). This period was a typical drought period and there were significant yield reductions in all three treatments. In this analysis, it was evaluated whether environmental parameters can be used to explain the yield reduction.
With all environmental parameters tested, sprinkler treatment did not show any significant relationship with any of the parameters. Drip irrigation showed a negative relationship with maximum temperature (r2=0.2, P=0.1) and to a lesser extent with vapor pressure deficit (r2=0.11, P=0.2). Both increasing temperature (r2=0.42, P=0.03) and vapor pressure deficit (r2=0.36, P=0.05) had strong negative relationship with rain-fed yield.
143 80
sprinkler 70
60 drip 2 2 50 r =0.42, p=0.03 r =0.17, p=0.2
40 control drip sprinkler control 30 33 34 35 36 37 3.2 3.4 3.6 3.8 4.0 4.2 4.4 Maximum temperature (0C) Solar radiation (MJm-2s-1)
80
70 Made tea (kg/ha/wk) tea Made
60 r2=0.36, p=0.05 2 50 r =0.17, p=0.2
40
30 0.8 0.9 1.0 1.1 1.2 1.3 2.6 2.8 3.0 3.2 3.4 3.6 3.8 Vapor pressure deficit (kPa) Potential evapotranspiration (mm/day)
Figure 6.15 Relationship between treatment yield and climate variables during January - March, 2009 (Only relationship between control yield and environmental parameters are given in equation)
Sprinkler irrigation produced highest yields during the drought as well during wet period of the year. However, during the wet period of the year, there were some months where rain-fed treatment produced higher yields than drip treatment. The hypothesis that sprinkler irrigation technology produce higher yield than drip irrigation can be supported by the findings presented in this experiment. Even though with a small drought period in 2008, there was a marked difference among key physiological parameters, like photosynthesis, among irrigation treatments. Sprinkler irrigation produced enhanced performance over drip irrigation.
6.4.3 Yield response to irrigation
The results here indicate that irrigation application can be used in either as sprinkler or drip system to maximize the tea production in low elevation tea growing area. Yield increases can be expected in even very wet years like 2008, where total rainfall is 20% higher than average annual rainfall. Even though irrigation was applied only during first two months of the year, yield increase was obvious even throughout the wet season as well. The reason for higher productivity of the irrigated plants can be attributed to better overall growth of the plant including root system. In irrigated plants root, system was more developed in the lower soil depths as well, specially the structural (>2mm) roots (Figure 6.13). Soil moisture level in top 144 soil layers, can be depleted during even a short dry spell as shown in April and December 2008 (Figure 6.3). But the deep soil layers may contain higher moisture content for the plant to use. Better growth in plants, perhaps devoid of drought related diseases as well, may cause the irrigated plants to perform better during the optimum climate occur for the high growth. In this regards, however, it can be considered that irrigating the plant from field planting itself, is the best way to achieve higher productivity in later years, than irrigation for the mature plants established without irrigation.
6.4.4 Soil moisture variation and plant growth
The variation of the soil moisture up to 60cm depth of the soil was illustrated in the Figure 6.3. There was a lower soil moisture concentration on the drip irrigated plots during irrigated months, even though both treatments are applied with same amount of water. For the low moisture content in the drip irrigation plots can be described as follows. Irrigation water is applied at a lower application rate (2.2mm/hr) than compared to sprinkler irrigation (4.4mm/hr). Application rate of irrigation has an impact on the infiltration of irrigation water to the soil (Clothier and Green 1994). At this rate, much of the applied water may leave the soil as soil evaporation in hot dry conditions. Soil evaporation is a critical factor in losing soil moisture during dry days. In a parallel experiment measuring soil evaporation in a tea field, it was found that average soil evaporation in January–February, 2009 period was 1.2(±0.1) mm/day in a rainless day. Usually drip irrigation was applied mostly during day time, as it needed more application hours than sprinkler irrigation. As a result, the moisture penetration to lower soil depth was limited under drip irrigation.
Sprinkler irrigation according to this experiment can be interpreted as beneficial for the maintaining higher physiological activities and yield during drought conditions in low elevation tea growing area. But attention should be given to energy consumption and total water requirement during the drought for this intensely grown crop.
6.5 Summary of results
Sprinkler irrigation has a higher ability to maintain 2-40C lower leaf temperature level than drip irrigation.
Pn was 15% higher in sprinkler irrigated plants than drip irrigated plants, gs remained
same in both treatments, though El 20% higher in drip irrigated plants than sprinkler.
Sprinkler irrigation yielded 6% higher than drip irrigation and 11% higher than rain- fed plants.
145 During wet season as well irrigated plants has shown 15-27% increase in yield in some months. However yield increase was consisted only with sprinkler treatment.
Yield drop in 2009 dry months were 53% for rain-fed plants and 43 and 44% for drip and sprinkler irrigated plants compared to wet months. Shoot extension rate was 11 and 45% higher in drip and sprinkler irrigation, than rain-fed plants. However, increase of 57% in harvestable shoot count only observed in sprinkler irrigated plots.
6.6 Conclusion
Experiment 5 was conducted with the aim of describing the effect of irrigation method on key physiological parameters, yield and plant growth and assessing the ability of different irrigation methods to alleviate the environmental limitations. It can be concluded that sprinkler irrigation resulted in higher yields than under drip irrigation, due to enhanced physiological activities. The reason for higher physiological activity is the lower leaf temperature under sprinkler irrigation during day time. Since these results were based on an experiment conducted in a very wet year, it is suggested that longer term evaluation under a range of annual rainfalls be undertaken.
146 Chapter 7
Effect of Short Term Water Stress and Raised Beds on Young Tea Plants
7.1 Introduction:
The importance of alleviating the moisture stress on tea by means of irrigation was discussed in earlier chapters. It was shown that irrigation maintains high growth rate, increased physiological activity during drought periods which result in enhanced annual yield (Chapter 5.0 and 6.0). All the experiments were conducted with mature tea, which were grown as irrigated plants from the field planting itself.
Other preliminary trials at the St Joachim Estate, Ratnapura, which are not reported as part of this thesis, applied drip irrigation to an established mature tea crop (>6 years old) during dry months of January to March. Under this situation it was not possible to deliver more than 20- 30% yield increase with irrigation (TRISL 2001). To get the maximum benefit from irrigation it is proposed that it must occur from the point of field planting. There could be two reasons for the poor response to irrigation by mature tea plants: plant characteristics and site characteristics. Repeated exposure to water stress during early growth stages can weaken the bush formation resulting in infection by various pathogens such as Phomopsis thea, which cause stem canker (Shanmuganathan and Rodrigo 1967; Carr 2010). Investigation already showed that rain-fed tea plants had a higher chance of infection with stem canker than irrigated plants in low elevation tea growing area (Liyanage and Bandara 2008). Drought stress will also lead to a poorly developed canopy which consist of relatively fewer shoot buds or plucking points (Wijeratne 1994).
Plant establishment in the field is done in accordance with the availability of monsoonal rains (Wadasinghe and Peiris 1987). Yet even within the monsoonal heavy rain periods, shorter duration droughts causes moisture stress to plant, as enhanced by low moisture retention in the soil (Ananda and Herath 2001). As described in the monthly soil moisture variation during January, 2008 – March, 2009 in Chapter 6.0, soil moisture storage does not rise immediately after the rainfall. So there is a chance for the delicate young plants being subjected to the moisture stress at any time of the year, in addition to the expected intense dry season of January – March each year.
Site characteristics may also explain the relatively poor performance to irrigation when it is applied to an established mature tea crop. The soil bulk density of low-grown tea areas are higher than that of high and mid elevation tea (Mapa, Somasiri et al. 1999). The natural higher bulk density accompanied with higher rainfall, floating stones, erosion of topsoil and
147 regular traffic can make low-grown tea area soils highly compacted and poorly aerated. Under such conditions in the tropics hard pans readily build up, reducing the availability of connected macro pores (Shougrakpam, Sarkar et al. 2010) and impeding infiltration of irrigation water to root zone (Kim, Chon et al. 2004). Conventional practice is to establish tea on levelled ground without any deep ripping of the soil profile (see Chapter 2). Raised beds have shown to significantly increase infiltration and yield in many and varied crops from wheat and maize (Verhulst, Kienle et al. 2011) to cactus pear (Labib 1998; Verhulst, Kienle et al. 2011). However, raised bed cultivation has not been evaluated for tea.
Three experiments are discussed in this chapter with the aims to:
1. quantify the effect of short term water stress on young tea growth,
2. assess the effect on partial irrigation on young tea growth, and
3. evaluate the interaction between raised beds and irrigation.
These aims are to achieve the objectives of:
a) understanding the limitations to young tea growth especially during the wet season, and
b) developing better land preparation techniques to achieve higher returns to irrigation.
This chapter describes two naturally lit, glasshouse experiments, one field experiment conducted at St. Joachim Estate, Ratnapura, and a desk-top analysis of historical rainfall data. Glasshouse experiments were conducted during 2007, using cultivar TRI 4042, while the field experiment was conducted during May 2008 to March 2009 period.
Experiment 5 is a glasshouse experiment evaluating the growth response of young tea plants subjected to range of irrigation intervals, from daily irrigation to bi-weekly irrigation.
Experiment 6 is a glasshouse experiment examining the effect of partial irrigation for 20 days on young plant growth.
Experiment 7 is a field experiment evaluating the irrigation effect on young plant growth when was exposed to short duration drought and examined the interaction between irrigation and raised-bed cultivation.
Ideally, Experiments 5 and 6 should have been conducted in the field. However, isolated rains can disturb drought treatments, thereby necessitating the experiments to be conducted in a glasshouse.
148 The desk-top analysis of historical rainfall data (section 7.5) serves to establish the frequency and duration of rain-free periods, and by applying a water stress coefficient, establish the fact that these periods of water stress can also occur even in the wet season.
149 7.2 Experiment 5: Effect of Water Stress Duration on Young Tea Plant Growth
7.2.1 Introduction
Short duration droughts of 5 days or more causes poor establishment and low productivity in perennial crops in wet zone of Sri Lanka including the low elevation tea growing area. (Sumanasena 2008), Such periods of short term water stress are considered to be a major impediment to the establishment of young tea in the field resulting in the industry being unable to achieve the required replanting rate of 2% (Illukpitiya, Shanmugaratnam et al. 2004). Similar drought casualties in replanting shrubs and trees in reforestation programs have been overcome through irrigation (Siles, Rey et al. 2010). While tea researchers in Sri Lanka are aware of this issue, the effect has not been quantified and thus no scientific basis on which to recommend short-term irrigation of young tea. The hypothesis for this experiment is that even very short term water stress negatively impacts on the growth and development of young tea plants.
7.2.2 Method
7.2.2.1 Plant material and water application
The experiment was conducted at a naturally lit glasshouse of Tea Research Institute, St. Joachim Estate, Ratnapura, from July 2007 to November 2007. Cultivar TRI 4042 was used for the trial. Selection of TRI 4042 was based on the readily availability of the plants and morphological characters (leaf size, leaf angle etc.,) are closer to previously tested cultivar TRI 2023. (Since the TRI 2023 is not advised for planting, young plants were not available in plant nurseries). The cultivar, TRI 4042 is also a broad leaf cultivar with flat horizontal leaf surface, producing high yield in the low elevation area and resist drought (TRISL 2002).
Table 7.1 shows the soil chemical constituents of the soil in the top 20cm layer of the soil in field where other field experiments were carried out. The same soil was used as the potting media in this experiment. The soil pH content is within the prescribed range of 4.5 -5.5 for tea (Zoysa 2008).
Ten month old tea plants of the cultivar TRI 4042 were grown in the estate nursery. They were first transferred to 12L plastic pots from nursery bags and kept inside the glasshouse for 4 weeks for acclimatisation. A black polythene layer covered the soil surface to minimize soil evaporation.
150 Table 7.1 Major soil chemical constituents of top 20cm soil layer in field no 01. Same soil is used as potting media for pot experiments
Parameter Amount
pH 4.6
Organic carbon % 1.8
Nitrogen % 0.3
Potassium(ppm) 90.0
Plants were divided into 5 blocks and 3 plants were assigned for each treatment within a block. Irrigation treatments were:
T1 - daily watering
T2 - watering at 4 day intervals
T3 - watering at 7 day intervals, and
T4 - watering at 14 day intervals.
The total amount of water applied to each treatment, during 112 days experiment period, is described in the Table 7.2. The watering regime was calculated to provide more than 3 mm of water per day for T1 plant (non water stressed). This is the average depth of water required for a field plant based on the long term weather data. Though, the actual water requirement for a potted plant inside the glasshouse could be lower, it ensured that the plants were not subjected to any water stress. To supply approximately 3mm water requirement, T1 plants were watered with 200ml/plant daily. There was no water drainage from the pots after watering with 200ml. For the other treatments, drainage was observed after watering. The amount of drained water was collected one hour after watering to calculate the actual amount of water retained in a pot. (Seepage ceased on all treatments after one hour).
151 Table 7.2 Water application rate and total amount of water applied in Experiment 5 Net water Total Application Rate Watering Seepage Treatment /application Water rate (ml) events (ml) (ml) (ml) (mm/day) T1 – daily 200 112 0 200 22400 3.4
T2 – 4 day 800 28 143 657 18410 11.2
T3 – 7 day 1400 16 426 974 15584 16.6
T4 – 14 day 2800 8 842 1958 15664 33.2
7.2.2.2 Measurements
Volumetric water content of the soil was measured using a single soil moisture probe (Delta-t MLX2 Theta probe – www.delta-t.co.uk) at the end of each treatment, prior to the next watering. One probe was used for all measurements. This is the time where all treatments were subjected to the maximum possible water stress. Base stem girth, plant height, total leaf number and number of branches were measured after the treatment period. Root weight, leaf weight and stem weight were measured after drying the samples in the oven at 850C for 24hrs. Results were analysed using SAS (Version 9.0) statistical software (SAS Institute Inc,).
7.2.3 Results
7.2.3.1 Soil moisture
Table 7.3 shows the moisture level maintained by each treatment at the end of the relevant irrigation interval. Volumetric moisture content was decreased by 10% between first 4 days. However, moisture depletion within next 3 days was 0.9%. From day 7 to day 14, (7 day) moisture depletion was only 2.2%. This suggests a reduction in transpiration with the increasing limitations of soil moisture.
7.2.3.2 Plant growth response
Plant girth at the base, showed a significance difference between treatments (Table 7.4). The pattern shows a significantly higher growth among well-watered plants. Girth of the daily watered plant is 26% higher than T4 plants. There is no significance difference among different treatments in plant height, though T1 plants showed a little higher growth. Plant height cannot be considered as a very good indicator for a bush type of plant like tea.
152 Table 7.3 Average volumetric moisture content at the end of Experiment 5 (T1-after 1 day, T2-after 4 day, T3- after 7 day, T4-after 14 day)
Treatment Volumetric moisture (%)
T1 – daily 18.7(±1.1)
T2 – 4 day interval 8.7(±0.4)
T3 – 7 day interval 7.8(±0.6)
T4 – 14 day interval 5.6(±0.4)
Both leaf number and branch number showed a significance difference among different treatments, which is important for bush formation. Leaf number of T1 plants was 86% higher than T4 plants at the time of sampling. T2 and T3 plants leaf number was almost similar. Leaf shedding was seen in the most stressed (T4) plants and it could be the reason for the high significant (P=0.002) difference in leaf number at the time of sampling. Branching of the plants was affected with the water stress of both availability and the duration. In the branch formation, both T3 and T4 plants showed no significance difference. T1 plants produced 53% higher branch number than T4 plants.
Table 7.4 Plant base girth, height, leaf number and branches at the end of Experiment 5 Height Leaf no Treatment Girth (mm) Branches (cm) (plant)
T1 - daily 7.7(±0.2)a 69.0(±2.9)a 63(±6.2)a 8.1(±0.9)(a
T2 - 4 day 7.1(±0.2)a 63.9(±2.2)a 46(±4.4)bc 6.4(±0.5)ab
T3 - 7 day 6.2(±0.3)b 64.2(±3.8)a 49(±5.1)ab 5.1(±5.1)b
T4 - 14 day 6.1(±0.2)b 65.7(±2.1)a 34(±4.4)c 5.3(±0.7)b
LSD (0.05%) 0.69 7.9 14.2 2.0
7.2.3.3 Stem and root growth
Table 7.5 shows the effect of watering frequency on stem and root growth. There is 65% increase in the stem growth when watering daily compared with the 14 day interval. The advantage of daily irrigation was more visible in the root growth, where root growth of T1 plants showing significant root growth (17±2.5g/plant) than other 3 treatments. By increasing the watering interval to 4 days from daily watering, root growth was lowered by 46%. The root: shoot ratio of 1.48 was observed in T1 plants.
153 Table 7.5 Plant stem and root weight and root:shoot ratio of Experiment 5 Stem Root Root: shoot Treatment (g/plant) (g/plant) ratio T1 - daily 25.3(±2.0)a 17.0(±2.5)a 1.5
T2 - 4 day 20.3(±1.8)b 9.1(1.0)b 2.2
T3 - 7 day 17.5(±2.1)b 7.5(±1.1)b 2.3
T4 - 14 day 15.3(±1.8)b 6.9(±0.8)b 2.2
LSD (0.05%) 5.5 3.7
7.2.4 Discussion
Experiment 4 simulated the results of subjecting the young plants to various irrigation intervals. In other words, it is like simulating the young plants in the field receiving 4 to 14 day duration of water stress. This is a common situation in low-grown tea areas. Plants were irrigated with regular intervals to provide necessary water requirement. There is a difference in total water applied among the treatments as the water seeps through the pots. Similar conditions are visible in the field itself, where sometimes heavy rain events are not effective because of runoff and deep drainage (Burman, Cuenca et al. 1983). For the mountain crops, soil moisture storage in the root zone is limited for most rain storms (Harden 2001; Pieri, Bittelli et al. 2007). The majority of the tea fields in Sri Lanka also lay in mountainous or hilly regions. So the results in this experiment can be described as reflecting the field condition itself.
