Effects of Water Deficits on Prosopis Tamarugo Growth, Water Status And
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plants Article Effects of Water Deficits on Prosopis tamarugo Growth, Water Status and Stomata Functioning Alson Time 1,2,* and Edmundo Acevedo 2,* 1 Programa Magister en Ciencias Agropecuarias, Facultad de Ciencias Agronómicas, Universidad de Chile, Santa Rosa, La Pintana, Santiago 11315, Chile 2 Laboratory Relation Soil-Water-Plant (SAP), Department of Agricultural Production, Faculty of Agronomic Sciences, University of Chile, Santiago 1004, Chile * Correspondence: [email protected] (A.T.); [email protected] (E.A.) Abstract: The effect of water deficit on growth, water status and stomatal functioning of Prosopis tamarugo was investigated under controlled water conditions. The study was done at the Antumapu Experiment Station of the University of Chile. Three levels of water stress were tested: (i) well-watered (WW), (ii) medium stress intensity (low-watered (LW)) and (iii) intense stress (non-watered (NW)), with 10 replicates each level. All growth parameters evaluated, such as twig growth, specific leaf area and apical dominance index, were significantly decreased under water deficit. Tamarugo twig growth decreased along with twig water potential. The stomatal conductance and CO2 assimilation decreased significantly under the water deficit condition. Tamarugo maintained a high stomatal conductance at low leaf water potential. In addition, tamarugo reduced its leaf area as a strategy to diminish the water demand. These results suggest that, despite a significant decrease in water status, tamarugo can maintain its growth at low leaf water potential and can tolerate intense water deficit due to a partial stomatal closing strategy that allows the sustaining of CO2 assimilation in the condition of reduced water availability. Keywords: water potential; drought; water depletion; twig elongation Citation: Time, A.; Acevedo, E. Effects of Water Deficits on Prosopis tamarugo Growth, Water Status and 1. Introduction Stomata Functioning. Plants 2021, 10, 53. https://doi.org/10.3390/ Water is one of the most limiting factors for plant growth. Thus, for fighting stress plants10010053 caused by water limitation, plants have developed mechanisms that allow them to cope with drought caused by water shortage. In presence of environmental stress whose influ- Received: 11 December 2020 ences exceeds the resistance limit for the survival of the woody plants, the plants could Accepted: 22 December 2020 activate some physiological and biochemical mechanisms that allow them to resist those Published: 29 December 2020 stressors. Nevertheless, that resistance may depend on the flexibility of these mechanisms, the compensatory abilities, the intensity and the duration of the stressors [1]. Presently, Publisher’s Note: MDPI stays neu- a great number of forest plants’ biomes have experienced an increasing mortality rate, tral with regard to jurisdictional clai- which is associated with rises in temperature and a greater incidence of drought [2]. Human ms in published maps and institutio- activities can also produce stresses in the future that can significantly affect the already nal affiliations. stressed environment [3]. This is the situation of the Pampa del Tamarugal (Atacama Desert, Chile), which is described as a hyper-arid desert mostly populated by Prosopis tamarugo. Tamarugo, which is a strict phreatophyte plant, is growing under a water table reduction condition due to groundwater extraction for the mining industry, agriculture Copyright: © 2020 by the authors. Li- censee MDPI, Basel, Switzerland. supply and urban areas [4]. This article is an open access article The effects of water deficit on plants are well studied. Reports demonstrate that water distributed under the terms and con- deficit can affect several variables and functions in plants, such as stomata functioning [5], ditions of the Creative Commons At- hydric traits such as water potentials and xylem hydraulic conductivity [6–8] and growth tribution (CC BY) license (https:// traits like twig growth, specific leaf area and leaf shoot ratio [9,10]. The functioning of these creativecommons.org/licenses/by/ parameters and other processes may determine the growth capacity and the survival of the 4.0/). plant [11]. Plants 2021, 10, 53. https://doi.org/10.3390/plants10010053 https://www.mdpi.com/journal/plants Plants 2021, 10, 53 2 of 11 The regulation of stomata in plants determines the consumption of and assimilation of CO2 and can influence the ability of plants to survive extreme conditions of water stress. In a water stress condition due to drought, the opening of the stomata is affected by the soil and plant water content. It has been demonstrated that stomata remain unaffected until the leaf water potential drops to some critical threshold value [12]. The closure of stomata in a water stress situation is considered one of the main factors that limit photosynthesis [13]. However, this depends on the plant’s stomatal behavior [11], as plants fall into two