INFLUENCE OF NUTRIENT SOLUTION AND SOLUTION pH
ON ONION GROWTH AND MINERAL CONTENT
by
CHAD D. KANE, B.S.
A THESIS
IN
SOIL SCIENCE
Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE
Approved
Chairj^rs(& of the Committee
Accepted
Dean of the Graduate School
August, 2003 ACKNOWLEDGEMENTS
I would like to thank my committee members: Dr. Green, Dr. Peffley, and
Dr. Thompson for their guidance, direction, and knowledgeable assistance during my graduate career, especially Dr. Green for his unlimited patience and understanding.
I appreciate the help Jay M., Jeremy J., Janet H., Brent W., Amanda B.,
Katie P., Clint S., Amanda H., Marci B. and Brad M. (and anyone else who assisted me in the greenhouse or lab) gave me throughout my career.
Thank you Vronka Stoker and Jennifer Collins. All your help with the little things made this research opportunity enjoyable and interesting. I don't believe that another group of people could have accomplished the things that we have, with all the distractions that we frequently encountered.
A very special thanks to Dr. Richard Jasoni for serving as a true mentor to myself I don't believe that I have learned as much valuable information from one person throughout my college career as I have from you. Thank you for being extremely patient, understanding, available, and for the endless guidance throughout these past two years.
I would also like to thank my family for their endless encouragement and support from the very beginning, especially Mom and Dad. A special thank you to my wife Christy for being exceptionally supportive and for the endless and diligent patience. TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
ABSTRACT v
LIST OF TABLES vii
LIST OF FIGURES viii
CHAPTER
I. LITERATURE REVIEW 1
Introduction 1
Onions 2
Beneficial Components 3
Hydroponics 5
Nutrient Solution 7
Objectives 12
II. MATERIALS AND METHODS 17
Growth Conditions 17
Phenotypic and Mineral Content Measurements 19
Experimental Design and Analysis 21
III. RESULTS 22
Phenotypic Variables 22
Mineral Content Variables 25
IV. DISCUSSION 36
ill V. CONCLUSIONS 40
REFERENCES 42
APPENDIX 47
IV ABSTRACT
This study is a component of a project designed to develop a management strategy for growing onions in a closed growth system on a vehicular space setting. The objective of this research was to evaluate the effects of hydroponic nutrient solution and solution pH on growth and mineral content of green onions. Three onion varieties, Allium cepa L.
('Deep Purple' and 'Purplette') and A. fistulosum L. ('Kinka'), were propagated in three nutrient solutions (Peter's Hydro-Sol, Hoagland's, or half strength Hoagland's), at two pH levels (5.8 and 6.5), in a three by two factorial design applied in a randomized block with three replications.
Seeds were germinated in Cropking's Oasis Horticubes™ under greenhouse conditions, and were irrigated with tap water. Once the seedlings reached the flag stage, the plants were placed into hydroponic units within the greenhouse and grown under ambient conditions. Plants were harvested 30 days after transplanting to the hydroponic units.
Based on efficient plant growth, the half strength Hoagland's solution is the preferred nutrient solution evaluated in this research.
However, Hydro-Sol generally produced onions with highest the mineral content. Mineral content varied with plant part,nutrient, nutrient solution, solution pH, and onion variety. Selection of an appropriate nutrient solution must consider both edible biomass production and mineral content. In the research reported here the solution that produced the greatest biomass did not produce plant material with the mineral content.
Future research may lead to the development of a modified nutrient solution that optimizes both edible biomass production and mineral content.
VI LIST OF TABLES
1.1 Baseline crops for advanced life support program 14
1.2 Percentage of the Daily Recommended Values (DRV) and Recommended Daily Intake (RDI) of mineral concentrations in onions 15
1.3 Hydroponic nutrient solution compositions 16
3.1 Main effects of nutrient solution, pH, and onion variety on neck diameter (ND), longest leaf midpoint diameter (LLMPD), pseudo-stem length (PL), longest leaf length (LLL), longest root length, and leaf number (LN) 30
3.2 Main effects of nutrient solution, pH, and onion variety on shoot mass (SM), bulb mass (BM), root mass (RM), total biomass (TB), edible biomass (EB), and percentage edible biomass (%EB) 31
3.3 Nutrient solution by pH interaction effects on shoot dry matter percentage, bulb Ca, and Bulb Zn 32
3.4 Main effects of nutrient solution, pH, and onion variety on dry matter percentage, ash percentage, and seleced minerals 33
3.5 Nutrient solution by variety interaction effects on bulb ash 35
VII LIST OF ABBREVIATIONS
ALS Advanced Life Support
AOAC Association of Analytical Chemists
B Boron
C Celsius
Ca Calcium
Ca(N03)2 Calcium Nitrate
CELSS Controlled Ecological Life Support System
CI Chlorine
CO Colorado
CT Connecticut
Cu Copper
CUSO4 Cupric Sulfate d Day dap Days After Planting
DRV Daily Reference Value
Fe Iron
FeS04 Iron Sulfate
GLM General Linear Model
H3BO3 Boric Acid ha Hectare
VIII HCI Hydrochloric Acid
JSC Johnson Space Center
K Potassium
KNO3 Potassium Nitrate
KOH Potassium Hydroxide
LaCb Lanthanum Chloride
Lat. Latitude
LLMPD Longest Leaf Midpoint Diameter
Long. Longitude
MA Massachusetts
Mg Magnesium
MgS04 Magnesium Sulfate
Mn Manganese
MnCl2 Manganese Chloride
Mo Molybdenum
MS Mississippi
N Nitrogen
Na EDTA Sodium Ethylenediamineteraacetic acid
Na Sodium
Na2Mo04 Sodium Molybdate
NaCI Sodium Chloride
NASA National Aeronautics and Space Administration
IX NH4*-N Ammonium Nitrate
NH4H2PO Ammonium Phosphate
Ni Nickel
NJ New Jersey
NOa" Nitrate
OH Ohio
PVC Polyvinyl Chloride
RDI Recommended Daily Intake
S Sulphur
s Second
SAS Statistical Analysis Software
TX Texas
U.S. United States
Zn Zinc
ZnS04 Zinc Sulfate CHAPTER I
LITERATURE REVIEW
Introduction
During extended space missions such as Mars exploration, or establishing bases on the lunar surface, humans will continue to need food, water, and air
(Lawson, 2003). It is not practical or economical to re-supply basic life support elements from Earth for these long duration missions (Lawson, 2003). The
National Aeronautics and Space Administration (NASA) needs to develop systems that produce food, purify the water supply, regenerate oxygen and remove undesirable components of the air (Lawson, 2003). In the late 1980s,
NASA developed a regenerative life support system to develop systems for long- term space flight (Barta & Henninger, 1996). The purpose of the controlled ecological life support system (CELSS) program is to develop a large-scale integrated testing bed for plant growth with physiochemical life support subsystems that would provide oxygen and food production for astronauts (Barta
& Henninger, 1996). Plant growth in controlled environments can be optimized by closely controlling environmental light intensity, photoperiod, temperature, and nutrient solution composition.
Scientists and engineers specializing in food production and processing and human nutrition within the Advanced Life Support (ALS) program comprised a list of candidate crops for development as food crops (Lawson, 2003). Candidate crops need to be highly productive over a short time period, contain high nutritional value, and be waste limiting (Lawson, 2003). The candidate crop list was to be limited in order to maximize the degree to which the readiness technology level of each individual crop could be improved. Among the list of candidate crops identified for space vehicle food systems was the onion (Allium cepa L.) (Table 1.1) (Lawson, 2003).