The girth of the plants was affected with the water application interval. Trunk growth of woody plants indicates the moisture stress experienced during the growth sage of the plant (Kozlowski and Pallardy 2002). Once the plants subjected to moisture stress even for few days, like 7 days, the plant stem growth reduces significantly. As a tree crop, avoidance of rainless days would be important for tea. There was no significant difference between the treatments of T3 and T4, though they were irrigated on two different frequencies with the same amount of water. Daily irrigated plants showed a higher number of branches per plant. Maintaining a favourable moisture level for the plant is important to increase the branch number as well the growth of the branches (Fordham 1969). Branching is key factor in formation of proper canopy. Low leaf number in water-stressed treatments, were due to the shedding of leaves. Leaf shedding is a common situation among tea plants during sever dry spells (Fuchs 1989). One of the clear observations in this trial is the significant difference in the leaf number
154 between, T1 and T2 treatments, where number of leaves observed in T1 was 46% higher than T2. Even a 4 day interval of watering could not maintain a high leaf number.
The total stem and root growth of T1 plants exceeded other treatments significantly. Increase in the branch thickness and root growth could be the reason for higher dry matter production. The ratio between shoot and root has a significant difference among treatments. The healthiest ratio of 1.5 (Bannerjee 1993) was maintained by T1 plants.
In conclusion, the results of this experiment support the hypothesis that even very short term periods without watering, as little as 4 days, has a significant negative impact on the growth and development of young tea plants. This information provides some confidence behind the advice for growers to maintain a frequent, even daily, watering regime when establishing a new tea stand in the low elevation growing areas.
7.2.5 Summary of Results
Daily watering produced 25% higher stem growth and 87% higher root growth than the plants with 4 day irrigation interval. However, stem growth, root growth, and branch formation did not have significance difference among T3 (7 day) and T4 (14 day) irrigation treatments. The water stress caused by keeping plants without watering for 7 days, cause significant growth affect similar to a two week dry period. Most sensitive for the irrigation interval is the leaf number. Even for the short irrigation interval like 4 day, plant lost 27% of its foliage.
155 7.3 Experiment 6: Effect of Partial Irrigation on Young Tea Growth
7.3.1 Introduction
The effect of exposing young tea plants to short periods of water stress, from 4 to 14 days, over a 3 month period was analysed in the previous experiment. This represents the field situation in low elevation tea growing areas well even in the wet season where there are similar rainless periods and many days with rainfall less than 3mm. Rainfall less than 3 mm is found to be ineffective in this region as much of it is intercepted by the canopy and evaporated. Nevertheless, growers usually identify such low-rainfall days as rainy days and assume that the crop receives enough soil moisture.
Following the recommendation from Experiment 5 for frequent, even daily, watering of young tea plants may exhaust local water resources and incur an unacceptably high energy cost. So a grower may consider frequent, but partial, irrigation as an option. This practice of partial irrigation to save water has been used successfully in other tree crops, e.g. almonds, without significant economic loss (Romero, Pablo et al. 2004). Similarly, if the growth or yield is not affected significantly partial irrigation would provide an alternative for tea plantations, where water is scarce for irrigation. Specifically this would provide an alternative way to protect young tea plants from short-term water stress during early stages in the field. To evaluate the effect of partial irrigation on the growth of young tea plant another experiment was conducted at a naturally lit glasshouse irrigating at different rates based on the total plant water requirement. The objective of this was to simulate the partial rain/irrigation sometimes experienced in the area and to evaluate the effect on young tea growth. The hypothesis for the experiment was that it is possible to maintain optimum young plant growth and development with frequent but partial irrigation. By ‘partial irrigation’ is meant irrigation that does not completely replace the plant’s water use in the period immediately before irrigation.
7.3.2 Method
The experimental design was a Randomised Complete Block with 4 treatments in 4 blocks. Each treatment consists of 3 plants. Ten month old plants of cultivar TRI 4042 were used. The prior treatment and potting media was similar to the experiment 5. The experiment was conducted for 20 days from 14th June to 4th July, 2007.
Plants were watered daily according to the following treatments. Treatments were applied as:
T1 – 100% water requirement,
T2 – 75% water requirement, 156 T3 – 50% water requirement and
T4 - 25% water requirement of the plant.
To calculate the daily water requirement 3 plants of T1 treatment were weighed at the same time each day. The loss of the weight in container was assumed to be the previous day’s water loss of the plant and irrigated accordingly.
At the end of the experiment period plant growth samples were taken from all plants. Base stem girth, plant height, total leaf number and number of branches were measured prior to getting plant destructive samples. As for the growth indicators, stem and root weight of the plants were measured after oven drying the samples for 24 hours at 850C.
7.3.3 Results
7.3.3.1 Plant growth response
Plant conditions, at the end of experiment period are shown in Figure 7.1. Plant growth characters according to each treatment are shown in Table 7.6. There is no significant difference in the plant height and branch number of a plant of the plant among treatments. However, for those parameters the highest value for each was recorded from T1 plants. The highest girth of the plant of 0.6(±0.04)cm was observed from T1 plants, which is 50% higher than T4 plants. Leaf number was affected significantly with 50% deficit irrigation. T1 plants produced 83% higher leaf number than T4 plants.
Figure 7.1 Picture of potted plants after treatment period in Experiment 6(T1 to T4 from left to right). T3 and T4 plants showed wilting and defoliation, while T4 plants showed most defoliation
157 Table 7.6 Plant growth characters of Experiment 6 (as affected by 4 different watering intensities)
Treatment Height(cm) Girth(mm) Leaf no. Branch no.
T1 – 100% 45.1(±3.3) 5.8(±0.4)a 33(±3.3)a 4.4(±0.6)
T2 – 75% 41.3(±2.1) 5.6(±0.5)a 28(±2.5)ab 3.5(±0.4)
T3 – 50% 41.2(±2.8) 4.8(±0.3)ab 21(±1.7)bc 3.8(±0.6)
T4 – 25% 39.5(±2.2) 4.5(±0.2)b 18(±2.3)c 4.0(±0.3)
LSD (0.05) ns 1.1 7.1 ns
7.3.3.2 Dry matter production and partition
Both stem and root growth was affected with decreasing irrigation intensity (Table 7.7). The highest stem weight of 8.6(±0.9)g/plant as well root weight 8.3(±1.7)g/plant were observed from T1 treatment. Increase in stem and root growth of T1 plants against T4 plants were respectively 110 and 124%. Plant root shoot ratio, which represent the vigour of the plant (Hajra 2001) was not significantly affected with water application rate.
Table 7.7 Plant root and stem weight at the end of Experiment 6 (under 4 different watering intensities) Stem Root Root : shoot Treatment (g/plant) (g/plant) ratio T1 – 100% 8.6(±0.9)a 8.3(±1.7)a 1.0(±0.3)
T2 – 75% 7.4 (±0.7)ab 5.6(±0.7)ab 1.3(±0.2)
T3 – 50% 5.5(±0.3)b 4.5(±0.5)b 1.2(±0.2)
T4 – 25% 4.1(±0.4)b 3.7(±0.4)b 1.1(±0.2)
LSD (0.05) 1.8 2.8
7.3.4 Discussion
Partial irrigation during drought period to save water and for better economic returns is a common practice with many tree crop cultivations (Romero, Pablo et al. 2004; Testi, Goldhamer et al. 2008) and in tea in Iran (Salimi and Latif 2008) as well. The experiment evaluated mainly the growth impact of deficit or partial irrigation during a short period under sun lit glasshouse condition. Even though the experiment was conducted for a short duration, the effect on the young plant growth is significantly high among treatments.
According to the results, 50% irrigation for a short period does not have a significant effect on growth. However, T3 or 25% irrigation results in significant reduction in the plant growth.
158 Both stem (r2=0.99, P=0.003) and root growth (r2=0.92, P=0.04) showed more linear relationships with water application rate.
These results support the hypothesis that it is possible to maintain optimum young plant growth and development with frequent but partial irrigation. Specifically, it is possible to maintain optimum growth when daily applying 75% of the water use of the previous 24 hour period. Daily irrigation under larger deficit regimes still has a negative impact on growth.
7.3.5 Summary of Results
Even though experiment was conducted for short time, there was a significant difference in leaf growth in plants. Young plants were sensitive for even 25% reduction in crop water requirement in canopy growth. Root growth was the most sensitive for the full requirement of crop water. Even with 75% partial irrigation root growth was affected by 32%. Branching, or height did not change with water application rate, perhaps due to shorter period of experiment.
159 7.4 Experiment 7: Effect of Irrigation and Raised Bed on Young Tea Growth
7.4.1 Introduction
The effect of short duration water stress and partial irrigation were examined in Experiments 5 and 6 respectively. These experiments established that: young tea plants grown under the hot humid conditions of low elevation areas required daily watering for optimum growth; and that a partial irrigation regime that replaces only 75% of the previous day’s water use is adequate to maintain optimal growth. These are important findings as poor establishment and growth in the initial stages of tea establishment can later cause considerable effect on the productivity and longevity of cultivation. In Experiment 2 (Chapter 5) it was reasoned that irrigation of young plants at field establishment underpinned the ability of irrigated plots to yield better than non-irrigated plots even in the wet season when irrigation water was not being applied. The stronger and healthier bush developed through irrigation at establishment could make more productive use of wet season rainfall.
Experiments 5 and 6 were by practical necessity conducted in a glasshouse and we need to be conservative in translating glasshouse results to the field. The main reason for this is that, even though the glasshouse pots used soil derived directly from the field, the bulk density of the glasshouse soil will be quite different from the field.
A key limiting characteristic of soils in the low elevation growing areas is their high bulk density. Such conditions inevitably restrict root growth and limit the potential of the tea bush to develop a canopy and root structure to make the most of available water. This was the interpretation for the relative low response to drip irrigation in a mature tea field, but had not been irrigated since establishment, in early years leading up to the current research project (TRISL 2005). In recognition that bulk density, and by extension infiltration and run-off, may be limiting the establishment, a field experiment examined the effects and interactions of irrigation and raised beds on young tea plant growth. The hypothesis of this experiment was that raised beds enhance the ability of young tea plants to productively use irrigation water.
7.4.2 Method
The experiment was conducted at St Joachim Tea Estate, Tea Research Institute Ratnapura during May 2007 to March 2007. Healthy ten month old plants of cultivar TRI 2023 were planted in the field in May 2007. May is the month recommended for field planting with the onset of south west monsoonal rains. It ensures young plants are not exposed to a severe dry season drought at least for next 8 months (Wadasinghe and Peiris 1987; Zoysa 2008). Also the relatively lower ambient temperature and absent of higher solar radiation facilitates young
160 plant growth. The experimental design was a Randomized Complete Block. Following are the treatments of the experiment.
T1 - 30cm raised beds with no irrigation
T2 - normal ground with irrigation
T3 - 30cm raised beds with irrigation
T4 - normal ground with no irrigation
There were 4 blocks and each plot consisted of 16 plants in two 8-plant rows spaced 1.2m between rows X 0.6m along rows. In between blocks and plots, 1.2m space (1 row) was kept vacant to separate the treatments. In the irrigated plots of treatment 2 and 3, a thick polythene layer was inserted into the soil, 60cm apart from the plant row, to a depth of 60cm. The polythene layer was inserted to prevent possible soil moisture seepage from irrigated plots to rain-fed plots. In the treatments 1 and 3, plants were established in the middle of soil beds with 60 cm wide, 5.6 cm long and 30cm in height. When constructing the beds, all large stones and boulders were removed manually. In the initial months, beds were reconstructed manually following heavy rains. However, once the soil got established, reconstruction was not necessary after rain. After planting, all plots were thatched with Mana (Imperata cylindrica) mulch to minimize any soil moisture loss. After nearly two months in the field, all plants were cut to 30cm height to encourage lateral branching in week 4 of July 2007. This is the recommended practice to encourage the lateral branch growth.
Drip irrigation system was established in June 2007, using Jain Turbo Tape, an in-line drip tape, with a emitter spacing of 60cm and discharge rate of 1.5L/hr/emitter at 10psi. The system was run by water from a deep tube well fitted with a submerged pump. Irrigation was applied in the first week of December, first and third weeks of January and the first two weeks of February, 2008. Unexpected heavy rains continued in following weeks until April, 2008.
Irrigation was applied as the difference between plant water requirement and the rainfall of the previous day. Irrigation was begun for 5 days after the rain event following the current industry recommendation (Sumanasena 2008). The plant water requirement was calculated based on the Class A pan evaporation of the day. A crop coefficient of 0.85 was used to calculate the plant water requirement (Laycock 1964). Rainfall and pan evaporation data was collected from the Estate weather station.
Plant height, number of branches and leaf area index per plant were measured using 4 plants per plot, prior to drought (January, 2008) and after drought (February 2008). After the drought period, destructive samples were taken to measure the total growth, selecting one plant from each plot. Plants, including the roots, were excavated carefully from the soil. 161 Additional care was taken to collect the all roots of the plant in the excavation. The plant sample was separated to stem and root compartments. Dead twigs were separated from the stem, as it was prominently available in some plants. Root samples were washed carefully to remove the soil. Root samples were further separated into fine (<2mm) and coarse (>2mm) roots. Roots samples were air dried before oven dried at 850C for 24 hours.
7.4.3 Results
7.4.3.1 Rainfall and soil moisture during the study
As the plants were grown during the onset of the rainy season (Figure 7.2), they received rain each week until the 30weeks after planting (third week of November, 2007) after the third week of May. This was followed by a completely rainless two weeks during January 2008 during which time irrigation was applied. February is normally a dry month too, but after February 2008, there was rain in each week. Plant potential evapotranspiration was around 20mm/week on average. It dropped to nearly 15mm/week during some weeks. The reduction in potential water use was observed in heavy rain months and in some comparatively dry months like January.
300 rain 25
ET0
250 20
200 15
150 (mm/week) 10 0 ET Rain (mm/week) 100
5 50
0 0 May Jul Sep Nov Jan Mar
Week (2007 May -2008 Feb)
Figure 7.2 Weekly rainfall and maximum potential evapotranspiration (ET0) since field planting
162 Week (2007 June - 2008 Feb)
May Jul Sep Nov Jan Mar 0
-20
50% available water -40 Water deficit (mm) deficit Water
-60 rainfed irrigated
Figure 7.3 Maximum potential soil water deficit in rain-fed and irrigated plots. Soil water retention in 60cm soil depth was 75mm
The change in the maximum soil water deficit (SWD) is given in Figure 7.3. SWD was calculated as the difference between rainfall and ET. The figure shows the soil water deficit up to 60cm depth of soil. Initially also there was around 20cm deficit of soil water due to absence of rainfall in the third week of May. However, there was no significant reduction in soil moisture until December. There were deficit and replenishment of soil water in December and from January 2008 followed rapid decline in soil moisture. During the last week of January soil moisture level plummeted to more than 50% of available soil moisture in the soil. This is a serious shortage in soil water availability.
7.4.3.2 Soil bulk density
Figure 7.4 shows the average dry bulk density of the soil up to 60cm depth in g/cm-3. The total depth was measured based on the average depth of 3 soil layers at 20cm interval. The bulk density of soil under the conventional flat land was 20% denser than raised beds. Among the two raised-bed treatments the irrigated bed had a slightly higher bulk density. This may be due to the irrigation water settling the soil particles more firmly in the loosely constructed earth bed.
163 2.0
1.8
1.6 )
-3 1.4
1.2
1.0
0.8
0.6 Dry bulk density (g/cm Dry bulk density
0.4
0.2
0.0 Raised, rainfed Flat, irrigated Raised, irrigated Flat, rainfed Treatment
Figure 7.4 Average plot soil bulk density (dry basis) up to 60cm depth (error bars shows standard error) 7.4.3.2 Plant height
All plants were cut to a height of 30cm after two months in field planting, and resultant growth during the non water stress period was measured at early January, 2008, prior to onset of drought. As shown in Figure 7.5, there was no significant difference between the treatments. Plants in all 4 treatments grew at a similar pace with respect to the height. It must be made clear that the two irrigated treatments were not actually irrigated at this early stage. Although the growth of plants in the raised bed treatments (Treatments 1 and 3) are nominally higher than the normal ground treatments the treatment effect was not significantly apparent at this early stage.
164 120 prior to drought after drought
100
80
60
Plant height (cm) 40
20
0 Raised, rainfed Flat, irrigated Raised, irrigated Flat, rainfed
Treatment
Figure 7.5 Plant height of the young tea plants before and after the drought. (error bars = standard error)
However, when height was measured again after one month drought period of 2008, and after irrigation applied to Treatments 2 and 3, the treatment effects became readily apparent (Figure 7.5). From this figure, highest increase in plant height was shown by Treatments 3 and 4, values of 11.4(±1.2) and 11.5(±2.0)cm/month. Both these treatments involved irrigation during the dry spell. The lowest plant height increase of 5.2(±1.1)cm/month was observed from Treatment 4, which involved neither land preparation nor irrigation during the dry spell. Both irrigation treatments produced similar height increases during this dry period; i.e. there is no interaction with the raised bed treatments (p=0.05).
7.4.3.3 Growth of branch shoots
Development of new branches to have a spread canopy for higher productivity and healthier canopy is very important in formation of the tea bush. The growth of new branches during the dry spell is presented in Table 7.8. Growth of new branches in the control plots (Treatment 4) was 1.6(±0.5) no/plant, which is significantly the lowest among the 4 treatments. The other 3 treatments cannot be statistically separated although the highest branch increase of 2.9(±0.6) no/plant was found in the treatment 3.