categories: isohydric and anisohydric. Under water stress, isohydric plants tend to close their stomata earlier than anisohydric plants, which delay their stomata closure, as well as enhance gas exchange and let the leaf water potential decrease as the soil water potential decreases [5,14]. Isohydric plants generally experience a negative carbon balance, while anisohydric plants experiment low water potential, which may provoke hydraulic failure by xylem cavitation and embolism. Prosopis tamarugo, a desert plant, has developed several mechanisms of adaptation that allow survival in the extreme environmental conditions where it grows. There are reports of mechanisms such as the capacity to maintain high stomata conductance when subjected to increasing temperature and atmospheric water demand [15,16], partial closure of stomata to decrease water loss, angle changes of its leaflets to avoid high levels of radiation in the afternoon [17] and osmoregulation, which is a solute accumulation in the cells allowing a positive turgor pressure development in spite of low water potential. Studying the aboveground growth response of tamarugo to the intensity and duration of water stress is of interest, as it may lead to guaranteeing the survival of tamarugo and save it from extermination provoked by induced groundwater depletion. Growth of tamarugo was studied under the semi-controlled condition in the An- tumapu Experiment Station of the Faculty of Agronomy of the University of Chile during the spring of 2016, with the objective of determining the effects of the water stress on the water status growth and the stomata functioning of P. tamarugo. 2. Results 2.1. Effect of Water Stress on Tamarugo Growth, Isotopic Discrimination of 13C and Enrichment of 18O A clear significant difference was observed for the twig growth rate in all watering levels. The well-watered plants had a high growth rate in all sampling dates (Table1). Table 1. Summary analysis of tamarugo growth parameters, Isotopic Discrimination of 13C and Enrichment of 18O under three water conditions. Means and standard errors (±S.E.) of twig daily growth rate, specific leaf area (SLA), branching architecture (BA) expressed as Apical Dominance Index (ADI), isotopic discrimination of 13C(D 13C) and enrichment of 18O (δ18O) of P. tamarugo observed under three water levels. Growth Parameters/Water TGR BA (ADI) SLA (cm2 g−1) D 13C (‰) δ18O (‰) Conditions (cm day−1) (cm−1) WW 0.80 ±0.06 a 123.3 ±3.14 a 0.06 ±0.01 b 19.2 ±0.15 a 28.8 ±0.17 b LW 0.34 ±0.06 b 113.3 ±3.14 b 0.04 ±0.01 b 17.7 ±0.15 b 29.4 ±0.17 a NW 0.18 ±0.06 c 117.9 ±3.14 b 0.08 ±0.01 a 17.8± 0.26 b 29.8± 0.17 a Different letters indicate significant difference for water conditions according to the DGC test (α = 0.05). WW: mean of well-watered; LW is the mean of low watered and NW is the mean of no watered. Tamarugo growth rate had a very high sensitivity to water deficit as shown in Figure1A. Growth detention was observed at between 15 to 20 days in the non-watered plants. By rewatering the non-watered plants with a liter of water, after the growth rate had reached zero in the fourth sampling date, growth resumed (Figure1A). PlantsPlants 20212021,, 1010,, x 53 33 of of 11 11 Figure 1. The Y axis shows growth and isotopic discrimination of 13C and enrichment of 18O as a function of three irrigation Figure 1. The Y axis shows growth and isotopic discrimination of 13C and enrichment of 18O as a function of three irrigation conditions between January and March 2016 (X axis). Twig growth (A), specific leaf area (B), isotopic discrimination of conditions between January and March 2016 (X axis). Twig growth (A), specific leaf area (B), isotopic discrimination of 13C 13 C D 18 E (CC(), tamarugo), tamarugo branching branching architecture architecture (D), enrichment ( ), enrichment of 18O of (E) O(and )leaf and water leaf water potential potential vs. twig vs. growth twig growth rate under rate underthree waterthree waterlevel and level six and sampling six sampling dates dates (F). ( (FF)). is (F a) issimple a simple linear linear regression regression of oftwig twig growth growth rate rate vs. vs. mean mean of of predawn predawn and and middaymidday water water potential potential corresponding corresponding to to the the same same evaluation evaluation da days.ys. Values Values represent represent the the mean mean of of twig twig growth growth rate rate for for six six samplingsampling dates dates of of the the evaluation evaluation period period as as a a function function of of mean mean leaf leaf water water potential. potential. Each Each point point represents represents the the mean mean growth growth raterate of of ten ten plants. plants. The The growth growth rate rate was was calculated calculated through through the the interpolation interpolation and and the the water water potential potential values values represent represent the the meanmean of of predawn predawn and and midday midday leaf leaf water water potential potential together together fo forr the the same same period period of of evaluati evaluation.on.