Onion is one of the least studied of the 15 ALS baseline crops. The onion is a good candidate crop because of its high productivity of edible biomass; furthermore, the carbohydrate storage bulb structure facilitates studies of source sink relationships in regulating plant response to environmental conditions.
Onion has unusual morphology, growth characteristics, and biochemical composition including essential oils, secondary metabolites, and nutritive phytochemicals. The data collected would fill gaps in the knowledge of the physiology and the controlled environmental production of onion.
Onions
Onions are in the genus Allium, in the Alliaceae family (Maynard &
Hochmuth, 1997). Onion ranks among the most important vegetable crops worldwide, with a production of 37 million tons in 1998 (Goldman & Schroek,
2001). The onion trails only tomato (Lycopersicon esculentum Mill.), potato
(Solanum tuberosum L.), and lettuce (Lactuca sativa L.) in value among all vegetable crops in the U.S. The U.S. is the third largest onion producer in the world, with 3 million tons produced on approximately 64,751 ha in 1998 and a value of $830 million (Schroek & Goldman, 2001). The annual value of the U.S. onion crop is $800 million at the farm level, and roughly $3-4 billion at retail
(National Onion Association, 2001) average annual onion consumption is approximately 6.2 kg of onions per person worldwide (National Onion
Association, 2001).
Beneficial Components
Onions contain a flavonoid (antioxidant compound) called quercetin, which delays oxidative damage to cells and other bodily tissues (National Onion
Association, 2001). Quercetin is the most abundant flavonoid in the human diet, and is mainly found in onions (Duthie & Dobson, 1998). Onions contain the highest amount of quercetin among commonly consumed fruits and vegetables
(Hertog et al., 1992), but other foods such as red wine, apples (Malus pumila), kiwifruit (Actinidia deliciosa), kale (Brassica oleracea L.), and green and black teas (Camilla sinesis) also contribute quercetin to the diet (Ahmad & Mukhatar,
1999; Dawes & Keene, 1999; Frankel et al., 1995; Goldberg et al., 1998). Other foods containing quercetin are lettuce, leeks (Allium ampeloprasum L. porrum group) and cranberries (Vaccinium macrocarpon) (Hertog et al., 1992; Hertog &
Hollman, 1996). Studies done at Wageningen Agricultural Unviersity, the
Netheriands, showed twice as much quercetin is absorbed from onion relative to tea, and three times as much quercetin is absorbed relative to apple (National
Onion Association, 2001).
Onions are a good source of vitamin C, K, dietary fiber, folic acid, Ca, Fe and contain a high protein quality, while at the same time are low in Na and contain no fat (National Onion Association, 2001). Table 1.2 shows the nutrition per serving of onions, and the percentage of the U.S. Daily Values or U.S.
Recommended Dietary Intakes for food labels (adults and children 4+ years)
(National Onion Association, 2001).
Previous studies have shown that long durations of microgravity can influence significant bone loss (Lane et al., 1999). The microgravity environment during space flight cause the bone mass and the levels of hormones that regulate Ca in the body to significantly decrease (Lane et al., 1999).
Furthermore element deficiencies such as Ca, Fe, Mg, and I create well-defined symptoms of illnesses in humans (FainA/eather-Tait & Hurrell, 1996). Recent studies have indicated that onions are a good source of Ca and other important minerals (Sanchez-Castillo et al., 1998). Higher concentrations of Ca were observed in three varieties of onion compared to other varieties, the onion bulb compared to other plant parts, and white onion compared to others (Sanchez-
Castillo et al., 1998). Fertilization, the age of the plant, and the chemical composition of the medium in which the crop is grown,can all affect the mineral composition of the plant (Sanchez-Castillo et al., 1998). Varieties and species differ in their ability to absorb nutrients, and these differences may be due to the differences of the root systems or the specific transport enzymes in cell membranes (Sanchez-Castillo et al., 1998).
Hydroponics
Hydroponics have been defined as the practice of growing plants using only water as a substrate with the addition of essential nutrients, and is one of many methods used in nutrient delivery systems (Goins et al., 1997; Resh,
1998). The word "hydroponic" is derived from the Greek words "hydro" meaning water, and "ponos" meaning labor (Jones, 1997; Resh, 1998). Hydroponics have been used for centuries, dating back to the Hanging Gardens of Babylon and the
Floating Gardens of the Aztecs in Mexico (Jones, 1997). Hydroponics have gradually evolved and were used during World War II to supply large amounts of fresh vegetables to troops stationed in and around islands in western Pacific
(Jones, 1997).
Since the 1980s hydroponics have been commercialized for vegetable and flower production, and recent reports show that there are over 60,000 acres of vegetables growing hydroponically in greenhouses worldwide (Jones, 1997).
At least one substantial hydroponic greenhouse industry can be found in almost every state (Resh, 1987).
Although hydroponics have been around for centuries, it is still considered a very young science. Hydroponics have only been used commercially for about
40 years (Resh, 1987). In a short time frame, hydroponics have been adapted to many different situations, ranging from outdoor field culture, indoor greenhouse culture, to a highly specialized culture to grow fresh vegetables for crews in atomic submarines (Resh, 1987). Hydroponics is a space age science, but at the same time can help in the development of the Third World (Resh, 1987). Places that were considered too barren to cultivate, such as deserts, the Arctic, and space can now be utilized by today's hydroponic fanner (Jones et al., 1998).
Several advantages and disadvantages of hydroponic culture were listed in 1981, which were still applicable in recent studies (Jones, 1997). The advantages include the following:
1. Crops can be grown where unsuitable soil conditions exists.
2. The labor costs for conventional practices are significantly reduced.
3. The system is economically feasible in high density and expensive
land areas, because maximum yields are achievable.
4. Conservation of water and nutrients is a feature of all systems.
Valuable chemicals are not lost, reducing pollution.
5. Soil-borne plant diseases are more readily eradicated in closed
systems, which can be totally flooded with an eradicant.
6. Environmental control is generally a feature of the system (i.e., root
environment, timely nutrient feeding or irrigation), and composition of
the air can be manipulated.
7. Water carrying high-soluble salts may be used if done with extreme
care. If the soluble salts in the water supply are over 500 ^imol mo!"'' an open system of hydroponics may be used if care is given to
frequent leaching of the growing medium to reduce the salt
accumulations.
8. The amateur horticulturist can adapt a hydroponics system to home
and patio-type gardens, even in high-rise buildings. A hydroponics
system can be clean, lightweight, and mechanized.
The disadvantages are the following: 1. Original construction costs per acre are great.
2. Trained personnel must direct the growing operation. Knowledge of
how plants grow and the principles of nutrition are important.
3. Introduced soil-borne diseases and nematodes may be spread quickly
to all beds on the same nutrient tank of a closed system.
4. Most available plant varieties adapted to controlled growing conditions
will require research and development.
5. The reaction of the plant to good or poor nutrition is unbelievably fast.
The grower must observe the plants every day.