165 Table 7.8 New branch growth during short dry spell 2008 Branch increase Treatment (no/plant)
1 Raised Bed No Irrigation 2.2(±0.3)ab
2 Normal Ground With Irrigation 2.6(±0.2)ab
3Raised Bed With Irrigation 2.9(±0.6)a
4 Normal Ground No Irrigation 1.6(±0.5)b
LSD(0.15) 1.28
7.4.3.4 Leaf area index (LAI)
Addition of new leaf to the plants is important to maintain healthier leaf area per plant. Leaf fall can be observed when the plants are subjected to drought conditions. During the study period leaf addition and leaf fall due to abscission from the plants were counted for each plot. Some plants showed a gain in leaf number meanwhile some other plants showed leaf loss in all 4 treatments. Change in leaf area index based on the addition of new leaf or loss (i.e. leaf abscission) for each treatment was calculated and presented in Figure 7.6. Irrigated treatments under both methods of land preparation had greater LAI than their respective rain- fed treatments. However, this difference was significant (P=0.05) in raised beds only. Both irrigated treatments showed higher LAI gains per plot, increasing more than one. Lowest LAI was observed from the raised bed, rain-fed plants.
166 0.10
0.08
0.06
LAI change 0.04
0.02
0.00 Raised, rainfed Flat, irrigated Raised, irrigated Flat, rainfed Treatment
Figure 7.6 Increase in leaf area index during dry spell (Error bar = standard error) 7.4.3.5 Development of stem parts and roots
The weight of the main stem and dead twigs are shown in Table 7.9 in dry weight g/plant. Stem growth was higher in Treatment 1 and 2, though significantly not different (p =0.05). Stem growth was similar in Treatment 3 and 4.
Table 7.9 Dry weight of main stem parts and twigs Stem Dead Twigs Treatment (g/plant) (g/plant) 1 Raised Bed No Irrigation 408(±27.1)a 10.5(±3.9)a
2 Normal Ground With Irrigation 411(±71.3)a 4.0(±1.0)ab
3 Raised Bed With Irrigation 353(±104.1)a 2.3(±1.4)b
4 Normal Ground No Irrigation 352(±66.5)a 4.3(±0.5)ab
LSD (0.05) ns 7.3
In the normal flat land plots irrigation facilitated 17% more stem growth than rain-fed normal flat land. The amount of dead twigs in the Treatment 1 was significantly higher than other three treatments (P=0.05). Drying of the soil and plant during dry days is likely to have caused the die back of plant parts in drought.
Development of the different size roots are shown in the Table 7.10. Soil bed preparation and irrigation facilitated more fine and coarse root growth. Irrigation alone resulted in more than 167 100% increase in the fine and coarse root growth. However, a combined effect of raised bed and irrigation was not observed in root growth.
7.4.4 Discussion
Land preparation technique, i.e. constructing raised beds of 30cm to plant young tea was able to reduce the soil compaction by around 10%. This is an alternative to remove soil constraints to improve productivity in this highly compacted soil in the region. Planting in the raised beds lowered the soil bulk density and increased the more feeder root development (<2mm) than flat non-irrigated treatment. Easy penetration of the roots in less compacted soil may cause for more smaller root development, which is vital for water and nutrient uptake. Similar results of root development also achieved with irrigation the plants in flat, normal lands as well. Drought usually impaired the growth of roots in woody plants, with less thickening of roots (Pace, Cralle et al. 1999). A similar increase was seen with the branch increase in the month long drought period.
Table 7.10 Fine and coarse root weight (g/plant) of Experiment 7 Coarse Fine (<2mm) Treatment (>2mm) (g/plant) (g/plant)
1 Raised Bed No Irrigation 40.3(±6.1)a 185(±29.9)a
2 Normal Ground With Irrigation 36.7(±4.8)a 180(±27.7)a
3Raised Bed With Irrigation 41.0(±1.7)a 195(±5.0)a
4 Normal Ground No Irrigation 18.3(±3.3)b 94(±19.4)b
LSD (0.05) 14.2 83.3
However, one of the main limitations for applying the raised beds for cultivation under rain- fed condition is the very low leaf development during the dry period, as indicated by the LAI increase. The reason for this can be attributed to the increased temperature conditions in an around the micro- climate of the young tea plant. This also confirmed with more dead twigs in the young tea plant in raised beds without irrigation.
Findings of this experiment showed that raised beds lower soil compaction. However, the observed effects of decreased soil compaction in raised beds, under irrigation were not conclusive as to whether it had a favorable affect on the growth of tea. Construction of raised beds is a technique that has to be applied very cautiously. There are many factors to be considered in constructing the raised beds for irrigation like, cost of establishment, long term maintenance of raised beds, weed growth and feasibility to apply in sloppy lands etc., 168 Nevertheless, lowering soil compaction is a recommendation for the tea growers, using techniques like rehabilitation grass cultivation prior to replanting, burial of pruning and envelope forking. So for maximizing the response to irrigation, it is advisable to pay attention the soil compactness of the field.
7.4.5 Summary of results
This experiment showed that Irrigation supports better growth of young tea regardless of ground preparation. Raised Beds may further improve some measures of performance of young irrigated tea, but it is worse than normal ground preparation under rain-fed conditions.
There was no significant treatment difference in height growth in the very early growth when plants were well watered by rainfall;
But during a short dry period irrigated plants grew on average 10% more than rain- fed. There was no interaction between irrigation and land preparation method;
During this dry period there was a statistical difference between treatments in the growth of branch shoots (P=0.15),
There was a strong treatment effect on leaf area index. The Raised Bed with Irrigation had highest LAI and retained most leaves, whereas the Raised Bed with No Irrigation had the lowest LAI. It lost even more leaves than the Normal Ground No Irrigation;
While there was no significant treatment effect on main stem weight and live twig weight, the Raised Bed with Irrigation had significantly less dead twig material. The Raised Bed No Irrigation had the most dead twig material;
Irrigated treatments both produced significantly more finer roots (<2mm) but there was no interaction with raised beds.
7.5 Conclusion
The aims of this chapter were to quantify the water stress effect on young tea growth, assess the effect on partial irrigation and evaluate the interaction between raised beds and irrigation. Experiment 5 evaluated the water stress effect and Experiment 6 evaluated the partial irrigation. In Experiment 7, interaction of raised beds and irrigation on young tea growth was evaluated in field. In summary, it can be concluded that plant growth is highest under daily rainfall or irrigation at young stage. However, 75% partial irrigation can be practiced without significant growth reduction. In the field, construction of raised beds can be helpful in
169 improving the plant response to irrigation. Raised bed construction for young tea plant is not suitable under rain-fed conditions.
170 Chapter 8
Financial Feasibility of Drip Irrigation in Low Elevation Tea Growing Area
8.1 Introduction
The preceding chapters have all dealt with the physiological and agronomic aspects of irrigating tea cultivation in low-elevation tea growing areas of Sri Lanka. A summary discussion of what the results from these studies mean for the practical establishment of irrigation will be given in Chapter 9. However, regardless of physiological and yield responses to irrigation, and cultivar differences in these responses, the final determinant of the feasibility of any innovation will be financial. This chapter presents such an evaluation.
The low-elevation tea growing areas produce around 60% of total tea production in Sri Lanka and as such are critically important for the generation of export income and provision of local employment (MPISL 2008). As described in Chapter 2 the demography and land-ownership pattern of this tea producing region is quite distinct from the mid and high elevation tea growing areas of Sri Lanka. The majority of the tea produced in this region is supplied from small-holder enterprises, where more than 95% of farmers own 1 ha or less as shown in Table 8.1 (TSHDA 2005). For reasons also elaborated in Chapter 2, this sector of the industry is also the most likely to adopt irrigation.
Table 8.1 Land extend distribution among tea small holder farmers (TSHDA 2005)
Extent (ha)
A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
The drought management tools currently practiced among growers are: selection of suitable cultivars, planting shade trees, and various agronomic practices like planting grasses to improve the moisture holding capacity of the soil (Ananthacumaraswamy and Amarasekera 1986; TRISL 2002; Zoysa 2008). Drought tolerant cultivars are sometimes less productive 171 than drought susceptible cultivars. Also during prolonged rainless periods, even highly drought tolerant cultivars are vulnerable. For this reason, small to medium tea growers are interested to adapt irrigation as a drought mitigation technique.
Generally, the choice of the irrigation system depends on many factors such as water source, area of land to be irrigated, plant canopy and root structure, topography and soil type (Çetin, Yazgan et al. 2004). However, drip irrigation is emerging as the most popular method in perennial crops because it confers better plant survival, greater yields, more efficient water use, more efficient distribution of nutrients and less plant stress (Çetin, Yazgan et al. 2004; Kigalu, Kimamboa et al. 2008). Accordingly, drip irrigation has become the only option currently considered in Sri Lanka despite earlier consideration of overhead sprinkler systems (Rogers 1959; Ananthacumaraswamy 1995).
Net Present Value and Internal Rate of Return analyses are the standard method to establish financial feasibility of investments. These methods are used to evaluate the financial feasibility of drip-irrigation technology in low-elevation areas using yield data for the two cultivars, TRI 3035 and TRI 2023, from the St Joachim Estate irrigation trial. In the previous chapters it became clear that there are significant differences in the responses of these two cultivars to irrigation. This chapter asks whether these differences have an impact on the relative feasibility of irrigating the two cultivars.
8.2 Methodology
The financial evaluation of implementing drip irrigation to the rain-fed tea cultivation in low elevation tea growing areas of Sri Lanka, based on the yield response irrigation trial at Ratnapura, is analysed here. Among the low elevation tea growing areas of Sri Lanka, the tested location of Ratnapura receives a higher annual rainfall. So the results can be applied validly to other tea growing areas like Galle and Matara, where annual rainfall is less. A drip irrigation system was selected for irrigating the tea fields as it is more suitable for the landscape of tea fields, which are mostly hilly and undulating.
8.2.1 Analytical methods
In the financial analysis net present value (NPV) and Internal Rate of Return (IRR) methods were used. The NPV method examines the cash flows of a project over a given time period and resolves them to one equivalent present date cash flow (Remer and Nieto 1995). NPV value is calculated based on following equation.
Net Present Value = PV value of benefits – PV value of costs (Turner and Taylor 1989).
This equation can be written as
172
Where, NPV is the net present value, R the net cash flows, f the interest rate (rate of return) and t the year (from zero to n). To calculate the NPV for 2006/07 period, interest rate of 10% was used. This is the minimum attractive rate of return (MARR) used largely evaluating agriculture projects in Sri Lanka (Liyanage 2009). The NPV method is suitable for analysing the drip irrigation investment, as the project inputs and outputs are fixed (Remer and Nieto 1995). Generally, if the NPV is positive, project is accepted and if NPV is negative, project is rejected (Beattie, Taylor et al. 1985).
Internal Rate of Return is a measure of investment worth which calculates the interest rate for which the present worth of a project equals zero (Park and Sharp-Bette 1990). This method does not represent any external factors like interest rate. However, accepting or rejecting the project depends on the available minimum attractive rate of return. If IRR is greater than MARR, then project is accepted, otherwise project is rejected.
As there is a chance to reduce the irrigation system cost with the expansion of the irrigation extent. Basically, using the same irrigation pump for higher irrigation extent leads to reduction in the investment cost. Similarly, irrigation companies provide discounted price for the growers who irrigate large extent (Herath 2011). This is specially applicable for the growers with low yielding cultivar.
At the end of the chapter, sensitive analysis was done for the two different cultivars based on different green leaf price and wage rate for discount rates of 0, 5, 10 and 15% respectively. These economic factors do have variation over the tea economics. Price of green tea depends on largely on the processing factory to which harvest is sold. However, external factors like break down in international demand, due to either economic or sociological reasons can have a negative effect on the price. Though wage rate is fixed on large company estates, some small growers pay above the nominal rate for the skilled workers to retain them in the property. Different discount rate would provide foresight at which rate the money should be borrowed, for the capital investment.
8.2.2 Irrigation system and cost estimation
All the data for yield, prices received, establishment and maintenance costs used in this analysis were gathered from the experience of growing irrigated tea on Field no 01, St. Joachim Estate in Ratnapura, Sri Lanka, during 1999-2009. The soil and topography of the site, generally flat with a 10m height difference and local well available, is indicative of the
173 majority of smallholders in the low-elevation areas. The establishment and maintenance costs were also no different than what a private smallholder would pay.
The installed irrigation system was a Netafim RAM 17D, dripperlines system from Israel. The irrigation system was installed during the field planting of 10 month old cultivar TRI 2023 and 3025 plants in May 1999. The young vegetatively propagated plants from the Tea Research Institute breeder Nursery of St. Joachim Estate, and hence they were according to standards laid by Tea Research Institute of Sri Lanka for young plants for the field cultivation.
Commencement of the plucking of the irrigated field commenced September, 2001. Hence, harvesting year was calculated from September to August next year. In each year drought prevailed during January–March period. The irrigation system was operated for 64 total hours during the dry spell of 2006/07 (during January – March) Total cost for operating the system 2006/2007 drought period was calculated from the field records of St. Joachim Estate. Based on the hourly operating cost of the system, operating cost of the previous years was calculated from irrigation records. Unit cost of electricity of electricity was Rs 30/kwh, and accordingly cost for operating the system for one hour is Rs 121. To calculate additional cost for the plucking, it is assumed that an average worker harvests 20 kg of green lead per day and worker wage was Rs. 405 per day (there was no change in worker wage rate from 2007 to 2009).
8.2.3 Green leaf price and wage rate
As the price of green tea, low elevation average green leaf price of 2009 was used (Rs 51.70/green leaf kg). For the sensitive analysis of irrigation investment, Net present value of irrigation investment under potential increase and decrease of the capital investment by 50% was calculated over long term green leaf average of Rs 51.10/kg and current wage rate of Rs 540/day. Possible increase or decrease in the investment of the drip system was to facilitate the investor’s ability to reduce the cost of investment through possible large area investment. Also there is a risk of increasing the cost of drip irrigation system as it depends on the import of materials.
Price variation of green leaf price (from Rs 40-70/kg) was then calculated for the 3 different wage rate scenario of Rs 405, 500 and 700 a day. This would facilitate the grower to select best price for the green leaf, under irrigation investment. Since, the green leaf price usually vary with tea factory, choice is there to select a best available price.
174 8.3 Results and Discussion
8.3.1 Drip irrigation system cost
Total cost for the irrigation system including the pump was Rs 515,840. Main items of the irrigation system include power source, filter, valves, main pipes, drip lines and other fittings (Table 8.2). The highest cost of the system was for the drip tube and accessories, which is 63% of drip irrigation system cost (excluding pump cost).
Table 8.2 Investment cost of drip irrigation system for 1 ha tea field (Jinasena Ltd 2010; Herath 2011)
Equipment Cost(Rs)
Filter system 20m3 15,000
Pressure gauge and air release valve 8,100
Drip tube, connectors and end caps 284,564
PVC pipes and fittings 63,240
Installation charge 20,400
Sub total 391,304
Value Added Tax (15%) 58,696
Total Irrigation cost 450,000
3.0HP electric pump with accessories 65,841
Total System cost 515,840
Most of the growers looking for irrigation system are the ones with permanent water supply source either like a river or well. The cost for irrigation well is not considered in this investment, as for the irrigation, an existing well is used. However, the cost of unit irrigation system can be brought down by installing more area than 1 ha at once.
Annual operating cost of the system was Rs 11776/ha, including the 10% of pump running cost allocated for annual maintenance (Table 8.3). The operating cost was significantly lower, when compared to capital investment cost. It was nearly a 3% of total investment cost.
Table 8.3 Total annual operational cost during 2006/07 dry season
Description Cost(Rs)
Operational cost for 64 hours 7,755
Labour cost 3,245
Repair and maintenance cost (10%) 776
Total 11,776
175
8.3.2 Green leaf yield
Table 8.4 shows the yield response for drip irrigation for two contrasting cultivars, from year 2001 to 2009. Highest yield response to irrigation is from the cultivar TRI 2023 throughout the period, which is a drought susceptible, fast growing cultivar, except 2009. Highest percentage yield increase was recorded in the 2001/02 period (first year in harvesting) Yield increases were 127 and 93% respectively for TRI 3025 and TRI 2023 cultivars in that year. Yield increase during the pruning years (2005/06 and 2008/09) was lower for both cultivars. Second year of the first pruning cycle (2002/03) recorded the highest yield increase in quantity wise for both cultivars. Though there is a difference in cultivar, there is no price difference for difference cultivar.
8.3.4 Net present value of installing a drip irrigation system
Net Present Value of installing a drip irrigation system for two cultivars is shown in (Table 8.5). According to the table, net return from the operating the irrigation system depends on cultivar and age of the pruning cycle (apart from climatic factors). For the cultivar TRI 3025, NPV at the end of 10 year operation was Rs -57898. The financial return after 10 years is a negative value and hence this investment cannot be considered as financially sound project for the cultivar on present green leaf price, wage rate and at a discount rate of 10% for a 10 year period.