System supports, water, nutrients and root aeration factors must be considered when using hydroponics, since plants are without soil (Jones et al.,
1998). In order for this to be accomplished, Oasis^"^ Horticubes (Smithers-Oasis,
Kent, OH) are often used because they are sterile, provide good drainage, are easy to handle, and have a stable pH (Resh, 1991). High plant growth rates that produce a constant yield may be maintained in a relatively small root zone by growing plants in hydroponically (Steinberg et al., 2000). Constant maintenance and pH monitoring are the main concerns when using hydroponic solutions.
Studies have shown that large differences in yields can occur between soil and soil-less vegetable production, with soil-less systems producing higher yields
(Bentley, 1959).
Nutrient Solution
Hydroponics are a good method for research under controlled conditions of nutrient availability (Lawson, 2003). Most modern hydroponic solutions are
based on the work of Hoagland and Arnon (1950) and have been adapted to
numerous crops (Whipker & Hammer, 1998). Seventeen elements are considered essential for normal growth and development of higher plants (Arnon
& Stout, 1939; Marschner, 1995). All of these elements are absorbed by the
roots through the root-zone media, except C, which is absorbed from the atmosphere by the shoots (Spomer et al., 1997). The elements Mg, Ca, K, P, N, and S are considered macronutrients because they are required in relatively
large concentrations in plant tissue (Spomer et al., 1997). The remaining elements (Fe, CI, B, Mn, Zn, Cu, Mo, and Ni) are considered micronutrients
because they are required in lower concentrations (Spomer et al., 1997).
Spomer et al. (1997) describes a complete nutrient solution formulation of the required nutrients commonly utilized for regular watering of plants grown in growth chambers. Spomer et al. (1997) recommends a nutrient solution equal to about one half the strength of the original Hoagland's nutrient solution (Hoagland
8 & Arnon, 1950). Hoagland's nutrient solution is a baseline nutrient solution, which was developed and published in a popular manuscript on the general subject of growing plants in nutrient solutions (Hoagland & Arnon, 1950).
Since many nutrients need to be added individually, preparing the
Hoagland's nutrient solution can be a laborious process. A premixed hydroponic fertilizer called Hydro-Sol (Peter's Hydro-Sol, Scotts-Sierra Horticultural Product
Co., Marysville, OH) is commercially available, and recent studies performed by
Texas Tech University Department of Plant and Soil Sciences have shown increased growth when plants were grown in Hydro-Sol.
To optimize growth both in the greenhouse and in the field, crops should be provided with appropriate levels of inorganic nutrients (Siddiqi et al., 1998).
Certain inorganic nutrients applied at excessive levels may be detrimental to plant growth (Siddiqi et al., 1998). Chen et al. (1997) found that the growth of lettuce was significantly increased when the NO3" concentration of the solution was reduced below the highest concentration being used by a local commercial hydroponic grower. It has been reported that the N, P, and K concentrations could be reduced to |JM concentrations without affecting the growth and accumulation of these ions in some crop species and pasture crops (Asher &
Ozanne, 1967; Clement et al., 1978; Siddiqi & Glass, 1983). Barker and Mills
(1980) reported that high concentrations of NH4*-N in solution can be toxic to plants, but other studies reported when NH4*-N was supplied at low concentrations or specific stages an increase in plant growth occurred. Conover and Poole (1986) reported that the grade, length, and height of several horticultural crops were increased when N sources contained 25% to 100%
NH4*-N. Whipker and Hammer (1998) reported that 12.5 % to 33% of the N in hydroponic solutions should be in the NH4* form. The modified Hoagland's used by Jasoni et al. (2002) contained 16.67% NH4'" and Peter's Hydro-Sol contained
100% NOa'.
Trewanas (1983) reported that NO3' frequently plays a role in plant developmental processes such as onion bulbing, which involve dormant structure formation and an increase in soluble carbohydrate to N ratio. Onion bulb weight to leaf blade ratio increased with a decrease in soil N levels (Brewster & Butler,
1989). Applications of N at early stages of growth promote onion bulbing, while lower levels delay it (Henriksen, 1987).
Excess N can encourage foliar growth and depress onion bulb grov\4h
(Brewster, 1990), but in later stages of the plant development, can result in formation of soft bulbs and may prolong the maturation process affecting product handling and post harvest quality of bulbs (Riekels, 1977). Stroehlein and
Oebker (1979) reported chili peppers (Capsicum frutescens L.) grown in excess
N (N > 280 kgha'"") produced excessive foliage, at the expense of fnjit production. Randle (2000) found that with an increased N concentration (0.97 g-L'^) in a hydroponic solution, onion bulb weight and firmness decreased, while yield increased. This study also determined that an increase in N content in hydroponic solutions used to propagate onions increased total N content.
10 decreased B, Ca, Mg, increased K contents, but showed no direct effect on Cu,
Fe, P, and Zn (Randle, 2000). Siddiqi et al. (1998) reported that when using hydroponic systems, NOs', P, and K concentrations may be reduced up to 25% of the concentrations that were currently being used in commercial greenhouses, without any adverse effects on tomato fmit yield or quality. No adverse effects on tomato morphology (e.g., dry matter, elemental composition, appearance, size, and shape) were observed for tomatoes grown at lower nutrient concentrations (Siddiqi et al., 1998).
Asher and Ozanne (1967) found that increased K in the nutrient solution increased K content and yield of both shoots and roots of several pasture crop species. Decreases were observed in rootishoot ratios and dry-matter percentage of fresh shoots and roots when K increased (Asher & Ozanne, 1967).
Increased N concentrations resulted in increased tomato plant height and leaf length, and increase flower number and marketable fruits per plant (Adams et al.,
1973). The percentage of K in leaves and total uptake of K by tomatoes were significantly controlled by the N concentration in the nutrient solutions (Adams et al., 1973).
In addition to nutrient concentration, plant growth can be influenced by pH.
Onion crops can be successfully produced on most fertile soils (Brewster, 1994), but soil pH in the range of 6-7 is usually recommended; on organic soils a lower pH is recommended (Brewster, 1994). Soil pH is very important in soil fertility due to its effects on the solubility and potential availability or phytotoxicity of
11 some plant nutrients and nonessential elements (Bloom, 2000). Soil pH has a direct relationship with the biological activity of plants and microorganisms
(Bloom, 2000). Bloom (2000) reported that most element's solubility increases as the soil becomes more acidic; exceptions included were P which is most available at a pH of 5.5 to7.5, and Ca and Mo which are more available at pH greater than 7. Brady and Weil (2000) reported that even with a difficulty in finding the relationship between soil pH and plant nutrient availability, the pH range of 5.5 to 7.0 might provide the most adequate plant nutrient levels.
A nutrient solution's pH is a property that is inherent to its composition
(DeRijck & Schrevens, 1997). Even though there is a broad range for optimal pH, a pH of 5.8 is best for optimal nutrient availability in hydroponics (Bugbee,
2003). Islam et al. (1980) reported that a pH range of 5.5 to 6.5 is optimal for the availability of nutrients from most nutrient solutions for most species. If the pH is not physiologically suitable, it can be adjusted by adding an acid or a base (De
Rijck & Schrevens, 1997). The availabilities of Mg, Ca, K, and P are slightly decreased at higher pH, while the availabilities of Mn, Cu, Zn and especially Fe are reduced (Bugbee, 2003).
Objectives
With limited information available on hydroponic solutions for onion production, there is a need to determine a suitable hydroponic nutrient solution that can be used for phenotypic and mineral content studies of onions. Due to
12 the ubiquity of the Hoagland's nutrient solution, the recommendation (Spomer et al., 1997) of a half strength Hoagland's solution, and the convenience of Peter's
Hydro-Sol, these nutrient solutions were selected for this study. Futhermore, the effects of nutrient solution and solution pH on onion growth and mineral content were investigated. The recommended pH levels of 5.8 and 6.5 were compared.
The objectives of this experiment were to assess the effects of nutrient solution
(Hoagland's, half strength Hoagland's, and Peter's Hydro-Sol), solution pH (5.8 and 6.5), and variety (A. cepa 'Deep Purple' and 'Purplette' [Johnny's Selected
Seeds; Albion, ME], and A. fistulosum 'Kinka' [Kyowa Seed Co., Ltd; Chiba
Japan]) on phenotypic variables and selected mineral concentration of onions grown hydroponically. The null hypothesis for this experiment is that nutrient solution, pH, and variety will not significantly influence onion growth mineral concentration.
13 Table 1.1: Baseline crops for advanced life support program, in order of readiness
Crop Scientific Name Space Setting Wheat Triticum aestivum L. planetary and vehicle White potato Solanum tuberosum L. planetary and vehicle Sweet potato Ipomoea batatas (L.) Lam planetary and vehicle Soybean Glycine max (L.) Merr. planetary and vehicle Dry bean Phaseolus vulgaris L. planetary and vehicle Peanut Arachis hypogaea L. planetary and vehicle Rice Oryza sativa L. planetary and vehicle Tomato Lycopersicon esculentum L. Vehicle Carrot Daucus carota L. Vehicle Cabbage Brassica oleracea L. Vehicle Spinach Spinacia oleracea L. Vehicle Chard Beta vulgaris L. Cicia group Vehicle Lettuce Lactuca sativa L. Vehicle Radish Raphanua sativus L. Radicula group Vehicle Onion Allium cepa L. Vehicle
(From Behrend, and Henninger, 2002)
14 Table 1.2: Percentage of the Daily Recommended Value (DRV) and Recommended Daily Intake (RDI) of minerals contained in mature onions.
Onion Nutrition Facts Amount/Serving (BOg) Percent Daily Values Calories (kcal) 30.0 Total fat (g) 0.00 0% Cholesterol (mg) 0.00 0% Sodium (mg) 0.00 0% Total carbohydrates (g) 7.00 2% Dietary Fiber (g) 1.00 6% Sugars (g) 5.00 Protein (g) 1.00 Vitamin C (mg) 5.00 9% Vitamin Be (mg) 0.10 5% Calcium (mg) 16.0 2% Iron(mg) 0.20 1% Folic acid (meg) 15.2 4% Potassium (mg) 126 4% Selenium (meg) 0.50 1%
Zinc (mg) 020 1%0
(From National Onion Association, 2001)
15 Table 1.3: Hydroponic nutrient solution compositions
Modified Modified Macronutrients (mM) Hoagland's 1/2 Hoagland's Hydro-Sol N 12.0 6.00 10.7 P 2.00 1.00 1.55 K 5.98 2.99 5.37 Ca 3.99 2.00 3.22 Mg 1.97 0.99 1.23 S 2.00 1.00 0.41 Micronutrients (pM) B 50.0 25.0 46.3 Mn 10.6 5.28 9.10 Zn 7.65 3.82 2.29 Cu 8.03 4.01 2.36 Mo 0.52 0.26 1.04 S 15.6 7.80 0 CI 121 60.7 0 Na 100 50.0 0 Fe 107 53.7 53.7
(From Jasoni et al., 2002)
16 CHAPTER II
MATERIALS and METHODS
Growth Conditions
Onions were grown hydroponically, in custom made hydroponic units.
The hydroponic units (152 x 81 x 66 cm) were constructed from polyvinyl chloride
(PVC) pipe and a fountain pump (115 V Versa Gold Series; Breckett Corp.,
Irving, TX), with gravitational flow producing a mean flow rate of 0.02 L s'\ The hydroponic units were placed within the Texas Tech University Horticultural
Gardens Greenhouse and Complex, Lubbock, TX (lat. 33''N, long. lOrW).
Nutrient solution was circulated past the plant roots and returned to a solution reservoir. Nutrient solution level was monitored daily, and maintained at 80 L of solution. The average daytime temperature was 29.5° C, and the average nighttime temperature was 18.6° C. The temperature was recorded every 960 s using a data logger (Hobo H8 series; Onset Computer Corp., Boume, MA). The temperature was controlled using fan and cooling pad units, and a 40% shade cloth. The average relative humidity ranged from nighttime 30.74% to daytime
63.49%. Three replications occurred between May 2002 and August 2002.
Three onion varieties ('Deep Purple,' 'Purplette' and 'Kinka') were propagated in three nutrient solutions (modified Hoagland's [Jasoni et al., 2002], half strength modified Hoagland's, and Peter's Hydro-Sol water soluble fertilizer), at two pH levels (5.8 and 6.5) in a three by two factorial applied in a randomized
17 complete block design with three replications. The composition of the modified
Hoagland's concentrate was 2 mM NH4H2PO4, 6 mM KNO3, 4 mM Ca
(N03)2-4H20, 2 mM MgS04-7H20, 50 pM H3BO3. 10 pM MnCl2-4H20, 7.6 pM
ZnS04-7H20, 8 pM CuS04-5H20, 0.40 pM Na2Mo04, 0.10 mM NaCI, 90 pM Na
EDTA, and 89 pM FeS04-7H20. The nutrient concentrations of the modified
Hoagland's solution were reduced to one half to prepare the half strength modified Hoagland's solution. The nutrient solution pH was adjusted using HCI and KOH and monitored daily using a pH meter (Piccolo; Hanna Instruments,
Bedfordshire, UK).
Onion seeds 'Deep Purple' and Purplette,' and 'Kinka' were germinated in
Cropking's Oasis Horticubes^'^ growing media. The Oasis Horticubes™ were cut to a two centimeter height with both top and bottom sides level, forming a six centimeter diameter puck. The Oasis Horticube™ pucks were autoclaved for two hours at 15 psi and 121°C prior to sowing. Six seeds were sown 0.75 mm deep in a circular arrangement in each puck. One set of 80 pucks was sown per onion variety. The pucks were placed in 38 x 53 cm trays in the greenhouse and irrigated with tap water. Once seedlings reached the flag stage (approximately
12 dap) they were randomly placed into the hydroponic units within the greenhouse. The onions were thinned to one plant per puck and grown under ambient conditions and treated for 30 days. Each hydroponic unit contained 12 plants of each onion variety.
18 Phenotypic and Mineral Content Measurements
The plants within the Oasis pucks were harvested at approximately 42
dap. The plants were individually placed in Ziplock™ bags and labeled
according to variety and treatment. The plants were removed from the Oasis™
and the following phenotypic data were collected: neck width, longest leaf mid
point diameter, pseudostem length, longest leaf length, longest root length, total
leaf number, total biomass, root mass, shoot mass and bulb mass (slightly
bulbing region for 'Deep Purple' and 'Kinka'). Data were collected on a fresh
weight basis. To provide adequate biomass for analysis, the plants were
composited into shoots, bulbs, and roots by variety and treatment. The plant
shoot, bulb, and roots were chopped into pieces less than five centimeters in
length and immediately frozen in liquid N2. After freezing, plant material was
ground in a coffee grinder for approximately 60 s (Mr. Coffee®, Hattiesburg, MS).