Table 8.4 Yield response of two tea cultivars to drip irrigation TRI 3025 - Green Leaf TRI 2023 - Green Leaf Year (kg/ha) (kg/ha) Control Irrigated Control Irrigated
2001/02 6318 14389 11232 21730
2002/03 16952 22824 24504 37306
2003/04* 16091 18000 19564 23467
2004/05 20061 23315 26716 32163
2005/06* 20357 20558 27966 29114
2006/07 15224 17830 20119 25437
2007/08 16543 17663 11634 21627
2008/09* 1451 4140 1756 3806 (* - plants were pruned during April in these years)
176 Table 8.5 Net Present Value of installing drip irrigation system for two cultivars (price in Rs)
Increased yield Income (add.) Cost (add.) Net revenue Discount NPV Year Factor TRI 3025 TRI 2023 TRI 3025 TRI 2023 TRI 3025 TRI 2023 TRI 3025 TRI 2023 (10%) TRI 3025 TRI 2023 Investment cost 1 -515,841 -515,841
1999/00 7433 7433 -7433 -7433 0.9091 -6757 -6757
2000/01 5011 5011 -5011 -5011 0.8264 -4141 -4141
2001/02 8071 10498 417271 542726 170027 219165 247244 323560 0.7513 185758 243096
2002/03 5873 12802 303634 661843 125192 265496 178442 396347 0.6830 121879 270710
2003/04 1908 3902 98644 201754 47381 87767 51263 113987 0.6209 31830 70777
2004/05 3254 5448 168232 281646 72349 116771 95883 164875 0.5645 54123 93067
2005/06 203 1148 10495 59349 7284 26419 3211 32930 0.5132 1648 16898
2006/07 2606 5318 134730 274923 64547 119458 70184 155465 0.4665 32741 72526
2007/08 1121 9993 57956 516623 28546 208198 29410 308425 0.4241 12473 130802
2008/09 2691 2052 139125 106088 65490 52550 73635 53539 0.3855 28390 20641
Net Present Value (Rs) -57898 391779
177 As shown in the table, except for 2009, net return was higher with cultivar TRI 2023 and the NPV value of cultivar TRI 2023 after the study period was Rs 391779. Higher positive value of the cultivar TRI 2023 suggests that installing a drip irrigation system for this cultivar is worthwhile. Higher yield production of this cultivar is the reason for positive NPV of this cultivar.
8.3.5 Internal rate of return (IRR)
As the investment of drip irrigation, only resulted positively with TRI 2023, Internal Rate of return (IRR), was to evaluate further economic return on the investment. IRR value for cultivar TRI 3025 is 7% and for cultivar TRI 2023, it is 23%. 7% discount rate is a lower value in the investment, and at present market condition need some kind of concessionary discount rate for the farmers to apply drip irrigation for low yielding cultivars like TRI 3025
8.3.6 Variation in capital cost
Cost of the drip irrigation system decrease with the increase of extent irrigating at once. Hence there is a higher chance to reduce the investment cost per hectare by irrigating more area. Same water pump can be used to irrigate more than 1 ha field. Sensitive analysis of irrigating two cultivars based on potential decrease in the cost of system installation. Green leaf price of Rs 51.10 was selected based on the long term average of the green leaf price in the area (Munasinghe 2010). The past trend of the green leaf price in the area is upward. However, there is a risk of sudden drop of the price in some years, due to the facts like global recession. Hence long term average green leaf value was used for this calculation. Present wage rate of Rs 540/day was selected as the wage rate.
Table 8.6 Net Present Value (Rs) of installing a drip system at different investment costs (Green leaf Rs 51.10/kg, wage rate Rs 540/day) (Herath 2011)
TRI 3025 TRI 2023 Extent System (ha) cost 0% 5% 10% 15% 0% 5% 10% 15%
1 515841 25255 -93529 -247476 -369983 638180 360259 13706 -244545
2 452921 88175 -30609 -184555 -307063 701101 423180 76626 -181624
5 413168 127927 9144 -144803 -267310 740853 462932 116379 -141872
10 363168 177927 59144 -94803 -217310 790853 512932 166379 -91872
20 343168 197927 79144 -74803 -197310 810853 532932 186379 -71872
178 According to the sensitive analysis (Table 8.6), irrigating cultivar TRI 3025 is not economical up to 20ha investment under the discount rate of 10 and 15%. If the grower is to irrigate more than 5 ha, it is economical at 5% discount rate. If the discount rate is 15%, irrigation is not economical even with cultivar TRI 2023. But at the discount rate of 105 or lower, irrigation is economical even for 1 ha field.
8.3.7 Sensitivity to variation in green leaf price and wage rate
Unlike capital cost of investment, there is a higher chance of variation in green leaf price, due to the constraints in export market and for the wage rate too. Even though, large plantation estates have the fixed wage rate for workers, with employer contract, small tea growers may face mostly increasing wage rate depend on the demand for the local labor. For the analysis Wage rates were selected as Rs 405, 500 and 700 day. Rs 405/day was the wage rate during 2007 to 2011. In 2011, wage rate was revised to Rs 540/day. However, there is a likely hood increase in the next wage revision to Rs 700/day
There is complete difference in the sensitivity of the two cultivars for different wage rates and green leaf price. Installing a drip system at the wage rate of Rs 405/day was economical at discount rates (0-15%). for cultivar TRI 3025, only if the price increases to Rs 65 per kg (Figure 8.1). To make the investment successful for the same cultivar at the 10% discount rate, green leaf price has to be keep at a rate of Rs 55 or higher. But in contrast for the cultivar, TRI 2023, keeping the wage rate above Rs 45 is sufficient to make the investment successful for discount rates up to 15% at wage rate of Rs 405.
At the present wage rate of Rs 540, financial feasibility is very weak for cultivar TRI 3025, even the price of green leaf reaches Rs 70, at higher discount rates of 10 and 15%. Financial feasibility further sinks for the same cultivar, with potential increase in wage rate to Rs 700.
In contrast, the other cultivar TRI 2023 shows a higher resilient to fluctuation in the wage rate and green leaf price in a wider range, thanks to its higher productivity. At the higher discount rate of 15%, it need however to reach a green leaf price of Rs 60 or higher to reach the positive NPV value.
The financial feasibility of installing a drip irrigation system from field planting upto 10 year period, was evaluated. It analysed the financial feasibility of mitigating the short term dry spells, tea plants are experiencing, specially during January – March period. Drip irrigation resulted in positive yield increase from both cultivars tested. Unlike in the case of introducing irrigation to mature cultivation, the yield response from young plant is significantly higher (Stephens and Carr 179 1991). As shown in Table 8.4, response to irrigation was lower during pruning years. However, cultivar selection is a crucial factor for the financial viability.
Rs 405/day 2000 TRI 3025 TRI 2023 0% 1500 5% 10% 1000 15%
500
0
-500 Rs 500/day 2000 TRI 3025 TRI 2023
1500
1000
500
0 Net Presen Value (Rs. 000) (Rs. Value Presen Net -500 Rs 700/day 2000 TRI 3025 TRI 2023
1500
1000
500
0
-500 35 40 45 50 55 60 65 70 75 35 40 45 50 55 60 65 70 75
Green Leaf Price (Rs/kg)
Figure 8.1 Variation of Net Present Value at different discount rates according to variation in green leaf price at different wage rates
Though there are significantly higher yield increases, only cultivar TRI 2023 was economically feasible for the irrigation investment, recording positive NPV value of Rs 391406. But for cultivar TRI 3025, investment was not economically successful, as the NPV value was Rs - 180 57898. Negative NPV means, it is not a financially acceptable project for installing a drip irrigation system for cultivar TRI 3025.
The yield gap between TRI 2023 and TRI 3025 can be used as a guide line for the growers to understand the selection for either suitable cost effective irrigation system. Similarly high yielding cultivar should be selected, if efficient irrigation system like drip is preferred. Still growers has a chance to run financially viable drip irrigation project with TRI 3025 cultivar, if he could install large extent of his field or if he could secure lower interest rate. (Even though irrigation of large extent is not feasible with small growers, these findings would be important for investment strategies of large plantation companies).
8.4 Summary of Results
Internal Rate of Return is 7% for TRI 3025 and 23% for TRI 2023. Only investing drip irrigation for TRI 2023 is justifiable.
Net Present Value for TRI 3025 is Rs – 57898 and for TRI 2023 is Rs 391779
At present economic conditions, drip irrigation investment is not justifiable for even for 20 ha field of TRI 3025
If interest rate rose to 15%, drip investment becomes uneconomical for cultivar TRI 2023 as well.
If the wage rate increases to Rs 700/day, to make investment economical at present interest rate (10%), for TRI 3025, green leaf price should be increased to Rs 70.00 and of for TRI 2023, green leaf price should be increased to Rs 54.00
8.5 Conclusion
When considering the above economic analysis, it can be concluded that though drip irrigation is associated with high capital cost with selection of high yielding cultivars like TRI 2023, significantly higher returns can be achieved for the investment. The irrigation can be used to increase the productivity of low-grown tea to a considerably higher level, if the capital investment cost can be subsidised.
181
182 Chapter 9
Discussion
9.1 Introduction
The low elevation tea growing areas of Sri Lanka have, over the last two decades, changed from being a minor to the major producer of the country’s tea. These areas are largely occupied by smallholder growers. The growing environment in these areas are quite different from the mid and high elevation growing areas in that crops are exposed to significant short-term water stress even under relatively humid conditions, and this regularly impacts on productivity. This study was conducted to assist the industry to mitigate the effects of this short term water stress through irrigation. Even though there were earlier detailed studies to understand the agronomy of rain-fed low elevation tea (Wadasinghe 1989; Wijeratne 1994), this is the first attempt to understand the agronomy of low elevation irrigated tea. This chapter discuss the key findings of this research program with interpretations and directions for future studies.
The aim of this thesis has been to evaluate the effect of short-term water stress on the agronomic and physiological characteristics of low-grown tea, the responses to irrigation, and the financial practicality of introducing irrigation in the low elevation tea growing areas of Sri Lanka. The strategy to achieve this aim was to research the 5 objectives listed in Table 9.1. This table also shows how the 7 experiments and financial analysis are linked to the objectives. The experimental hypotheses are re-stated and the final column of the table indicates in which sections of the chapter this work is discussed.
Following this Section 9.6 more generally discusses the seasonal variation in soil moisture in the field trials. This section serves to explain an important finding; that soil moisture build up in irrigated plots even in times of the year when irrigation was not being applied. As this is the first detailed program of tea irrigation research in Sri Lanka, many questions emerged that call for further study. Special attention is given to these new study areas in Section 9.7.
A lot of information has been generated in the course of this study and Table 9.2 gathers all the summary points for Chapters 5 to 8.
183
Table 9.1 Study objectives and relevant hypothesis Objectives Chap Ex Hypothesis Supported/Not Supported
5.2 1 H1.There is a cultivar difference in physiological and Accepted, yield response to irrigation A. To quantify the changes in physiology and yield as affected by the water stress and 5.2 1 H2.Air temperature is the main environmental factor Accepted, recovery by irrigation determining yield 5.4 3 H4.Cultivar selection is, by itself, an inadequate Accepted strategy to cope with water stress B. To evaluate the water use of tea and environmental parameters that govern the H3.Transpiration is closely related to the plant 5.3 2 Accepted water use in low elevation tea growing productivity and air temperature is the key areas environmental factor controlling transpiration
C. To evaluate plant performance in response to different micro-irrigation methods 6.0 4 H5.Different micro-irrigation methods differ in their Accepted effect on tea physiology and productivity
7.2 5 H6.For young tea, even short duration water stress Accepted retard the plant growth Only up to 75% can be D. To quantify the effect of soil moisture 7.3 6 H7.Optimal growth of young tea can be maintained limitation on young tea plant growth under partial irrigation allowed H8.Effect of irrigation on plant growth can be 7.4 7 enhanced by lowering the soil compaction in accepted growth bed. E. To undertake a simple valuation of the Under what financial conditions is the irrigation of Only TRI 2023 profitable 8.0 8 practical financial feasibility of irrigating tea two contrasting tea cultivars feasible in the low under current conditions elevation growing areas of Sri Lanka?
184 Table 9.2 Experimental summary of results Summary of Results
Difference in Ψdawn was observed among two cultivars, irrespective of irrigation.
TRI 2023, showed difference in Ψnoon according to irrigation treatment. th Rain-fed TRI 2023, showed the lowest Ψnoon by 10 week, closer to permanent wilting point.
Decline in Pn rate with drought, did not recover even after rain occurrence for rain-fed plants for both cultivars.
For TRI 3025, gs, increase did not increase El
TRI 2023 responded favourable to changes in gs. This factor further indicates the need for more water usage for highly productive cultivar and the stomatal activity of TRI 3025 was lower than TRI 2023, even under irrigated conditions. 0 Chapter 5.2 Tl increased above 35 C for couple of hours during the day even within short dry periods for rain-fed plants. Experiment 1 Experiment Irrigated plants too showed drop in monthly yield, in dry months, as compared to wet months. The cultivar difference in the production gap is visible and there is no interaction between irrigation and cultivar selection Advantage of irrigation in yield is reflected prominently even during wet period for this leafy crop. Stem canker infection was higher in TRI 2023 even under irrigation. However, irrigation was effective in controlling the disease.
E followed ET0 during wet season. Daily changes in evaporative demand were not reflected immediately. Dry matter production in wet season closely followed the transpiration pattern. During wet season of the yearly (nearly 9 months), increase in ambient temperature drives the transpiration.
Chapter 5.3 During dry period, water use of irrigated plants was double than that of rain-fed plants. But it was lower in dry periods, than wet periods. Experiment 2 Experiment
TRI 4049 showed 17% higher Pn and 4% higher El than TRI 3025.
TRI 3014 and TRI 4049 showed significantly higher Wi than TRI 3025.
TRI 4049 showed a higher Wi and Pn was resilient to increasing temperature.
Chapter 5.4 Chapter 5.4 Maintenance of favourable leaf temperature during hot dry days is not significantly different among cultivars. Experiment 3 3 Experiment
Sprinkler irrigation has a higher ability to maintain 2-40C lower leaf temperature level than drip irrigation.
Pn was 15% higher in sprinkler than drip. gs remained same in both treatments, though El , was 20% higher in drip than sprinkler. Sprinkler yielded 6% higher than drip and 11% higher than rain-fed. During wet season, irrigation increased yield by 15-27% in some months. However it was consisted only with sprinkler. Yield drop in 2009 dry months were 53% for rain-fed and 43 and 44% for drip and sprinkler, compared to wet months. Chapter 6.0 Experiment 4 Experiment Shoot extension rate was 11 and 45% higher in drip and sprinkler, than rain-fed plants. 57% increase in harvestable shoot count only observed in sprinkler.
185 Daily watering produced 25% higher stem growth and 87% higher root growth than the plants with 4 day irrigation interval. However, stem growth, root growth, and branch formation did not have significance difference among 7 and 14 day irrigation treatments. Most sensitive for the irrigation interval is the leaf number. Even for the short irrigation interval like 4 day, plant lost 27% of its foliage Chapter 7.2 Chapter 7.2 Experiment 5 5 Experiment
Even though experiment was conducted for short time, there was a significant difference in leaf growth in plants. Young plants were sensitive for even 25% reduction in crop water requirement in canopy growth. Root growth was the most sensitive for the full requirement of crop water. Even with 75% partial irrigation, root growth was affected by 32%, though not significantly different with full irrigation. Chapter 7.3 Experiment 6 Experiment Branching, or height did not alter with water application rate, perhaps due to shorter period of experiment
This experiment showed that Irrigation supports better growth of young tea regardless of ground preparation. Raised Beds may further improve some measures of performance of young irrigated tea, but it is worse than normal ground preparation under rain-fed conditions. Specifically it showed that There was no significant treatment difference in height growth in the very early growth when plants were well watered by rainfall. But during a short dry period irrigated plants grew on average 10% more than rain-fed. There was no interaction between irrigation and land preparation method. During this dry period there was statistical difference between treatments in the growth of branch shoots. There was a strong treatment effect on leaf area index. The Raised Bed with Irrigation had highest LAI and retained most leaves, whereas the Raised Bed with No
Chapter 7.4 Chapter 7.4 Irrigation had the lowest LAI. It lost even more leaves than the Normal Ground No Irrigation. Experiment 7 7 Experiment While there was no significant treatment effect on main stem weight and live twig weight, the Raised Bed with Irrigation had significantly less dead twig material. The Raised Bed No Irrigation had the most dead twig material. Irrigated treatments both produced significantly more finer roots (<2mm) but there was no interaction with raised beds
Internal Rate of Return is 7% for TRI 3025 and 23% for TRI 2023. Only investing drip irrigation for TRI 2023 is justifiable. Net Present Value for TRI 3025 is Rs – 57898 and for TRI 2023 is Rs 391779 At present economic conditions, drip irrigation investment is not justifiable for even for 20 ha field of TRI 3025 If interest rate rose to 15%, drip investment becomes uneconomical for cultivar TRI 2023 as well. If the wage rate increases to Rs 700/day, to make investment economical at present interest rate (10%), for TRI 3025, green leaf price should be increased to Rs Chapter 8.0 Chapter 8.0 Experiment8 Experiment8 70.00 and of for TRI 2023, green leaf price should be increased to Rs 54.00
186 9.2 Tea Plant Response to Water Stress and Irrigation
Objective A of this study is concerned with the quantification the changes in physiology and yield as affected by water stress and recovery by irrigation. The hypotheses raised in experiment 1 were supported by the evidence and it is now time to discuss this evidence and explain the processes involved. This discussion covers physiological responses, yield responses and cultivar differences.
Experimental evidences to support or reject the hypothesis H1- H3 (Table 9.1) are discussed in this section. In summary it can be stated that above three hypotheses can be supported according to the outcomes. Superior physiological and yield response to irrigation by TRI 2023 over TRI 3025 proved the fact that there is a cultivar difference in response to irrigation. In the yield terms, maximum temperature was found to be the most influential environmental factor for irrigated and rain-fed plants. The third hypothesis, about the evaluating the effectiveness of cultivar selection as a drought mitigation strategy can be accepted based on the fact that there is no significant difference in leaf temperature among tested cultivars in dry days.
9.2.1 Physiological response
As plants enter periods of water stress, diminished activity is observed in the leaves as a defensive mechanism before large water deficits occur in the root zone (Chaves, Pereira et al. 2002). It is therefore important to examine the physiological response to the rapidly imposed water stress at leaf level when needing to understand the causal relationships between water stress and its impact on yield.
The water potential of the two cultivars tested in Experiment 1 showed clear differences at dawn as well mid-day (Figure 5.3), with the greatest difference was observed in Ψdawn. The effective water status of a plant is mostly reflected in its Ψdawn (Barros, da Se Mota et al. 1997). One of the most notable observations was that there was no difference among Ψmidday in irrigated and rain- fed plants of the TRI 3025 whereas there was a difference in TRI 2023. As the water stress period progressed Ψdawn continued to decline so-called drought tolerant cultivar TRI 3025 even when it was under irrigation. In contrast the Ψdawn or irrigated TRI 2023, a drought susceptible cultivar, remained steady.
TRI 3025, which had been bred for constrained rain-fed conditions, could not take advantage of the applied water; whereas TRI 2023, which had been developed for unconstrained rain-fed conditions, appeared to take advantage of applied water well. In response leaf shedding was
187 observed in TRI 3025 in the latter part of the water stress period due to its very low water potential. In contrast, TRI 2023 maintained a higher transpiration rate (Figure 5.6) under both rain-fed and irrigated regimes and the leaves showed relatively higher leaf water potential.
Photosynthesis is the most sensitive physiological activity to water stress in tea (Jeyaramraja, Raj kumar et al. 2003). Water stress caused decrease in the Pn of both cultivars in Experiment 1. At the end of the 10 week long study period, Pn dropped by 45% and 75% respectively for rain-fed
TRI 2023 and TRI 3025, despite some scattered rain during the period. Reduction of Pn with increasing ambient temperature (Figure 5.10) was more significantly related in rain-fed plants (P=0.005) than irrigated plants (P=0.03). Favourable, relatively low, leaf temperatures for photosynthesis were found to be maintained by drip and sprinkler irrigation in Experiments 1 and 4 (Figure 5.8 and 6.9). Among the two micro-irrigation methods (Experiment 4), sprinkler irrigation was more efficient in lowering leaf temperature (Figure 6.8). Irrigation was helpful preventing plant reaching dangerously high temperature levels in the short hot humid period without rain. This is very interesting when compared with tea production in the cool dry Tanzanian Southern Highlands where irrigation is used to raise the leaf temperature (Smith, Burgess et al. 1994).
During periods of water stress photosynthesis is reduced either through direct influence on the metabolic and photochemical processes in the leaf, or indirectly via stomatal closure and cessation of leaf growth which results in decreased leaf surface area (Chen, Zhuang et al. 2010). Among the above two mechanisms, reduction in the photosynthesis in low elevation tea seems to be mainly caused by the influence of metabolic and photochemical process in the leaf rather than stomata closure. This was evident in Experiment 1 results. There it was found that Pn rate of even drip-irrigated plants dropped during water-stress period. The decline of the Pn continued during the drought period. At week 5, though there was an increase in gs, photosynthesis did not increase with a proportionate level in both cultivars. If stomatal closure had had a significant role in reducing Pn during the dry period, Pn should have increased with the increase of gs in week 5.
Between the two cultivars, TRI 2023 responded more to the increase in gs.
Further evidence is in the photosynthetic light response of tea cultivars after experiencing water stress for several weeks (Figure 5.9). Among the irrigated plants the light saturated maximum photosynthesis rate (Pmax) of TRI 2023 cultivar was 62% higher than TRI 3025. For both cultivars, Pmax was more than 100% higher in irrigated plants than the rain-fed plants. Damage to photosystem II is the probable cause for the less efficiency in the utilising the captured energy (Melis 1999). As water stress reduces the capacity of leaves due to water stress, reactive oxygen 188 species are produced at the cellular level (Smirnoff 1998). These reactive oxygen species cause oxidative stress as well as intra-cellular signalling (Finkel 1998) further lowering photosynthesis.
Other evidence of lowering Pn, by processes other than stomatal control was reported in grapevine. Maroco et al (2002) found a reduction in the activity of various enzymes of the Calvin cycle proportional with the intensity of water stress. The above cited evidence and relatively lower changes in gs in tea leaves (Figure 5.5) suggest that the water-stress induced reduction of Pn in Experiment 1 was more due to direct influence of metabolic and photochemical process, than stomatal control.
Instantaneous leaf transpiration (El) changes over the water stress period in Experiment 1 showed significant cultivar difference. Drought resistant TRI 3025 showed 40% reduction in the El under irrigation and a 70% reduction when rain-fed during the 10 week period. In contrast the reduction in the El in TRI 2023 was stronger in rain-fed plants (20% reduction under irrigated cf 48% reduction under rain-fed). However, such variation was not observed in stomatal conductance (gs) in either cultivar. Strong stomatal control of El was not evident during the water stress period. Tea is a high density plant and the low elevation growing areas generally have low wind speeds. As a result, the plant canopy is poorly coupled to the atmosphere (DaMatta and
Ramalho 2006). El hence becomes more dependent on solar radiation than the vapour pressure deficit between stomata and ambient air (DaMatta and Ramalho 2006). This predominance of solar radiation was also illustrated in Experiment 4 where the gs steadily declined over the diurnal period while El showed an increase up to 1200 hours in irrigated and rain-fed plants (Figure 6.7).
9.2.2 Yield response
Yield depression is the most visible, and of course commercially significant, effect on productivity caused by dry spells. In Experiment 1 the average weekly yield fell below more than 50% during the January – April period compared with the rest of the year. Drip irrigation was able to increase the yield by 92 to 118% respectively for TRI 2023 and TRI 3025 during the driest months of February and March 2007. However these yields were still 62 to 70% lower than the monthly average yield over the rest of the year. A similar pattern emerged in Experiment 4 in the comparison of drip vs sprinkler technology (Figure 6.11). The reasons for the yield improvements during dry months in irrigated plants can be attributed to:
1. relatively increased Pn (Figure 5.4)
2. higher shoot weight (Figure 6.9)
3. increased shoot extension rate (Figure 6.10). 189 However, irrigation was not effective in increasing the harvestable shoot count in the plant
(Figure 6.10). As mentioned earlier, high ambient temperature, which negatively affects Pn of even the irrigated plants (Figure 5.10) explains yield decline of irrigated plants in dry months.
One of the most notable features in these irrigation experiments was the relatively higher yield of irrigated plants even during the wet season when the irrigation was not being applied. This was evident in both Experiments 1 and 4. This wet season yield increase of the irrigated plants is an additional advantage of irrigation during dry months. The carry-over effect of irrigation can be attributed to the following reasons:
1. Plants grown under irrigation from the field planting grow larger in terms of number of shoot-bearing branches than rain-fed plants resulting in higher yields at maturity. Similar results were observed from other tree crops like Japanese Plum, grown under irrigation for 4 years (Intrigliolo and Castel 2005).
2. Irrigated plants had a higher leaf area index and non-irrigated plants higher leaf senescence during the dry season in mature plants (Figure 4.4) as well as with young plants (Figure 7.5). So the net effect of more photosynthetic area in irrigated plants translated to higher yields even during wet months. High LAI translates as a more favourable source to sink ration for dry matter partitioning (Li, Yang et al.)
3. Rain-fed plants were infested with a higher percentage of stem canker disease as observed in Figure 5.12. This disease, which is pre-disposed by water stress, is a common cause of die-back and death of tea (Carr 1974; Bannerjee 1993).
Further to this list is a more developed explanation in Section 9.7 which discusses the higher soil moisture content in the irrigated plots throughout the year.
When analysing the effect of environmental factors that influenced the weekly yields during 2007 (Table 5.5), it was found that maximum temperature showed the most significant negative relationship with weekly yield. In particular it was the maximum weekly temperatures (>35°C), the first three months of the year that have the greatest impact, even surpassing that of rain (Table 5.3). This maximum temperature effect was visible in both cultivars in rain-fed as well as irrigated conditions; although drip irrigation did soften the effect to some extent.
190
In the dry season the differences between maximum and minimum weekly temperatures are greater than in the wet season. Lower minimum temperatures at night (~21°C) are usually observed in the months of December to February, than in other times in the year. These lower temperatures will affect shoot initiation and extension. In Experiment 1 increases in minimum weekly temperature showed a positive relationship only with irrigated plants of both TRI 2023 and TRI 3025 (P=0.02 and 0.06 respectively). Rain-fed plants did not respond to changes in minimum temperature over the whole year. So in summary, irrigated plants were protected from extreme maximum weekly temperatures and could take advantage of increases in minimum weekly temperatures.
9.2.3 Tea Cultivars for irrigation and drought mitigation
Large differences in physiology, growth and yield among tea cultivars (Othieno 1978) make it difficult to recommend tea irrigation as a blanket application. It was found in the Experiment 1 that benchmark cultivars with contrasting drought characteristics, responded differently to water stress and irrigation. These differences between the benchmark cultivars pave the way for selecting cultivars to tolerate water stress under rain-fed conditions or cultivars better suited for irrigation. Such cultivar selection can be based on the similarity of physiological response to environmental parameters.
Transpiration control is one mechanism of tree crops to mitigate water stress periods (Nguyen- Queyrens and Bouchet-Lannat 2003; DaMatta and Ramalho 2006). Drought tolerant cultivars show low transpiration during water stress periods. As a result, most genotypes, showing drought tolerant capabilities tend to be less productive (Monclus, Dreyer et al. 2006). These physiological traits were visible in the benchmark drought tolerant cultivar, TRI 3025 in Experiment 1. The effort in Experiment 3 was to evaluate the drought resistant and productive traits of some selected new tea cultivars.
Among the tested cultivars, TRI 4049 showed higher productive characters (17% higher Pn than
TRI 3025) with marginal increase of transpiration (4% compared to TRI 3025). Increasing Pn under water deficit conditions is one of the most sought after physiological character for improving plant productivity under drought conditions (Blum 2009). Increasing the instantaneous water use efficiency (Wi) is one way of achieving higher productivity in dry land agriculture (Condon, Rebetzke et al. 2002). TRI 4049 showed 27% increase in Wi (Figure 5.22).
TRI 3014 also showed similar increase in Wi as compared to TRI 3025. However, Pn of TRI 191 3014 was significantly affected by increasing ambient temperature (Table 5.10) during short water stress period. This factor makes TRI 3014 more vulnerable to high temperature, water stress periods in low elevation areas than TRI 4049.
High leaf temperatures (43-460C) were observed in all tested cultivars including TRI 4049 (13 February in Table 5.9). There was no significant difference among the cultivars with respect to high leaf temperature. Leaf temperature build up was nearly 60C more than maximum temperature of that particular day (~360C). In two other observation days, leaf temperature of the cultivars ranged from 34-390C, with significant differences among cultivars. The reason for low leaf temperature level could be low air temperature (20 February) and soil moisture improvements after rain (06 March). Usually for the C-3 plants, including tea, increasing leaf temperature above 300C, inhibits photosynthesis (Schrader, Wise et al. 2004). In addition to photosynthetic inhibition, increase in leaf temperature causes large water vapour pressure deficits on the leaf surface (Maherali, DeLucia et al. 1997) and leaves have to transpire more and may consequentially lose water (Mitamura, Yamamura et al. 2009). As the ambient temperature increase is an associated factor with low elevation water stress period in January – March, high leaf temperature increases during hot dry days make all tested cultivar vulnerable for photo inhibition and increased water stress.
Accordingly cultivar selection is an inadequate measure for mitigation of hot humid water stress periods in low elevation areas. But an additional measure of establishment of high shade trees (Falcataria mollucana) would control high leaf temperature build up. So, high shade trees and drought susceptible cultivar combination would produce a formidable drought mitigation strategy in low elevation areas, in addition to irrigation.
9.3 Water use of tea in low elevation area
Whole plant water transpiration (E) of the benchmark highly productive TRI 2023 was measured in a wet season and dry season in Experiment 1. Transpiration is very important as it plays a key role in the hydraulic cycle of the plant and only 1% of the water taken by plants is involved in metabolic processes (Rosenberg, Blad et al. 1983).
Some very significant seasonal differences in plant water use were observed. Plants showed higher E during wet season than dry season (Figure 5.14 & 5.17). Transpiration reduction in rain-fed plants was 45% in dry season as compared to wet season. (As a parallel observation yield reduction in dry season with compared to wet season is closer to 50%). There was an increase in potential evapotranspiration (ET0) during the dry season. ET0 was 24% higher in dry 192 season than wet season as calculated by daily values. This is similar for long term climate average calculations as well. ET0 in months of February and March is considerably higher than rest of the year (Figure 4.5). Increase in ET0 was mainly driven by increased solar radiation, ambient temperature and vapor pressure deficit. The average E of irrigated plants in the dry period was 2.7mm/day, which is 0.75 of ET0 and it was 109% higher than rain-fed plants. The increase in drip irrigated plant was largely due to night time transpiration of the plants as shown in Figure 5.18.
During the wet season of the year, the dominant driving factor for E is maximum temperature (Figure 5.16). So the favorable temperature during this season can be considered as the main reason for the increased productivity of tea in this region. Transpiration processes are closely related to productivity as shown in our study (Figure 5.15) and in many previous studies (Yang, Short et al. 1990; Ananthacumaraswamy, De Costa et al. 2000). The wet season of the year lasts for nearly 9 months of the year (Figure 4.8). Maximum temperature also does not reach more than 340C during this period. As a result, there is no threat for the photosynthetic mechanism of the leaves or growth of the plants including shoot extension with high temperature (>350C) (DaMatta and Ramalho 2006) as experienced in dry months of January to March. During the wet months the maximum temperature is a stimulant for higher productivity without restriction of soil moisture due to adequate rain. On the other hand, for the rain-fed plants, there is a restriction of soil moisture to drive transpiration and to moderate the leaf to air temperature (Yang, Short et al. 1990).
It is clear, based on the results of Experiment 2 that transpiration and plant productivity are closely correlated during wet season of the year. Wet season lasts for nine months of the year from April to December. The main environmental factor that determines transpiration is maximum temperature, which does not exceed 340C on average.
9.4 Irrigation System Selection
Irrigation can be applied as either drip or sprinkler. Drip is the preferred method for hilly and undulated tracts for a crop like tea (Sivanappan 1994). However, as the sprinkler is also popular among farmers, due to ease of operation and low cost, performance of tea physiology and productivity was evaluated under drip and sprinkler irrigation in Experiment 4. It was found that increase of leaf temperature in dry days was reduced by 2-40C under sprinkler irrigation less than drip irrigation. Sprinkler irrigation increased tea photosynthesis by 15% during the dry spell and the total annual yield by 6% compared with drip irrigation. These results confirmed the
193 hypothesis that different irrigation techniques have different effect on tea physiology and productivity.
Sprinkler irrigation resulted in lowering the leaf temperature (Figure 6.8) with increased photosynthetic rate (Figure 6.4). The mean for lowering leaf temperature in sprinkler irrigated plants is the artificial cooling to tea leaves. Lowest transpiration was reported in sprinkler irrigated plants among three treatments (Figure 6.6 and 6.8). In contrast with Experiment 1, there was little difference between leaf temperatures in drip and rain-fed plants from 0600hr to 1800hr (Figure 6.8). However leaf temperatures of both drip and sprinkler irrigated plants exceeded the air temperature (which did not increase exceed 330C) during that period. Another reason could be the geographical location of the Experiment 4, as compared to Experiment 1. The site for Experiment 4 had more open space, being exposed to an adjacent public road (Figure 4.2). As such the site probably received more scattered solar radiation which increased ambient temperature.
Among irrigation treatments, sprinkler gave a 6% higher yield increase than drip. Reasons for this could be: higher photosynthetic rate (Figure 6.4), higher shoot extension rate and increased harvestable shoot number (Figure 6.10). These increased yields in the sprinkler irrigated plants remained throughout the experiment period (Figure 6.11), unlike drip. This better growth under sprinkler irrigation (Figure 6.12) translated into a sustained yield advantage across the dry and wet seasons.
Experiment 4 showed that while sprinkler irrigation has the ability to maintain better leaf temperature during dry days, high shoot density and high shoot extension rate, it could not establish a yield advantage in a very wet year. However, it is possible that significant differences between the treatments may occur in a dry year. Experiment 4 comparing different micro- irrigation techniques, was a parallel small experiment, established along main drip irrigation trial (Experiment 1). Clearly, this is an experiment that needs data from a few more years. Sprinkler irrigation is also a cheaper option to install than drip irrigation, so growers will naturally prefer the most cost-effective option.
The selection of the proper irrigation systems is a focal point for the direction of the Sri Lankan tea industry. Application efficiency of irrigation water is a critical factor with diminishing water resources in the global irrigation industry. Based on long term research in India, it was found that on-farm irrigation efficiency of properly designed and managed drip irrigation system was >90% as compared to 65-70% of sprinkler irrigation (Sivanappan 1994). This factor had a huge
194 impact on preserving depletion of ground water resources and for optimal utilization of surface water sources in India. This factor is also very important for the low elevation tea growing areas of Sri Lanka (Eriyagama, Smakhtin et al. 2010). While the dry zone in the north of Sri Lanka is developed for water storage in ancient and modern tanks (large earthen dams), and also supplied from channels from the Mahaweli River, the wet zone area of Sri Lanka has no infrastructure to harvest rain water (Punyawardena 2004).
Given the extent of tea cultivation in the low elevation area, the long term viability of sprinkler technology is questionable. Already some sprinkler-irrigated tea cultivations in East and Central Africa are facing increased pressure on water resources and looking for drip irrigation as the alternative method (Möller and Weatherhead 2006). Unlike in Experiment 4, Möller and Weatherhead (2006) cited significant yield increase in drip irrigation as compared to over head sprinkler irrigation, accounting from number of unpublished papers and industry communications.
Drip irrigation was introduced in late 1990’s as the most suitable micro-irrigation system for tea. In addition to the advantage of water saving, there are some other advantages as well. Ability to apply fertilizer to root zone is a much awaited practice specially for some larger tea fields. I addition to more beneficial applications of daily or more split applications, it would save huge cost on manual labour for fertilizer application. Fertigation is a something that cannot be practised with sprinkler irrigation for tea. Drip irrigation allows, undertaking field operations, like mechanical or manual harvesting, tipping infilling and weeding (Möller and Weatherhead 2006).
Hence in the longer run it is important to select drip irrigation as the suitable micro-irrigation system for tea in Sri Lanka.