The ground plant material was immediately placed into Ziplock^*^ bags and
placed into a -20 "C-freezer.
To determine dry matter percentage, duplicate samples of approximately
one gram of frozen plant material powder was placed into a small pre-weighed
ceramic crucible and placed into a vacuum oven and dried for at least 16 hours
at 100 °C AOAC (1990). Samples were removed from the oven and placed into
a desiccator to allow for cooling. Once samples cooled, duplicate samples were weighed to determine dry weight. Percentage dry matter was calculated by
19 dividing the dry weight by the fresh weight, and multiplying by 100 to report as a percentage. Duplicate samples were averaged.
To determine percentage ash approximately one gram of frozen plant material was placed into a pre-weighed ceramic crucible. Samples were dried and then placed into a muffle furnace for at least 16 hours at 500 "C following
AOAC method 900.02 (AOAC, 1990). Samples were removed, placed into a desiccator for cooling, and then weighed. The weight recorded minus the weight of the cnjcible, was recorded as the ash weight. To calculate percentage ash, the ash weight was divided by the fresh weight and multiplied by 100. Duplicate samples were analyzed when sufficient plant material was available.
Mineral (Ca, Mg, K, Zn, and Na) content was determined as described by
Perkin Elmer (1976) on a 2380 Atomic Absorption Spectrophotometer (Nonwalk,
CT). The dry ash was allowed to dry for five hours, in accordance with AOAC method 900.02 (AOAC, 1990). After the sample was dissolved in 15 ml of 20%
HNO3, the solution was filtered through Whatman 40 grade ashless filter paper
(Clifton, NJ) and diluted to 100 ml with distilled water; duplicate samples were prepared when possible. One milliliter of each sample was placed into one of two separate tubes and 10 ml of distilled water was added. To one set of the tubes, 0.5 ml of 5% LaCb was added as releasing agent. This tube was used for
Ca and Mg quantification.
20 Experimental Design and Analysis
The experimental design was a three by two factorial applied in a randomized block design. Data were analyzed by the GLM procedure in the SAS statistical software, and treatment differences were separated using Duncan's multiple range test at the 5% level (unless othenwise indicated). The main effects of nutrient solution, solution pH, and onion variety on phenotypic and mineral content variables were evaluated.
21 CHAPTER III
RESULTS
Phenotypic Variables
Neck diameter was significantly (P < 0.0001) affected by variety (Table
3.1). 'Deep Purple' produced a significantly larger neck diameter (5.49 mm) than did 'Purplette' (4.71 mm) and 'Kinka' (4.08 mm)(Table 3.1). Plant neck diameter was not significantly affected by nutrient solution or pH (Table 3.1).
LLMPD was significantly affected by nutrient solution (P < 0.005) and variety (P < 0.0001) (Table 3.1). The onions grown in half strength Hoagland's produced a significantly greater LLMPD (4.56 mm) than did plants grown in
Hoagland's (4.09 mm) and Hydro-Sol (3.96 mm) (Table 3.1). 'Deep Purple' produced a significantly greater LLMPD (5.01 mm) than did 'Purplette' (4.10 mm) and 'Kinka' (3.49 mm) (Table 3.1).
Pseudostem length was not significantly affected by nutrient solution, solution pH, or onion variety (Table 3.1). The total leaf number was significantly
(P < 0.005) affected by variety (Table 3.1). 'Deep Purple' (4.81) and 'Kinka'
(4.72) produced a greater total leaf number than did 'Purplette' (4.38).
The longest shoot length was significantly affected by solution pH and onion variety (P < 0.0001) (Table 3.1). The onions grown at pH 6.5 produced a significantly longer shoot (30.31 cm) than did onions grown at pH 5.8 (28.05 cm)
22 (Table 3.1). 'Deep Purple' produced a significantly longer shoot (32.52 cm) than did 'Purplette' (28.29 cm) and 'Kinka' (26.73 cm) (Table 3.1). Nutrient solution did not significantly influence longest leaf shoot (Table 3.1).
Longest root length was significantly (P < 0.0001) affected by variety
(Table 3.1). 'Deep Purple' produced significantly longer roots (25.76 cm) than did 'Purplette' (20.86 cm) and 'Kinka' (13.48 cm) (Table 3.1). Longest root length was not significantly affected by nutrient solution or solution pH (Table 3.1).
The largest shoot mass was significantly affected by nutrient solution, solution pH, and variety (P < 0.0001) (Table 3.2). Onions grown in half strength
Hoagland's produced significantly greater shoot mass (4.55 g) than did onions grown in Hydro-Sol (3.60 g) (Table 3.2). Onions grown at pH 6.5 produced a significantly greater shoot mass (4.41 g) than did onions grown at pH 5.8 (3.82 g)
(Table 3.2). 'Deep Purple' produced significantly greater shoot mass (5.68 g) than did 'Purplette' (3.98 g), and 'Purplette' produced significantly greater shoot mass than did 'Kinka' (2.68 g) (Table 3.2).
Bulb mass was significantly (P < 0.0001) affected by variety (Table 3.2).
'Purplette' produced significantly larger bulb mass (0.73 g) than did 'Deep Purple'
(0.50 g) or 'Kinka' (0.28 g) (Table 3.2). Bulb mass was not significantly affected by nutrient solution or solution pH (Table 3.2). 'Purplette' is the only variety that is an actually bulbing onion. This result is to be expected since 'Deep Purple' and 'Kinka' are non-bulbing onions.
23 Root mass was significantly (P < 0.0001) affected by variety (Table 3.2).
'Deep Purple' produced a significantly greater root mass (1.82 g) than did
'Purplette' (1.11 g) or 'Kinka' (0.60 g) (Table 3.2). Root mass was not significantly affected by nutrient solution or solution pH (Table 3.2).
Onion total plant biomass was significantly affected by nutrient solution, solution pH, and variety (P < 0.0001) (Table 3.2). Onions grown in half strength
Hoagland's produced a significantly greater total plant biomass (6.39 g) than did onions grown in Hydro-Sol (5.13 g) (Table 3.2). Onions grown at pH 6.5 produced a significantly greater total plant biomass (6.21 g), than did onions grown at pH 5.8 (5.37 g) (Table 3.2). 'Deep Purple' produced significantly greater total plant biomass (7.99 g) than 'Purplette' (5.83 g), which was significantly greater than 'Kinka' (3.55 g) (Table 3.2).
Edible biomass was significantly affected by nutrient solution, solution pH, and variety (P < 0.0001) (Table 3.2). Onions grown in half strength Hoagland's produced a significantly greater edible biomass (5.11 g) than did onions grown in
Hydro-Sol (4.05 g). The onions grown at pH 6.5 produced significantly greater edible biomass (4.31 g) than did onions grown at pH 5.8 (4.31 g) (Table 3.2).
'Deep Purple' produced significantly greater edible biomass (6.17 g) than did
'Kinka' (2.95 g) and 'Purplette' (4.72 g) (Table 3.2).
The percentage edible biomass was significantly (P < 0.0001) affected by variety (Table 3.2). 'Purplette' (82.51%) and 'Kinka' (83.27%) produced a significantly greater percentage edible biomass than did 'Deep Purple' (78.12%)
24 (Table 3.2). The percentage edible biomass was not significantly affected by nutrient solution or solution pH (Table 3.2).