9.5 Effect on Young Tea Growth
Development during the post-nursery stage is very important for the long term productivity of tea plants. Chapter 4 showed how short, acute water stress periods impact on the young plant establishment in the field. There are several water stress periods within a year. For example, the average number of rainless periods of > 5 days per year, was more than 10 during 1986-2010. In Chapter 7 the adverse effect of exposing post-nursery plants for both short term drought and partial supplement of crop water requirement were studied in glass house and in field (Experiments 5-7).
195 9.5.1 Effect of water stress interval on young tea
The effect of different water application intervals (daily to 14 day interval) was tested in Experiment 5. The effect of daily irrigation on stem and root growth was significantly different from other treatments (4, 7 and 14 day intervals). Water application at 4 day intervals reduced stem growth by 25% and root growth by 87% as compared to daily irrigation (Table 7.5)2. This differential in the suppression of shoot and root growth can be expressed as the root:shoot ratio which increased with increasing length of water stress periods. All trees have an optimal root:shoot ratio to match their physiological ability to access water with the availability in the environment (Steinberg, Miller(Jr) et al. 1990). For tea, the optimal root:shoot ratio is identified as 1.5 (Bannerjee 1993). Under daily irrigation this ratio was maintained at 1.5 but with water stress it increased to about 2.
This glasshouse experiment used the same soil as in the field experiments and this soil is particularly low in organic carbon (Table 5.1). Soils with low carbon content are prone to high soil compaction (Gomez, Singer et al. 2002). The presence of gravel content can also be seen in this soil. These two factors can cause the soil temperature to increase in this hot humid climate, which slows the root growth (Lopushinsky and Max 1990).
After a 14 day watering interval only 55% of leaves remained on the plants (Table 7.4). Even under a 4 day watering regime plants only 73% of leaves remained on the plants. Similarly leaf loss after short watering periods has been observed in mature commercial fields under sprinkler irrigation near Ratnapura. The physiological reason for this response could be as follows. Ethylene is the main hormonal factor controlling the physiological process promoting leaf abscission after a period of water stress (Tudela and Primo-Millo 1992). The ethylene is generated by the oxidation of 1-aminocyclopropane-1-carboxylic acid (ACC) which originates in the roots. However, in addition to ethylene, abscisic acid (ABA) is also implicated in the process of leaf abscission (Goren 1993). Water stress induces both ACC and ABA accumulation in roots and arrested xylem flow (Gómez-Cadenas, Tadeo et al. 1996). Shortly after rehydration root ABA and ACC returns to pre-stress levels and restores normal xylem flow promoted ABA and ACC transport to leaves, triggering leaf abscission (Tudela and Primo-Millo 1992; Gómez- Cadenas, Tadeo et al. 1996).
2 This sensitivity of young tea to short periods of water stress was evident in another experiment, not reported within the body of this thesis (Appendix 2), where water application rates were studied in potted tea plants (C.sinensis var sinensis) in a growth room. Transpiration was reduced with increasing water stress particularly under higher ambient temperature (35°C).
196 9.5.2 Effect of partial irrigation on young tea
As described earlier, the wet zone of Sri Lanka does not have rain water harvesting structures like irrigation tanks. As a result, severe water shortages even for domestic purposes are common during prolonged rainless periods. One option for larger estates is to irrigate the young tea fields by transporting water by tankers attached to field tractors (Figure 9.1). In such scenario, controlled application of irrigation water is essential to reduce irrigation cost.
Figure 9.1 Water tanker used for sprinkler irrigation commercial tea field in Ratnapura, Sri Lanka January, 2010 (Note: Shade plants were not grown up to provide satisfactory shade in the drought)
Partial or regulated deficit drip irrigation is an irrigation strategy based on limiting wastage from soil evaporation and drainage and applying water so that plant water deficits occur when adverse effects on productivity are minimized (Goldhamer and Viveros 2000). It was widely used in fruit crops like peach and pear (Domingo, Ruiz-Sánchez et al. 1996). Partial irrigation has also been used for cost-effective tree establishment on degraded lands (Khamzina, Lamers et al. 2008).
Experiment 6 simulated partial irrigation regimes on young tea in a glasshouse. In growth terms, both plant stem girth and leaf number were significantly affected when the daily irrigation application was reduced from 100% to 25% of plant water requirement (Table 7.6). Stem girth was reduced by 33% and leaf number reduced by 45%. However, there was no significant difference in branch development, perhaps due to short period of study.
197 In the total plant development also, both stem growth and root growth were significantly affected under partial irrigation. However, significant reduction occurred only when the irrigation amount was reduced to 50% from 100% (Table 7.7). Stem growth reduction was 36% and root growth reduction was 46% for the reduction of irrigation level from 100 to 50%.
This glass house experiment suggests that 75% partial irrigation can be applied without significant negative effects on the growth of young tea. Translating this finding into field plantings requires some caution especially in the sensitive first year. The glasshouse is a much gentler environment with partial shade, protection from wind and relatively stable humidity. The practice of partial irrigation when establishing tea needs to be further developed in the field.
9.5.3 Raised beds to enhance irrigation in young tea
Conventional practice is to establish tea on flat ground. Soil compaction is observed in tea fields due to frequent human traffic to the field, especially for harvesting. Soil compaction decreases total porosity and increases volumetric water content and soil strength (Greacen and Sands 1980). Experiment 7 was conducted to evaluate the performance of post-nursery plants in the first year of field planting under raised beds and irrigation. The raised beds constructed in the experiment reduced soil compaction (as measured by dry bulk density) by 12% under rain-fed treatment compared with normal flat ground. After only one month of irrigation the soil settled so that the bulk density was only 6% lower than flat ground.
Raised-bed planting even improved the root growth significantly under rain-fed conditions. Fine root content was increased by 120%, while coarse leaf content increased by 96% over rain-fed flat ground. Increased porosity may be the reason for better root growth under raised beds. On the rain-fed raised-beds a high leaf loss and dead twigs was observed in many plants. As a result net leaf area gain is low (Table 7.9). This is likely due to the enhanced exposure to radiation and advection coupled with low soil moisture content.
In this simple experiment, it showed that there are some advantages in planting young plants in raised-beds, to enhance the irrigation effect. Raised beds were able to lower the soil compaction, and it was reflected in improved root growth in raised beds. So lowering of soil compaction is a matter for concern to achieve optimum soil bed irrigation. However, this is a practice that should be adapted with cautious. Essentially, this type of practice is more suitable for flat lands, as with possible soil erosion in sloppy lands. There are some issues associated with system, like additional cost for establishment, potential more weed emergence, interaction with tea pluckers and potential adverse effect in case of if irrigation could not be applied in dry season. 198 9.5.4 Final comment on irrigation in young tea plants
The importance of mitigating water stress during the post nursery period was shown through Experiments 5-7. Better root development and prevention of water stress induced leaf abscission are the two major gains of irrigation. Growers in Sri Lanka are concerned that irrigating tea from field establishment will encourage surface rooting, making them ‘softer’ and more vulnerable. However, we now know this concern is unfounded. The root growth of 9-year old tea in Experiment 4 which had been irrigated since establishment was 18-34% better than the rain-fed plants. This makes sense as root growth is favoured by high nutrient and moisture availability (Persson 1978). It is possible that plants irrigated from establishment may become less tolerant to water stress if irrigation is discontinued when the plants mature. This has also been shown to be false, at least for oak afforestation programs in the Mediterranean (Siles, Rey et al. 2010). In Sri Lanka, which struggles with maintaining an optimal replanting rate (Anon. 2011), irrigation in the first one or two years of establishment should become a standard recommendation. This may be done with portable sprinkler systems that can be moved onto another re-planting field. However, this research program has shown that there is great merit in establishing permanent drip irrigation systems.
9.6 Further agronomic considerations
The sections in this Chapter hitherto discussed the results of field and glass house experiments conducted at Ratnapura, Sri Lanka. The Section 9.6 is dedicated for discussing some of the significant observations, apart from formal results and irrigation planning.
9.6.1 Carry-over effects into the wet season
Maintenance of optimum soil moisture in the plant root zone is very important for the tea crop, as it produces new shoots for harvesting throughout the year. Also exposure of plants to dry spells can induce a reproductive phase (Mueller, Scudder et al. 2005). The irrigation systems proposed for the low elevation growing areas are to only apply water over the January to March period when these intense dry spells occur. An important observation in this research was that yields of irrigated plots were elevated even in the wet season when the pump was turned off (Experiment 1, Figure 5.11). Similarly both yield and elevated soil moisture was observed in irrigated plots over the wet season in Experiment 4 when the pump was off (Figure 6.3). How can this be explained?
199 In Experiment 4 monthly soil moisture of rain-fed and irrigated plots was measured up to 60cm depth in 2008 January - 2009 March period (Figure 6.3). In this soil type, 50% moisture depletion of available water content(AWC) limits the root water intake (Anandacumaraswamy 2008). Some however suggest that for tea crop, this limit is 40% (Allen, Pereira et al. 1998). Reduction in the AWC to 50% was observed in some rainy months like April, July and December (Figure 6.3). Even without irrigation in such months, it is important to cite possible reasons for the moisture build up in irrigated plots;
1. Irrigated plants showed a higher canopy growth (Figure 6.13) than rain-fed plants. Since tea is cultivated as a bush, high canopy means, more stem water harvesting in rain events (Llorens and Domingo 2007).
2. Irrigated plants showed a higher structural root density, particularly in lower soil depths of 40-60 cm (Figure 6.14). Higher root density ensure more rain water infiltration to the root zone (Martinez-Meza and Whitford 1996).
3. Irrigated plants have a higher leaf area growth, which prevents soil evaporation, preserving soil moisture during non rainy days.
9.6.2 Irrigation scheduling
Irrigation scheduling in this trial was based on the Class A Pan evaporation data and 5 rainless was the trigger to start the irrigation. This method was followed in the absence of any previous research on irrigation scheduling in tea in this hot humid environment. The gap of 5 consecutive non-rainy days is just a locally assumed indication of ‘a short dry spell’ for other perennial crops, such as cocoa, coffee, pepper and nutmeg in the wet zone of Sri Lanka (Sumanasena 2008). It has been selected on the basis of letting the soil dry for around 25-30% of reduction in the available soil moisture in the root zone. However, the current research program found that, in addition to the soil moisture replenishment, irrigation plays an important role in maintaining favourable leaf temperatures. The effect of even 4 day water stress period cause a significant leaf abscission in Experiment 5. For this reason it may be advantageous to commence the irrigation within two or three days of rainfall ceasing to get the best advantage of irrigation in tea.
Instead of scheduling irrigation based on plant water use and rainfall, measurement of leaf temperature can also be used successfully to identify crop water stress (Howell, Hatfield et al. 1984). When considering the difficulties of operating equipment (e.g. soil moisture meters ) in this compacted soil, leaf temperature measurements can be developed as a quick method to identify the plant water stress at a rapid time. 200 The nominal water application rate of Netafim RAM 17D drip dripper is 2.2 mm/hr. The wetting pattern of the root zone was not tested under this water application rate in the study. As there is a chance under drip irrigation for wet and dry regions to develop in the root zone (Bielorai 1982), plants may still have a chance to experience water stress (Romero, Pablo et al. 2004). So it is necessary to measure several water application rates to identify the correct water application rate according to the soil type.
9.6.3 Shade trees
The sites for both Experiment 1 and 4 were without shade trees. Planting of shade trees is recommended practice under rain-fed tea cultivation in Sri Lanka. In Latin America, similar plantation crops such as coffee and cocoa used to all have shade trees in the early 20th century, but in some sites shade-free intensively managed monocultures have been found to work (Alvim 1977). In 1969 shade trees were removed from a significant area of highland tea estates in Sri Lanka. This led to an immediate yield increase, but then a gradual decline resulting from sunburn and dieback (Fuchs 1989). In low elevation growing areas, the relative merit of shade trees has not been tested. This region has a higher cloud cover than the highlands and it is possible that tea could still be grown without shade trees. Some small holders would prefer not to have shade trees because of problems such as wasp attack and falling limbs. Nevertheless, shade trees are still the standard practice. This practice is typically using medium sized trees of Gliricidia spp which are pruned to height of 3-4m during rainy months.
The experiments in this research program were designed intentionally without shade to exclude the competition to light, water and nutrients (Beer and Catie 1987), and also to minimize the disturbance to water application in the sprinkler irrigation trial. Shade trees would particularly confound results in the wet season of the year, when it is already quite cloudy. However, as this research program has revealed the significance of high temperatures on low elevation tea, it would be appropriate follow-on research to study the interaction of shade trees, which will moderate air temperature, with drip irrigation.
Importance of shade tea presence to harvest rainwater as stemflow and throughfall in tea field is discussed in Appendix 1. Breaking precipitation as stemflow is important for minimizing the soil erosion (Morgan, Quinton et al. 1998). But the larger water drips, dropping as throughfall disturb soil particles (Hidalgo, Raventos et al. 1997) and cause erosion (Figure 9.2). Accordingly, albizia plant plays a major role in harvesting more rainwater as stem water, due to its higher relative dominance (with high basal area). This is particularly important in collecting rain water
201 to soil moisture storage during isolated rains in dry periods. However, there are no reports to analyze the competition of albizia roots on tea plant for soil moisture during water limited dry seasons.
Figure 9.2 Presence of soil particles can be seen at tea stems after heavy rain events
9.7 Financial Evaluation
Financial evaluation was conducted to answer the question under what financial conditions is tea irrigation feasible in Sri Lanka’s low elevation tea growing areas. The evaluation was based on drip irrigation with two different cultivars with the result that under present economic conditions, only TRI 2023 (the benchmark high yielding but drought susceptible cultivar) is profitable for a high investment like drip irrigation.
The Internal rate of return for TRI 3025 is 7% as against 23% of the TRI 2023. IRR analysis gives a quick comparison for the farmer to decide on investment by comparing it with available lending rates. The maximum available rate of return is 10% for commercial agriculture ventures in Sri Lanka. However, sometimes under special agriculture project finance schemes, there is a chance for a grower to obtain an agriculture loan at a subsidised rate of 8%. In tea such loans are available for new planting fields but not for irrigation. The main reason for lack of credit facility for irrigation is the lack of awareness about the positive results of irrigation, among growers as well as among lending agencies. The interest rate of 8% usually comprise of 2-3% 202 implementation charge by the local banks. Donor agencies like Asian Development Bank, Central Bank of Sri Lank or World Bank dispatch agriculture loans usually at interest rate of 5- 6%. Still there is a chance to claim financial viability for drip irrigation, if there is a mechanism to finance the investment by donor agencies to farmers directly through a mechanism like farmer organizations at a rate of 5-6%. This would be very beneficial for the small farmers who own less than one or two hectares.
Net present value of drip investment for TRI 2023 is Rs 391779 and for TRI 3025 it is Rs -57898. The index of profitability of the NPV technique is to accept all projects with positive NPV (Cuykendall, White et al. 1999). Based on current climatic conditions, only investment for TRI 2023 is financially viable. In the present analysis, NPV was calculated based of the drip irrigation yields from 2001-2009. There were no major droughts in this period like there were in 1983 and 1992. However, based on the evidences of climate change, there is a high vulnerability for rain-fed cultivations to experience more frequent severe water stress periods, with increasing ambient temperature (Mongi, Majule et al. 2010). So it is worthwhile to calculate future irrigation investment in tea, considering the additional drought stress possible.
Cultivar difference in response to irrigation is a known factor in tea (Salardini 1978; Stephens and Carr 1991; Kigalu, Kimamboa et al. 2008). In Experiment 1, yield increase under irrigation is 26% for TRI 2023 as compared to 16% of TRI 3025. Nevertheless, 16% yield increase under irrigation is significant achievement for higher productivity. Currently tea growers are not using TRI 2023 because it is no longer recommended for rain-fed cultivation. They are using less productive tea clones which have been bred for drought tolerance. This research program has shown that growers considering drip irrigation would be better advised to either: bring back
TRI2023; or introduce TRI 4049 which was shown to have relatively higher Pn and Wi in Experiment 3. In any case to continue to irrigate with currently used drought-tolerant varieties is financially sub-optimal.
The main limitation for financially feasibility for drip irrigation is the very high investment cost. The investment cost for the drip irrigation system alone (without pump) is Rs 450,000 per hectare. This is approximately 30% of the average land price of one hectare of mature tea in the area. There is a chance to lower the high investment cost of drip irrigation for farmers with large extent of land and plantation companies who owns regional large tea estates, by investing in large areas. Irrigation cost will be lower for larger extent since certain component cost (e.g. pump and filters) remain same irrespective of area covered (Sivanappan 1994).
203 Increase in labour wage rate is a threat for future investment. During the period from 2000 to 2009, average agriculture wage rate increased by 213%. For the workers attached to plantation companies, there is a fixed wage rate which usually determined by every two year. But for the medium scale tea farmers, who has the most potential for irrigation, pays the wage rate usually based on the market wage rate. Such farmers sometimes tend to pay little higher wage rate than average to attract workers for the work and to ensure long term service. So it is important to determine the financial feasibility of such growers, based on their actual wage rate.
In summary it can be concluded that for TRI 3025, IRR is lower and positive Net Present Value was obtained only from TRI 2023. Among the variable factors associated with financial analysis, wage increase is the most possible scenario. In this case, if the wage rate is increased to Rs 700/day, as requested by some labour unions, an irrigated tea cultivation would not be financially feasible unless the farm gate green leaf price also increases to Rs 54/kg for TRI 2023 and Rs 70/kg for TRI 3025.
9.8 Summary
This chapter has discussed the tea plant response to irrigation in low elevation tea growing areas, in physiological, yield and financial terms. It has focussed on explaining the direction and variations in which the experimental data took, and possibilities for future research. The following and final chapter will conclude the thesis with an explication of how these 8 experiments satisfy the aim and objectives of this program of research.