Mineral Content Variables
Shoot dry matter percentage was significantly affected by a nutrient solution by pH interaction (Table 3.3). The onions grown in Hydro-Sol at pH 5.8 produced a significantly greater shoot dry matter percentage (6.8%) than did onions grown in Hydro-Sol at pH 6.5 (5.96%), Hoagland's at pH 5.8 (5.9%), 6.5
(5.78%), and half strength Hoagland's at pH 5.8 (5.55%) (Table 3.3). Variety had no significant influence on the shoot dry matter percentage (Table 3.4).
Bulb dry matter percentage was significantly affected by nutrient solution and onion variety (Table 3.4). Onions grown in Hydro-Sol produced a significantly greater bulb dry matter percentage (9.13%) than did onions grown in half strength Hoagland's (8.32%) (Table 3.4). 'Kinka' produced significantly (P <
0.005) greater bulb dry matter percentage (9.33%) than did 'Deep Purple'
(8.18%) and 'Purplette' (8.48%) (Table 3.4). Solution pH did not significantly influence bulb dry matter percentage (Table 3.4).
Root dry matter percentage was significantly affected by variety (Table
3.4). 'Kinka' produced a significantly (P < 0.05) greater dry matter percentage
(5.38%) than did 'Deep Purple' (4.68%) (Table 3.4). Root dry matter percentage was not significantly affected by nutrient solution composition and pH (Table 3.4).
25 Percentage shoot ash was significantly affected by nutrient solution (Table
3.4). Onions grown in Hydro-Sol produced significantly greater shoot ash percentage (1.01%) than did onions grown in Hoagland's (0.81%) and half strength Hoagland's (0.85%) (Table 3.4). Shoot ash percentage was not significantly affected by pH or variety (Table 3.4).
Bulb ash was significantly affected by a solution by variety interaction
(Table 3.5). 'Deep Purple' grown in Hydro-Sol produced a significantly greater bulb ash percentage (1.07%) than did 'Deep Purple' grown in Hoagland's or half strength Hoagland's, as well as 'Kinka' or 'Purplette' grown in Hoagland's, half strength Hoagland's, or Hydro-Sol (Table 3.5). Solution pH had no effect on bulb ash percentage (Table 3.4).
Root ash percentage was significantly affected by variety (Table 3.4).
'Deep Purple' produced significantly greater root ash percentage (0.74%) than did 'Kinka' (0.62%) (Table 3.4). The root ash percentage was not significantly affected by nutrient solution or solution pH (Table 3.4).
Shoot Mg concentration was significantly (P < 0.0001) affected by nutrient solution (Table 3.4). Onions grown in Hydro-Sol produced a significantly greater shoot Mg concentration (11.40 mg lOOg'^) than did onions grown in Hoagland's
(6.34 mg lOOg"^) or half strength Hoagland's (6.83 mg 100g"^)(Table 3.4). Shoot
Mg concentration was not significantly affected by pH or variety (Table 3.4).
Bulb Mg concentration was significantly (P < 0.005) affected by nutrient solution and onion variety (Table 3.4). Onions grown in Hydro-Sol produced a
26 significantly greater bulb Mg concentration (21.52 mg 100g"^) than did onions grown in Hoagland's (16.29 mg lOOg"^) or half strength Hoagland's (15.81 mg lOOg"^) (Table 3.4). 'Kinka' produced a significantly greater concentration of bulb
Mg (20.25 mg lOOg-^) than did 'Deep Purple' (17.62 mg lOOg'^) and 'Purplette'
(15.75 mg lOOg"^) (Table 3.4). Solution pH did not significantly influence bulb Mg concentration (Table 3.4). Root Mg concentration was not significantly affected by nutrient solution, pH, or variety (Table 3.4).
Shoot Ca concentration was not significantly affected by nutrient solution, solution pH, or variety (Table 3.4). Bulb Ca concentration was significantly affected by a solution by pH interaction (Table 3.3). Onions grown in Hoagland's at pH 6.5 (50.25 mg lOOg^) produced a significantly greater bulb Ca concentration than did onions grown in Hoagland's at pH 5.8 (40.14 mg lOOg'"*), half strength Hoagland's at pH 5.8 (39.14 mg lOOg^), half strength Hoagland's at pH 6.5 (36.12 lOOg-^), and Hydro-Sol at pH 5.8 (34.16 mg lOOg'^) and pH 6.5
(27.39 mg lOOg'"*) (Table 3.3). Bulb Ca concentration was not significantly affected by variety (Table 3.4). Root Ca concentration was not significantly affected by nutrient solution, solution pH, or variety (Table 3.4).
Shoot Zn concentration was significantly affected by nutrient solution
(Table 3.4). Onions grown in half strength Hoagland's produced significantly greater shoot Zn concentration (1.36 mg lOOg"^) than did onions grown in
Hoagland's (0.67 mg 100g'^) (Table 3.4). Shoot Zn concentration was not significantly affected by pH or variety (Table 3.4).
27 Bulb Zn concentration was significantly affected by a solution by pH interaction (Table 3.3). Onions grown in Hydro-Sol at pH 6.5 (0.987 mg lOOg'^) and Hoagland's at pH 5.8 (0.969 mg lOOg'') produced significantly greater bulb
Zn concentrations than did onions grown in half strength Hoagland's at pH 6.5
(0.489 mg lOOg"^) (Table 3.3). Bulb Zn was not significantly affected by variety
(Table 3.4).
Root Zn concentration was significantly affected by nutrient solution (Table
3.4). Onions grown in half strength Hoagland's (1.42 mg lOOg'^) and Hydro-Sol
(1.56 mg lOOg'^) produced significantly greater root Zn concentration than did onions grown in Hoagland's (0.90 mg lOOg'^) (Table 3.4). Root Zn concentration was not significantly affected by pH or variety (Table 3.4).
Shoot Na concentration was significantly affected by nutrient solution
(Table 3.4). Onions grown in Hoagland's produced a significantly greater shoot
Na concentration (40.16 mg lOOg'^) than did onions grown in half strength
Hoagland's (33.07 mg lOOg'^) (Table 3.4). Shoot Na concentration was not significantly affected by pH or variety (Table 3.4).
Bulb Na concentration was significantly affected by solution pH (Table
3.4). Onions grown at solution pH 6.5 produced a significantly greater bulb Na concentration (22.32 mg lOOg'"*) than did onions grown at solution pH 5.8 (17.26 mg lOOg'^) (Table 3.2). Bulb Na concentration was not significantly affected by nutrient solution composition or variety (Table 3.4). Root Na concentration was not significantly affected by nutrient solution, pH, or variety (Table 3.4).
28 Shoot K concentration was significantly affected by nutrient solution (Table
3.4). Onions grown in Hydro-Sol produced a significantly greater shoot K concentration (121.18 mg lOOg'^) than did onions grown in half strength
Hoagland's (103.21 mg lOOg"^). Shoot K concentration was not significantly affected by solution pH or variety (Table 3.4).
The bulb K concentration was significantly affected by variety (Table 3.4).
'Kinka' produced a significantly greater concentration of bulb K (193.18 mg lOOg-"") than did 'Deep Purple' (148.82 mg lOOg""") or 'Purplette' (137.13 mg lOOg'^) (Table 3.4). Bulb K concentration was not significantly affected by nutrient solution composition or pH (Table 3.4). Root K concentration was not significantly affected by nutrient solution composition, pH, or variety (Table 3.4).
29 Table 3.1: Main effects of nutrient solution, pH, and onion variety on neck diameter (ND), longest leaf midpoint diameter (LLMPD), pseudo-stem length (PL), longest leaf length (LLL), longest root length (LRL), and leaf number (LN).
Treatment ND LLMPD PL LLL LRL LN —(mm)-— —(cm)- Nutrient Solution Hoagland's 4.65 4.09 b* 3.91 28.8 20.0 4.61 1/2 Hoagland's 4.99 4.56 a 4.20 29.9 20.7 4.77 Hydro-Sol 4.64 3.96 b 3.79 28.9 19.5 4.53 SE 0.73 0.52 0.61 2.83 3.68 0.33 pH 5.8 4.66 4.16 4.01 28.1 b 19.4 4.60 6.5 4.86 4.25 3.92 30.3 a 20.7 4.67 SE 0.73 0.52 0.61 2.83 3.68 0.33 Variety Deep Purple 5.49 a 5.01 a 4.13 32.5 a 25.8 a 4.81 a Kinka 4.08 c 3.50 c 3.84 26.7 b 13.5c 4.72 a Purplette 4.71 b 4.10 b 3.93 28.3 b 20.9 b 4.38 b SE 0.073 0.52 0.61 2.83 3.68 0.33
"Means of each variable and main effect followed by different letters are different P < 0.05 (n = 3).
30 Table 3.2: Main effects of nutrient solution, pH, and onion variety on shoot mass (SM), bulb mass (BM), root mass (RM), total biomass (TB), edible biomass (EB), and percentage edible biomass (%EB).
Treatment SM BM RM TB EB %EB ln\... —\9) Nutrient Solution Hoagland's 4.19 a' 0.5 1.16 5.85 ab 5.11 a 82.26 1/2 Hoagland's 4.55 ab 0.56 1.28 6.39 a 4.69 ab 81.2 Hydro-Sol 3.6 b 0.46 1.08 5.13 b 4.05 b 80.44 SE 0.83 0.22 0.41 1.56 0.955 9.01 pH 5.8 3.82 b 0.49 1.06 5.37 b 4.31 b 81.3 6.5 4.41 a 0.52 1.28 6.21 a 4.93 a 81.29 SE 0.83 0.22 0.41 1.56 0.955 9.01 Variety Deep Purple 5.68 a 0.5 b 1.82 a 7.99 a 6.17 a 78.12 b Kinka 2.68 c 0.28 c 0.6 c 3.55 c 2.96 c 83.27 a Purplette 3.98 b 0.74 a 1.11 b 5.83 b 4.72 b 82.51 a SE 0.83 0.22 0.41 1.56 0.955 9.01
'Means of each variable and main effect followed by different letters are different P_< 0.05 (n = 3).
31 Table 3.3: Nutrient solution by pH interaction effects on shoot dry matter percentage, bulb Ca, and bulb Zn.
Variable 5.8 6.5 Shoot Dry Matter (%) Hoagland's 5.90 be* 5.78 be 1/2 Hoagland's 5.55 e 6.24 ab Hydro-Sol 6.80 a 5.95 be BulbCa (mg lOOg'^) Hoagland's 40.1 b 50.3 a 1/2 Hoagland's 39.1 b 36.2 be Hydro-Sol 34.2 be 27.4 e BulbZn(mg100g'') Hoagland's 0.97 a 0.73 ab 1/2 Hoagland's 0.85 ab 0.49 b Hydro-Sol 0.73 ab 0.99 a
"Variable Means of each nutrient solution and pH followed by different letters are different P< 0.05 (n = 3).
32 Table 3.4. Main effects of nutrient solution, pH, and onion variety on dry matter percentage, ash percentage, and selected minerals.
Shoot Dry Matter Ash Mg Ca Zn Na K
(%)• t„ ig iOOg- 1) 1" Nutrient Solution Hoagland's 0.81 b* 6.34 b 63.1 0.67 b 40.2 a 109 b Vi Hoagland's - 0.85 b 6.83 b 67.6 1.36 a 33.1 b 103b Hydro-Sol - 1.01 a 11.4a 67 0.96 ab 35.6 ab 121a SE 0.62 0.18 2.95 12.9 0.7 8.41 20.5 PH 5.8 0.93 7.75 65.9 1.13 34.5 112 6.5 - 0.85 8.62 65.9 0.86 38.1 109 SE 0.62 0.18 2.95 12.9 0.7 8.41 20.5 Variety Deep Purple 5.27 b 0.88 7.23 61.3 0.90 35.2 113 Kinka 7.17a 0.84 9.18 69.5 1.12 36.2 105 Purplette 5.67b 0.95 8.14 66.9 0.96 37.4 114 SE 0.62 0.18 2.95 12.9 0.7 8.41 20.5 Bulb Treatment Nutrient Solution Hoagland's 8.54 ab - 16.3 b - - 19.8 162 Vi Hoagland's 8.32 b 15.8 b - 19.4 142 Hydro-Sol 9.13 a 21.5 a 20.2 176 SE 0.79 0.01 11.9 9.73 0.35 6.41 49.3 PH 5.8 8.52 0.72 18.0 17.3 b 159 6.5 8.81 0.73 17.7 22.3 a 160 SE 0.79 0.01 11.9 9.73 0.35 6.41 49.3 Variety Deep Purple 8.18 b - 17.6 b 37.0 0.77 20.2 148 Kinka 9.33 a 20.2 a 35.3 0.89 19.8 193 Purplette 8.48 b 15.8 b 41.5 0.69 19.4 137 SE 0.79 0.01 11.9 9.73 0.35 6.41 49.0 Root Treatment Nutrient Solution Hoagland's 4.88 0.66 7.03 37.1 0.90 b 49.0 96.8 Vz Hoagland's 5.15 0.70 5.72 44.8 1.42 a 44.8 101 Hydro-Sol 5.10 0.71 8.44 37.1 1.56 a 47.8 111 SE 0.651 0.02 18.2 302 0.46 110 478
33 Table 3.4. continued
Root Dry Matter Ash Mg Ca Zn Na K irr Treatment {%) [n ig lOOg-1) PH 5.8 5.01 0.68 6.84 45,9 1.39 47.8 98.2 6.5 5.05 0.70 7.29 40.9 1.19 46.6 108 SE 0.65 0.02 18.2 302 0.46 110 478 Variety Deep Purple 4.68 b 0.74 a 5.79 39.2 1.06 48.6 107 Kinka 5.38 a 0.62 b 8.81 46.4 1.31 44.7 94.6 Purplette 5.03 ab 0.71 ab 6.59 44.6 1.51 48.3 107 SE 0.65 0.02 18.2 302 0.46 110 478
Means of each variable and main effect followed by different letters are different P_<_0.05 (n = 3).
34 Table 3.5: Nutrient solution by variety interaction effects on bulb ash.
Variable Deep Purple Kinka Purplette Bulb Ash (%) Hoagland's 0.88 b 0.47 d 0.63 c 1/2 Hoagland's 0.78 be 0.63 e 0.74 c Hydro-Sol 1.07 a 0.63 c 0.73 c
*Variable means of each nutrient solution and variety followed by different letters are different P<0.05(n = 3).