204 Chapter 10 Conclusion This research program has evaluated the effect of short-term water stress on the agronomic and physiological characteristics of low-grown tea, the responses to irrigation, and the financial practicality of introducing irrigation in the low elevation tea growing areas of Sri Lanka. The study revealed: the particular significance of high air temperatures in low-elevation tea; the importance of cultivar selection for irrigation; and the importance of reducing water stress of young establishing tea plants. In summary it has shown that, within well-defined practical and financial parameters, there are strong grounds for promoting drip-irrigation of tea in this region.
There were five specific objectives to this study and the conclusions to each of them are as follows: A. To quantify the changes in physiology and yield as affected by the water stress and recovery by irrigation Water stress during the main seasonal dry season, January to March, caused key physiological processes (photosynthesis, transpiration, stomatal conductance and leaf water potential) which are related to productivity, to be depressed even under drip irrigation. Cultivar TRI 2023 (benchmark for high productivity, low drought tolerance) responded better to irrigation in maintaining high physiological activity, than TRI 3025 (benchmark for moderate productivity, high drought tolerance). Overall, drip irrigation resulted in 21% annual yield increase (P<0.001) over rain- fed cultivation. Higher physiological response to irrigation of TRI 2023 was materialized as higher yield increase, resulting in 16% higher yield than TRI 3025 under drip irrigation. Cultivar difference in response to irrigation is a decisive factor for irrigated tea cultivation. New cultivars for irrigation can be screened on the basis of their physiological responses under high ambient temperatures in humid, but water-stressed, conditions. For example in this study, TRI 4049 is a strong contender as a more productive clone in hot humid drought periods with 27% higher water use efficiency than TRI 3025.
B. To evaluate the water use of tea and environmental parameters that govern the water use in low elevation tea growing areas Air temperature is the dominant parameter governing transpiration in the wet season (r2=0.62, P<0.0001), which is the key for higher productivity in the region. 205 However, high air temperature suppresses physiological activities in the dry season. There was a strong difference in tea plant transpiration in dry and wet seasons. Average daily transpiration of rain-fed plants was 2.3(±0.3) and 1.3(±0.2) mm/plant in the wet and dry seasons respectively. Irrigated plants showed 109% increase in transpiration than rain-fed plants in the dry season.
C. To evaluate plant performance in response to different micro-irrigation methods Sprinkler irrigation produced 6% higher yield than drip irrigation, based on the very wet year of 2008. Higher assimilation rate (15%), shoot extension rate (31%) and higher shoot count (51%) were observed than drip, translating into higher yield in sprinkler-irrigated plants in dry month. Maintenance of 2-4% lower leaf temperature in midday hours under sprinkler irrigation, compared with drip irrigation, was the key for a higher assimilation rate. Irrigation treatments showed to counter the negative effects of hostile climate parameters (temperature, irradiance, vapor pressure deficit and potential evapotranspiration), which increase during dry season. However there are practical problems with sprinkler irrigation in the low elevation tea growing areas; undulating topography and presence of shade trees affect the uniformity of application.
D. To quantify the effect of soil moisture limitation on young tea plant growth. Irrigation proved effective in establishment of the young tea plant establishment through glasshouse and field experiments. Ideally, young plants require daily irrigation during the dry season. Decreasing irrigation frequency from daily irrigation to 4 day intervals reduced the stem growth by 20% and root growth by 46%. In the glasshouse, it is possible irrigate young plants to 75% of plant water requirement with no significant effect on growth. However, deficit irrigation of 50% reduced stem growth by 34% and root growth by 45%. Caution should be applied when applying deficit irrigation in the field. Establishing young plants on raised beds in the field enhances growth through increased soil porosity. Irrigated plants on raised beds grew more roots (12 and 8% increase in fine and coarse roots) and a 60% increase in leaf area, compared with irrigation on flat ground. Rain-fed plants on raised beds tended to shed more leaves possibly due to greater exposure to radiation and advection.
206 Together, these field and glasshouse experiments with young tea point to the possibility of an ideal establishment regime to be: daily irrigation but irrigating to 75% of plant water requirement; on raised beds, or ground otherwise prepared to increase soil porosity.
E. To undertake a simple valuation of the practical financial feasibility of irrigating tea. Financial evaluation of drip irrigation showed that an investment in drip irrigation is feasible under the assumed parameters of production cost and green leaf price. The choice of the right cultivar, i.e. one that responds well to irrigation, is crucial. Cultivars selected for drought tolerance are not advised. Under the present economic climate (considering 2007 as base year) and field yield data from 1999 to 2009, drip-irrigated TRI 2023 cultivar returned Net Present Value of Rs 391,779 and Internal Rate of Return of 23%. Sensitivity analysis showed that there is good flexibility in the face of fluctuations of the main cost components. The systems is feasibly against a possible wage rate increase to Rs 700/day (2011 rate = Rs 540/day), and where TRI 2023 should fetch a farm gate price of Rs 54/kg (2011 price range = Rs50 – 65 /kg).
This research program showed that irrigation can be applied as a successful tool in the field to increase productivity and to protect the crop during acute short water stress periods. Climate change predictions for the region are more rain, greater variability of rainfall and higher temperatures. As this research has shown that it is high temperature during short dry periods that impact most on yield, irrigation will still have a future in Sri Lanka even under a predicted higher annual rainfall regime. As the wet zone does not have water storage infrastructure, the next step in promotion of irrigation in the low elevation growing areas will be to demarcate the fields which already have water resources for irrigation and the areas that have potential to develop water resources.
207
208 Appendix 1
Rain Partitioning in a Low Elevation Tea Field
A1.1 Introduction
Tea is grown almost entirely in Sri Lanka as a rain-fed crop. So the precipitation is the main contributor, recharging soil moisture storage. The precipitation is redistributed in tree environment as through fall and stemflow (Martinez-Meza and Whitford 1996). Precipitation, intercepted by leaves and branches collects in the canopy until it is evaporated, drip to soil or routed through canopy (Johnson and Lehmann 2006). Evaporated fraction is termed as interception and varies by storm and tree species characters, sometimes ranging up to 30% of precipitation (Rutter, Kershaw et al. 1972).
Stemflow is the water that collect in canopy and routed down the trunk (Johnson and Lehmann 2006). Throughfall is the water that routed towards trunk but falling to ground before reaching trunk, due to blockages or discontinuities along flow paths (Crockford and Richardson 2000). Understanding throughfall and stemflow is important to understand the above ground process of partitioning rainfall in tea environment.
The processes important in relation to soil moisture storage of plant root system are deep drainage, soil evaporation, runoff, subsurface flow and capillary rise (Allen, Pereira et al. 1998). While the subsurface flow and capillary rise can be considered to have a minimal impact in tea environment, it is important to understand the drainage and runoff to estimate amount of rainfall which is termed as “effective”(Dastane 1974).
Even though there are many attempts to understand the above ground rain partition and water balance in forests and in other annual and perennial crops, there was no attempt to understand the rain water partition and water balance in low elevation tea growing areas, where mixed cultivation of shade and tea cultivations are available. This is an attempt to understand the above ground rain partitioning, runoff, soil evaporation and deep drainage in a low elevation tea field.
A1.2 Materials and Methods
A1.2.1 Study Area
The study was conducted at Field No 01, St Joachim Estate, Ratnapura Sri Lanka (6040’ N, 80025’E, 29 m amsl). Details of the location and climate were given in previous chapters. The
209 field consist of more than 15 years old mature tea, belongs to cultivar TRI 2026. This is a cultivar with large leaves and very popular among growers in the region. The field consisted of large area of Gliricidia maculata (Gliricidia) as medium shade tree and another separate block consisted of Falcataria molucana (Albizia) as high shade tree. Tea plant stemflow and throughfall were however measured at open spaces, where shade canopy was not intercepting the rainfall. Data collection was conducted during South West monsoon period of 2007 and 2008 in between May-June months. Soil evaporation was measured during 11 rainless days in between January- March, 2008.
A1.2.2 Rainfall
Rainfall data was collected from an Automatic Weather station (Measurement Engineering, Australia) installed at the field site. Weather station was placed in between Experiment 1 and 4 sites (described in Chapter 5 and 6), where shade trees were not available. Rainfall data measured through a tipping bucket rain gauge and recorded to Starlogger data logger. Rain gauge was placed above the tea canopy to receive full amount of precipitation. Rainfall amount was measured after each rain event during day time in some selected days. When two rain events were measured in a day, a gap of at least 4 hours was observed for the second event in the day. Four hour gap is considered to be enough for the plant to dry water of the previous rain event.
A1.2.3 Stemflow
Stemflow was measured in 24 tea plants, 10 gliricidia plants and 6 albizia plants. Tea plants grown with gliricidia as a shade plants was selected for the data collection as it has a minimal effect of rainfall interception than albizia. To measure stemflow plastic circular collar was attached to the stem of trees, covering full circle. It was placed tea trunk (before branching) at tea plants and at a 0.5-1m height at gliricidia and albizia plants. Plastic collar was stapled and sealed to tree trunk using non leaking adhesive. Water flowing through was collected in plastic containers. The container was to be buried into soil near the tea plants as the collar was fixed at lower height. After each rainfall event, the amount of water collected was measured and the container was emptied. The canopy cover of each plant was measured approximately to calculate the unit rainfall.
A funnelling ratio (F) is used to express the plants ability in collecting stemflow. It shows the tree canopy divergence of rain water to trunk. The funnelling ratios of tea and gliricidia were plotted for some rain events according to the following equation (Herwitz 1986):
210 ܵܨ ൌ ݒ݈ܨ ܤܣ ൈ ܲ݃
Where SFvol is the stemflow volume, BA is basal area of the tree and Pg is the incident rainfall at the top of canopy.
The average stem areas of the plants were used to calculate the basal areas. The average stem areas at breast height (~1.2m) were calculated for 6 albizia plants and 12 gliricidia plants to calculate the basal area. For tea, 24 plants were sampled at the just above base to calculate average stem basal area.
A1.2.4 Throughfall
Throughfall was measured 12 collectors placed beneath tea canopy, under gliricidia shade. Throughfall collectors were made of PVC pipes with 12cm diameter and 40cm height. Collectors were randomly placed in the field and location of each collector was changed daily after measurements to minimize sampling error.
A1.2.5 Soil evaporation
Soil evaporation was measured using 16 micro-lysimeters, distributed in the site during 11 rainless days. The micro-lysimeters were made up of steel cylindrical tube with 10cm diameter and 15cm height. There were openings at either side of the tube. Tubes were driven carefully into soil in a nearby field, where measurements were not taken. They were then removed carefully with intact soil inside the tubes. For that surrounding soil was removed and a sharp cut was made at the bottom to detach the soil. Once removed, the bottom side was covered with a muslin cloth. Micro-lysimeters were then buried beneath tea bushes carefully with a minimum disturbance to soil in tube or surrounding. Water loss was calculated as the evaporation from the soil by weighing the micro-lysimeter at 800 hours each day and reducing the previous days’ weight. Solar radiation of the day was recorded separately as the water loss through the soil was related to the solar radiation available during the day (Hanks 1991).
A1.2.6 Drainage
A flux meter was buried at a depth of 60cm below the soil surface. It consisted of large opening with a 25cm diameter in a conical shape. The bottom of the opening consisted of a funnel and funnel neck was completely filled with a cotton rope. Before burying in the soil the top part of the flux meter was filled with soil from the same site. This arrangement facilitated soil water to 211 seep down through the cotton rope and a spoon type flow gauge was fixed beneath to measure the water flowing down. A flow gauge was fixed to a data logger and drainage was measured after each rain event.
A1.2.7 Runoff
Surface runoff after rain events was measured using two steel runoff plots. The runoff plots were rectangular steel structures, with 20cm walls. The lower wall of each plot had a flume leading the runoff water out. Runoff collectors were buried 50 cm into soil and large containers were placed in a soil pit beneath the openings. Water collected to as the runoff was measured after each rain event.
A1.3 Results
The industry-recommended number of tea plants and shade plants and their basal area is given in Table A1.1. Albizia showed the highest basal area in the tea field. Since gliricidia plant too had small diameter stem (only 3 times larger than tea stem), it showed the lowest basal area.
Table A1.1 Standard plant density in a one hectare tea field and basal area of plants Stem area Basal area Plant Density/ha (m2/plant) (m2/ha)
Tea 12500 2.06 X 10-3 25.81
Gliricidia 260 6.27 X 10-3 1.63
Albizia 60 4.97 298.04
Table A1.2 shows the frequency of plants and relative dominance of the plants as the percentage of the total basal area of the field, under three different shade plant combinations. Since tea is a densely planted crop, it has the highest frequency under three different shade scenarios. Tea shows a 94% dominance when it is grown under gliricidia. But in other two shade combinations, albizia shade plant is dominant in basal area, though its frequency is small.
Relationship between funnelling ratio of tea and gliricidia plants and rainfall is given in Figure A1.1. Tea clearly shows a reduction in funnelling ration with the increase of rainfall depth. Though it is not quite significantly visible as for tea, funnelling ratio of gliricidia plant too decreased with rainfall depth.
212 Table A1.2 Frequency of plants (%) and relative dominance (%) according to basal area under different shade plant combination in a tea field
Gliricidia only Albizia only Gliricidia + Albizia Shade Relative Relative Relative Frequency Frequency Frequency type dominance dominance dominance % % % % % % Tea 98.0 94.0 99.5 8 97.5 7.9
Gliricidia 2.0 6.0 - - 2.0 0.6
Albizia - - 0.5 92 0.5 91.5
250
tea gliricidia 200
150
100 Funneling ratio
50
0 0 5 10 15 20 25 30 35 Rain (mm)
Figure A1.1 Relationship between funnelling ratio and rainfall according to tree species
Table A1.3 shows the relationship between rainfall and different rain partitioning components in a tea field. In a completely covered tea field, where chances are few for direct incidence of rainfall receiving the soil, only around 40% of rainfall is collected to soil. Out of the soil that received to soil, nearly 8% is lost as the drainage from root zone. Though soil evaporation was measured in a dry season due to practical limitations in the wet season, it also showed a considerably high value in wet season. The average solar radiation of the 11 rainless days, sampled, was 207(±8.5) Wm-2. Accordingly, average soil evaporation during dry season in >1mm even within a tea canopy.
213 Table A1. 3 Relationship between rainfall and partitioning and soil moisture movement in a tea field. Throughfall and stemfall is given only for tea plants. n= no. of rain events sampled/. (Note: Soil evaporation is expressed in relation to solar radiation measured in Wm-2)
Parameter Slope r2 Probability
Throughfall (n=36) 0.106 0.70 0.0001
Stemfall (n=35) 0.322 0.82 0.0001
Drainage (n=22) 0.084 0.43 <0.0001
Runoff (n=21) 0.038 0.51 <0.0001
Soil evaporation (n=11 days) 0.017 0.74 0.0007
A1.4 Discussion
The study evaluated the pattern of different tree species in a tea field, contributing to stemflow of the precipitation. It gives an idea about rain partitioning and effective rainfall in the field. Stemflow is the main rain contributor to the soil moisture storage. Different tree species contribute differently to stemflow, based in their canopy characters. Among them, basal area is a most significant character (Herwitz 1986). According to basal area of shade and tea plants, when the tea is grown with gliricidia, its contribution to stemflow is much lower than tea. But in the presence of albizia as the shade tree, it represents the largest basal area contributing much stemflow. Even though, the relationship between basal area and stemflow was not studied in this study, relationship between basal area and stemflow was established for other trees like, red oak, sugar maple and American beech (Carlyle-Moses and Price 2006). Funnelling ratio of the tea and gliricidia shows a negative relationship with increasing rain event. This is can be due to dripping of more water from the plant before diverting to the stem, due to heavy water flow in high rain storms. Because in higher intensity rainfalls result in increased flow velocities along branches that may exceed the flow capacities of the branches (Herwitz 1986). In compared to gliricidia, tea plant was more efficient in harvesting small rain events with higher funnelling ratio. Presence of comparatively (with stem sizes) higher crown area, branch inclination and shorter distance to travel may contribute to higher capacity of tea plant to collect more stem flow (Steinbuck 2002). The simple water balance model shows a picture about the effective rainfall in low-grown tea field. According to the findings, nearly 40% of the rain enters soil as the stemflow and throughfall. However, in this study, the rain events measured were <40mm. Heavy rain events were very difficult to sampling with over flowing water collectors. The contribution in such
214 large rain events (>40mm) may be higher. Also the rain falling on the open spaces, like in between rows need to be considered. Even under small rain events, >10% is lost from root zone as drainage and runoff. Even though measures like mulching soil with organic matter can be used to minimize the runoff, there is no mechanism to control the drainage. Soil mulching has another advantage to reduce the high soil evaporation in dry days. This is important for irrigated tea field as well, since there is a high chance to lose irrigation water as soil evaporation in dry season. This analysis is important in calculating the soil water balance especially in some inter-monsoon dry periods, when isolated low rain events are occurred. In such cases it is worthwhile to arrange mechanisms to harvest maximum rain amount to root zone and minimize losses.
A1.4 Summary of Conclusions
Albizia as a shade tree has higher dominance in harvesting precipitation as stemflow. Tea plays a major role in producing more stemflow in low rain events. Funnelling ratio reduced with expanding the rain event. After rain event, only 40% of rain reaches soil as stemflow and throughfall. Runoff and drainage losses accounts for nearly 10 of the soil moisture balance in tea root zone. During, dry days, soil evaporation accounts for >1mm moisture loss.
215
216 Appendix 2
A2 Tea Plant Behavior under Water Stress on Different Temperature Regimes
A2.1 Introduction
Tea is mainly grown in high rainfall areas of the tropics and subtropics with its preference to warm wet climates. Other than rainfall, the next most important factor in deciding the geographical distribution of the crop is air temperature. An ambient temperature regime of 18- 300C is considered to be optimal for the best growth and yield of tea (Squire and Callander 1981).
Tea is consumed mainly as either black (fermented), green (non-fermented) or oolong (semi- fermented) beverage (Hampton 1992; Kamunya, Wachira et al. 2009). The production type of the tea is mainly spread around different tea producing countries. Sri Lanka is mainly producing orthodox black tea. Among total Sri Lankan tea production, 94% belong to orthodox black tea category and 98% of total tea production of low elevation tea growing areas of Sri Lanka is orthodox black tea, in 2008 (SLTB 2009).
The price of the tea is determined by the quality that is determined through the chemical composition of the black tea (Wright, Mphangwe et al. 2000). To determine the quality of the made tea, chemical composition of the fresh green leaf can be used satisfactorily (Robertson 1992; Obanda and Owuor 1995; Wright 2005). The quality of the black tea is determined by (a) environmental influences on the quality, (b) manufacturing practices and (c) genetic make-up of the cultivar (Ramaswamy 1964; Astill, Birch et al. 2001; Wright 2005).
An experiment was conducted examining the hypothetical interaction of soil moisture and air temperature, under the controlled conditions. It studies the relationship in Chinese variety tea plants, (Camellia sinensis var. sinensis) rather Assam origin tea (Camellia sinensis var. assamica) as appropriate stock was not available in Australia at the time. The experiment measures the plant growth, photosynthesis, transpiration and changes in quality components water use efficiency of green tea grown over a range of four water stress regimes by two temperature regimes.
A2.2 Method
A2.2.1 Plant material
For the experiment, 15 month old Camellia sinensis var. sinensis plants were used. Plants were provided by a shade house nursery in NSW, Australia (Paradise Plant Nursery of Kulnura, New
217 South Wales). After the delivery to experiment site, plants were transplanted into 12 litre containers filled with University of California mixture (pH 5.5) and grown for two months in a glass house prior to imposing the temperature treatments. Two weeks before commencement of the treatments, plants were uniformly pruned to 60cm height.
A2.2.2 Experimental design
The experimental design was a randomized complete block design where four water stress treatments were assigned for four blocks in two contrasting temperatures. Each treatment consisted of 3 plants in a plot and 5 blocks were prepared inside the growth room. This would ensure equal light availability for all treatments and plots. As the growth chamber (manufactured by Phoenix Biosystems, Australia), could only be set at one temperature regime at a time, the two separate sets of plants were used for the two temperature regimes, each for 21 days.
A2.2.3 Temperature regimes
A low and a high temperature regime were applied which simulate the minimum and maximum temperature levels experienced in low-grown tea areas of Sri Lanka. For the low temperature regime, the growth chamber temperature was kept at 250C during day time (0900 -1900 hrs) and at 180C during night period (1900 – 0900 hrs). For the high temperature treatment, the growth chamber temperature was kept at 330C during daytime (0900 -1900 hrs) and at 260C during night time (1900 – 0900 hrs).
A2.2.4 Water stress regimes
Water stress was applied using different irrigation frequencies, from daily irrigation to zero irrigation for the entire trial period of 20 days. The objective of this water frequency is to simulate the different level of short term drought spells available in low elevation tea growing areas of Sri Lanka, where rain normally falls as few isolated rain events in dry season. Accordingly the four water stress regimes were: 1. Watering the plants daily 100ml (W1) 2. Watering plants at 5 day interval 500ml (W5) 3. Watering plants at 10 day interval 1000ml (W10) 4. Plants were not watered for 20 days (W20).
218 A2.2.5 Light regime
Six light bulbs of 400 watt lamps (Phillips) suspended 0.4m above the tea plants provided the light to the growth chamber. Day length was set as 10hrs and 30 minutes and the light intensity level was varied across the day to approximately simulate diurnal fluctuations in the field (Table A2.1). Maximum Photosynthetically Active Radiation(PAR) of 600µmolm-2s-1 was received by the plants for 7 hours during day time.
Table A2.1 Available Photosynthetically Active Radiation from 0900 to 1930 hours during the treatment period Time PAR Light Intensity (hours) (µmolm-2s-1)
0900 – 1030 60% 360
1030 – 1730 100% 600
1730 – 1930 60% 360
A2.2.6 Measurements
Leaf physiological activities of photosynthesis (Pn), leaf transpiration rate (El), stomatal conductance (gs) and leaf temperature (Tl) were measured at two day intervals using ADC LCApro gas exchange system. (Results are given as average value for the entire period). Topmost mature leaf of the plant was used for the measurements, recorded between 1300-1400 hours. Three samples were taken from each replicate, making 15 records for each treatment. Plant water use of the water stressed plants (W20) was measured by weighing the pots each day.
To measure the shoot growth, shoot extension of selected active shoots were measured at the start and end of trial period. At the end of 20 days, harvestable young shoots were plucked and analysed for the quality components after drying in a micro wave oven, using HPLC. The samples were freeze-dried and packed in dry ice before sending to the Gosford Horticultural Research Centre, New South Wales Department of Primary Industries for analysis. Samples were analysed for theanine, caffeine and catechins.
SAS (Version 9.s) statistical software (SAS Institute, Inc,) was used for statistical analysis.
219 A2.3 Results
A2.3.1 Leaf photosynthesis (Pn)
Both increasing irrigation interval and air temperature negatively affected the leaf photosynthesis (Figure A2.1). There was a significant difference among W1 and W5 treatments under both temperature regimes, showing the importance of daily rainfall or irrigation. When the air 0 temperature level was kept at 25 C during day time, plant Pn was higher for all irrigation intervals. However, the negative effect of increasing irrigation interval acted on a similar pattern in both air temperature levels.
4.5 low temperature (r2 = 0.85, P = 0.07) high temperature (r2 = 0.85, P = 0.07) ) 4.0 1 - s 2 - m 2 3.5
mol CO 3.0
2.5
2.0
Photosynthesis rate ( rate Photosynthesis 1.5
1.0 0 5 10 15 20 25 Irrigation interval (day)
Figure A2.1 Average photosynthetic rate of mature tea leaves at mid-day during treatment period, according to watering frequency
A2.3.2 Stomatal conductance (gs)
Higher stomatal condition was observed under high air temperature level in all 4 irrigation treatments (Figure A2.2). Under both temperature regimes, well watered plants showed the highest gs. Other irrigation intervals, did not show significance difference among each other in both high and low air temperature levels. Even the 5 day water stress caused significant reduction in gs than daily watering plants.
220 0.045 low temperature (r2 = 0.63, P = 0.2) 2
) high temperature (r = 0.75, P = 0.1) 1 -
s 0.040 2 - O m 2 0.035
0.030
0.025
0.020 Stomatal conductanceH (mol
0.015 0 5 10 15 20 25 Irrigation interval (day)
Figure A2.2 Average stomatal conductance of mature tea leaves during midday, according to frequency of watering
A2.3.3 Leaf transpiration (El)
1.2 low temperature (r2 = 0.6, P = 0.2) high temperature (r2 = 0.9, P = 0.05) ) 1 -
s 1.0 2 - O m 2
0.8
0.6
0.4 Transpiration rate (mmol H
0.2 0 5 10 15 20 25 Irrigation interval (day)
Figure A2.3 Average transpiration rate of mature tea leaves during the study period at mid-day, according to different watering frequency
Similar to gs, high air temperature has caused more transpiration of water from leaves (Figure A2.3). Water consumption increased more than 100% when increasing the ambient temperature level. In both temperature regimes, well watered plants showed highest average El. However effect of irrigation interval was not significantly different among each other.
221
A2.3.4 Leaf temperature (Tl)
Leaf temperature (Tl) response to irrigation interval was similar in both air temperature levels (Figure A2.4). In both occasions, lowest leaf temperature was recorded from well watered plant. 0 0 However, when the air temperature level was kept at 25 C, Tl was kept above 2-3 C, above ambient temperature level. But at 330C air temperature level, daily irrigated plants showed lower 0 Tl than air temperature. All other irrigation treatments kept Tl above 33 C. Deviation of Tl from air temperature level (330C) was however confined to <10C.
36
34 330C 32 C) 0 30
28
26 0
Leaf temperature ( temperature Leaf 25 C 24
low temperature 22 high temperature
20 W1 W5 W10 W20 Treatment
Figure A2.4 Leaf temperature of mature tea leaves during mid-day during treatment period according to various water frequency. Reference line shows the high (330C) and low (250C) air temperature
A2.3.5 Water use efficiency (Wi)
Instantaneous leaf water use efficiency (ratio of photosynthesis to respiration) is shown in Figure
A2.5. Highest Wi was shown in low air temperature regime for all irrigation intervals. Except 0 for W20, for all other irrigation intervals, Wi was >100% higher at 25 C. Difference in Wi among irrigation intervals were lesser at high ambient temperature levels.
222 0.8
2 0.7 low temperature (r = 0.9, P = 0.03) high temperature (r2 = 0.8, P = 0.1)
0.6
0.5
0.4 Water use efficiency 0.3
0.2
0.1 0 5 10 15 20 25 Irrigation interval (day)
Figure A2.5 Average instantaneous water use of tea plants according to water application frequency
A2.3.6 Shoot extension rate
2.0
1.8 low temperature high temperature 1.6
1.4
1.2
1.0
0.8
0.6 Shoot extension (mm/day) 0.4
0.2
0.0 W1 W5 W10 W20 Treatment
Figure A2.6 Shoot extension rate during the experiment period for two air temperature levels
223 At high ambient temperature level of 330C, shoot extension rate was >100% higher for all irrigation intervals. There was no significant difference among irrigation intervals at either air temperature levels. On average, well watered plants showed a higher shoot extension rate.
A2.3.7 Quality parameters
Composition of the some of the chemical constituents that are important in determining made tea quality is shown in (Table A2.2). They are caffeine, total catechin level and catechin gallate to catechin ratio [epicatechin gallate(ECG) + epigallocatechin gallate(EGCG) to epigallocatechin(EGC) ratio]. In general the under the high air temperature, concentration of the caffeine was higher than under low air temperature. Though statistically not significant at P=0.05 level, watering interval showed some effect increasing leaf caffeine. More caffeine content was detected in plant leaves when grown at high air temperature levels. However, amount of total catechin was lower at high air temperature levels. Total catechin concentration at low temperature was 28.9(±0.7)mg/g while with high temperature it was 22.1(±1.0)mg/g.
The ratio between ECG+EGCG/EGC varied differently with air temperature. There was no significant difference among watering treatments and the catechin gallate to catechin ratio under low temperature conditions. But with high temperature conditions, watering frequency had a significant relationship with watering frequency. Catechin gallate to catechin ratio increased >300%, when the irrigation interval was increased from daily to 5 day interval at high temperature. At the same high temperature level, when the plants were kept without watering for
20 days, the catechin gallate to catechin ratio increased to nearly fivefold level.
Table A2.2 Main components of made tea quality according to different watering treatment in mg/g
Caffeine Total Catechin (ECG+EGCG)/EGC Treatment low high low high low high
W1 11.9(±0.9) 15.4(±0.9) 30.0(±3.5) 24.9(±2.2) 5.0(±0.5) 8.4(±1.2)
W5 12.5(±0.7) 16.5(±0.9) 29.9(±4.1) 21.9(±1.3) 6.1(±1.1) 27.0(±4.7)
W10 14.0(±1.0) 16.7(±0.8) 28.7(±2.1) 21.7(±1.7) 4.8(±0.4) 25.9(±5.1)
W20 13.3((±0.8) 18.0(±0.6) 27.1(±2.7) 20.1(±1.1) 5.6(±0.7) 37.0(±3.1)
LSD (0.05) ns ns ns ns ns 11.8
224 A2.4 Discussion
In past, experiments were conducted to evaluate the suitable temperature for the tea plants photosynthesis using different clones (Barbora 1994; Joshi and Palni 1998). Also the effect of water stress and seasonal changes in photosynthesis has been studied (Squire 1977; Smith, Burgess et al. 1993; Lin 1998; Hajra and Kumar 1999). However in this experiment, the effect of higher temperature and water stress were studied together.
A2.4.1 Effect on physiology and growth
Photosynthesis is an indicator of plant stress (Nilsen and Orcutt 1996). Both increasing irrigation interval and air temperature had a negative effect on Pn. Since there was similar PAR on both temperature regimes, air temperature can be considered as the environmental parameter that negatively affected Pn. The results support the argument that at high air temperature levels, Pn is controlled due to changes to leaf photosynthetic capacity (Lin 1998). The results contradicts the argument that Pn is controlled by stomatal activities (Barbora 1994). However, plants are very sensitive to irrigation frequency under both air temperature levels. Best irrigation management to keep higher Pn is the daily irrigation or rain.
But in overall, Pn was lower than in field conditions, as reported in field experiments and experiments in elsewhere (Hajra and Kumar 1999). This could be due to the low availability of PAR within the chamber. The available average light intensity inside the chamber leaf surface is 370(± 7.2) µmolm-2s-1, which is much lower than the saturation light level of 735 µmolm-2s-1 (Sakai 1987).
High ambient temperature caused the plants to lose more water, as shown by high gs and El. So the correct moisture maintenance is more crucial in hot environments like, low elevation tea growing area. Also higher transpiration is related to higher dry matter production in the plant, which ultimately resulted in high yield. This was evident in this experiment through increased shoot extension rate and in previous experiments. But even 5 day irrigation interval caused transpiration process to reduce significantly than daily watering.
Leaf temperature, which is an important parameter controlling photosynthesis and the yield, when reached to excessive levels (Hadfield 1968), have behaved differently under two temperature levels. In both temperature levels, well watered plants have been able to maintain a lower leaf temperature value. However when the air temperature was kept at 330C, leaf temperature has been higher than air for water stressed plants. But when the air temperature is
225 kept at 250C, all plants leaf temperature values have been kept above the air temperature. Daily plant watering is important to maintain a favourable leaf temperature level, in areas with high ambient temperature like low elevation tea growing areas of Sri Lanka. Otherwise leaf temperature could reach to critical values of 350C or above as reported by (Bannerjee 1993). Under the 5 day watering treatment (W5), the leaf temperature seems to be little higher than other treatments, under both temperature levels. The plants of W10 and W20 treatments, may have adapted other techniques like, folding the leaves to minimize the temperature damage. With relation to irrigation under high temperature levels, it can be hence assumed that regular watering is the best way to minimize the leaf temperature increase.
According to experiment, it showed that Wi was lower and less sensitive to irrigation interval at high ambient temperature. But, in contrast, daily watering has leads to higher water use efficiency under low temperature. So in low temperature tea growing locations, like mid and high elevations of Sri Lanka, daily irrigation during dry season can be used to increase low productivity. Shoot extension rate was not affected by the irrigation interval at either air temperature levels. This may be due to short duration of the experiment period. But, the high temperature increased shoot extension rate.
A2.4.2 Effect on quality
The shoot chemical composition of the young tea shoot grown under two different temperature levels were studied as a response to different irrigation intervals. The results indicate a significantly different behaviour of main important chemical constituents, with relation to their growing environment and moisture stress as imposed by different watering regimes. In overall the results indicate the quality of the black tea produced can be controlled by controlling soil moisture and air temperature. One popular method of quality improvement in made tea was accomplished by different cultivar selection within a given region (Wright 2005; Owuor, Obanda et al. 2008). But this experiment, quality aspect of made tea can be altered with the maintaining the water stress level in the field.
The caffeine content is the chemical compound that involves in tea cream and it may influence the briskness of the tea (Robertson 1992). Though there is a trend of increasing caffeine content with increasing duration of water stress, difference was not significant among watering interval. However, as there is a consumer concern about the amount of caffeine content in tea, irrigation can be used to control the increasing caffeine content under hot growing conditions.
226 The quality index of ECG+EGCG/EGC was found to be directly related to sensory properties of green tea and is used to assess the quality of tea in China (Liang, Liu et al. 1990). It is also used for assessing the seasoning variation of fresh shoots (Yao, Caffin et al. 2005). Under high temperature conditions, watering frequency, the ratio increased significantly. The high ratio was quite extra ordinary except for daily watering treatment, as such high variation were not found among seasonal variation in field level (Yao, Caffin et al. 2005). However, this ratio is not so significant in black tea production as it is mainly used to detect the quality of green tea (Harbowy, Balentine et al. 1997). Even though, low elevation tea growing areas of Sri Lanka produce almost entirely black tea, understanding the quality changes are important in future product diversification.
A2.4.3 Effect of physiological process on quality
Caffeine content changes according to Pn and El is given in Figure A2.7. Both increasing, Pn and
El, reduced the caffeine content. As the concentration of caffeine is higher under high air temperature, irrigation interval is more important in hot growing areas. As the transpiration process had little impact from the irrigation interval, maximizing Pn by daily irrigation is the best possible way to reduce the caffeine content. Also in field conditions, as the measurement of Pn and El is easy and not damaging the plant, detection of Pn and El can be used to predict the made tea quality. Even large area can be sampled with minimal damage in a short time.
19 (a) (b) low temperature (r2 = 0.69, P = 0.1) 18 high temperature (r2 = 0.92, P = 0.03)
17
16
15
14 Caffeine (mg/g) Caffeine
13 low temperature (r2 = 0.93, P = 0.02) high temperature (r2 = 0.98, P = 0.005) 12
11 12340.4 0.6 0.8 1.0 1.2
Photosynthesis rate (mol CO m-2s-1) Transpiration rate (mmol H O m-2s-1) 2 2 Figure A2.7 Relationship between average photosynthesis during the experiment and caffeine content (a) and transpiration and caffeine content (b)
227 A2.5 Summary of Conclusions
Plant photosynthesis rate had significant negative relationship with increasing water stress days under high temperature conditions When the water stress days were increased, stomatal conductance too fell, but did not show a strong relationship. However transpiration had a close linear relationship with watering frequency under high temperature condition. Plants showed a high water use efficiency under low temperature conditions, and there was a strong negative relationship too with watering frequency under low temperature levels. The shoot extension was significantly higher under high ambient temperature levels. Made tea quality has strong relationship with drought stress in high temperature levels.
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