35 CHAPTER IV
DISCUSSION
To be considered for use in the ALS program, the candidate crop selected needs to be highly productive over a short time period, contain high nutritional value, and waste limiting (Lawson, 2003). High plant growth rates that produce a constant yield may be maintained in a relatively small root zone by growing plants in hydroponic solutions (Steinberg et al., 2000). Most modern hydroponic solutions are based on the work of Hoagland and Arnon (1950) and have been adapted to numerous crops (Whipker and Hammer, 1998). The research discussed here compares growth of three onion varieties in three readily available hydroponic solutions: Hoagland's, Half-strength Hoagland's, and Hydro-
Sol, at pH 5.8 and 6.5
Under the conditions of our study biomass production was greatest for onions grown in Half-strength Hoagland's solution (Table 3.2). This finding agrees with Spomer et al. (1997) who recommended a nutrient solution equal to about one half the strength of the original Hoagland's nutrient solution (Hoagland and Arnon, 1950). Biomass was greatest for plants grown at pH 6.5, which is within the range of 5.5 to 6.5 considered optimal for the availability of nutrients from most nutrient solutions for most species (Islam et al., 1980). The variety producing the greatest biomass was 'Deep Purple'.
36 In addition to total biomass produced, percentage edible biomass is an important variable that influences waste generation. Lawson (2003) recommends that the candidate crop be waste limiting. Therefore, a plant that produces a large root mass (inedible) and a low percentage edible biomass would not be considered waste limiting; onions producing relatively high amounts of shoot and bulb mass are desirable. Onions grown in half strength Hoagland's or at pH 6.5 produced significantly greater shoot mass than did onions grown in half strength Hoagland's or Hydro-Sol and pH 5.8 (Table 3.2). 'Purplette' produced significantly greater bulb mass than did the other varieties (Table 3.2).
This is explained by the fact that 'Purplette' is a bulbing onion while 'Deep Purple' and 'Kinka' are non-bulbing onions. 'Deep Purple' had a significantly greater shoot mass than did 'Purplette' or 'Kinka' (Table 3.2). 'Deep Purple" produced a significantly greater root mass than did 'Purplette' and 'Kinka' (Table 3.2). As discussed previously production of a large root mass is undesirable in a waste
limiting system.
Shoot mass and bulb mass combine to provide the overall percentage of the plant edible portion (percentage edible biomass). Onions grown in half
strength Hoagland's produced the greatest percentage edible biomass (Table
4.2). In the research reported here, the highest edible biomass was produced with the most dilute nutrient solution. This represents relatively higher nutrient
utilization efficiency and potentially minimizes nutrient requirements.
37 Furthermore, this potentially reduces energy costs associated with wastewater recycling
Nutrient concentrations within the onions varied with plant part and nutrient. Hydro-Sol produced the highest concentrations of Mg in the shoots and the bulbs. However, concentrations in the root were unaffected by nutrient solution with no significant effect present in the roots (Table 3.4). No significant effect on shoot and root Ca existed (Table 3.4). Hoagland's produced the highest concentration of bulb Ca (Table 3.4). The bulb Zn concentration was significantly influenced by solution by a pH interaction; Hydro-Sol at pH 6.5 and
Hoagland's at pH 5.8 produced the greatest concentrations of bulb Zn (Table
4.3). The onions grown in Hoagland's produced the highest concentrations of shoot Na, the onions grown at pH 6.5 produced the highest bulb Na concentrations, with no effect on the roots (Table 4.4). Onions grown in Hydro-
Sol produced the highest shoot K concentrations, and 'Kinka' had the highest bulb K concentrations (Table 4.4). The Oasis™ medium in which the plants are grown is chemically inert so it does not interfere with the mineral absorption, allowing the plant to have unlimited uptake of minerals increasing the overall ash content of the onion plant (Resh 1991). Sanchez-Castillo et al. (1998) reported that the addition of fertilizer, the age of the plant tissue, and the chemical composition of the medium in which the crop is grown, could all have an affect on the mineral composition of the plant material. Randle (2000) stated that increasing N decreased Ca and Mg content and increased K. In the research
38 reported here, our solutions differed not only in N concentration, but also in concentration of other elements. Therefore, the effects N and the other elements cannot be separated. Furthermore, varieties and species differ in their ability to absorb any given nutrient, and these differences may be due to the differences of the root systems or the specific transport enzymes in cell membranes (Sanchez-
Castillo et al., 1998). Given the lack of consistent results, selection of appropriate nutrient solution may need to be based on factors other than nutrient content, or nutrient solutions can be modified based on additional research designed to optimize mineral content.
39 CHAPTER V
CONCLUSIONS
The results from this study indicated nutrient solution, pH, and variety significantly affected several plant physiological variables. Total biomass and edible biomass were greatest in plants grown in half strength Hoagland's nutrient solution, or at a solution pH of 6.5. 'Deep Purple' produced a significantly greater overall total biomass than did 'Purplette' or 'Kinka.'
Onions grown in Hydro-Sol produced a significantly higher dry matter percentage and percentage ash, and Mg and K concentrations. Onions grown in half strength Hoagland's produced significantly greater Zn concentrations and onions grown in Hoagland's produced significantly greater Ca and Zn concentrations. 'Kinka' produced significantly greater dry matter percentage, Mg, and K, and 'Deep Purple' produced significantly greater percentage ash than did the other varieties.
The half strength Hoagland's solution is the preferred nutrient solution evaluated in this research. However, Hydro-Sol tended to produce onions with highest mineral content. Mineral content varied with plant part, nutrient solution, solution pH, and onion variety. Selection of an appropriate nutrient solution must consider both edible biomass production and mineral content. In the research reported here the solution that produced the greatest biomass did not produce plant material with the highest mineral content. Future research may lead to the
40 development of a modified nutrient solution that optimizes both edible biomass production and mineral content.
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46 APPENDIX
July 37, 19f5 X. Jaaonl antrlwt toivtlM ChaalcaX Melaoular Nutriant Solution llaaantal Ceapoaltion WaighC {9/L) wM Klaaanc tM ppm
1. NH^HjPO^ US 46 a NB4-N 3.0 28 MO,-N 10.0 140
a. xMo, 101 131 6 P 3.0 63 X 6.0 334
3. Ca(NO,)}'4BjO 336 1(9 4 Ca 4.0 160 Kg 2.0 48 4. KgSO^-7IljO 246 99 a s a.o 64
Ka 0.10 3 HjBO, 63 0.62 .05 CI 0.10 4 0.09 6
MnCl2-4R,0 198 0.43 .01
lnSO,-7H20 SM 0.44 .0076
CuSO^-SHjO aso 0.40 .0080
Ma^a04-2H20 342 0.03 0,0004
Kaci 5S 1.17 0.1
rira't dlaaolvo: Ha -nXTA 372 6.7 .09
than add: T•SO^•^n,o 276 5.0 .089 7. KOB 56 4.0 .357 Md S BL oC BoXutloBa 1-6 to aaob L of da-ioaliad vatar. Mjuat pa with aolutlon 7 (IM) to pa 7.ai 30 BL XOH/L of Nutriant Solution L ot Nutriant Solution -»h of KOH Naadad for pK 7.2 1 20 3 40 3 60 4 80 5 100 6 130
Figure A.I: Ingredients for Jasoni's modified Hoagland's
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