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SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY

Calcium Effects on Soybean Seed Production, Elemental Concentration, and Seed Quality

^a Dep. of Agronomy, P.O. Box 830915, Univ. of Nebraska, Lincoln, NE 68583 USA
^b Pioneer Hi-Bred International, Inc., Research and Product Development, P.O. Box 85, Johnston, IA 50131 USA
^c Dep. of Horticulture and Crop Science, 2021 Coffey Road, The Ohio State Univ., Columbus, OH 43210 USA

mikeb{at}unlserve.unl.edu

	ABSTRACT

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Soybean [Glycine max (L.) Merr.] plants produce fewer and lower quality seeds when grown in conditions that decrease seed Ca concentration. Indeterminate soybean was grown in modified hydroponic culture to ascertain the effects of Ca deficiency on leaf dry matter, seed production and elemental concentration, and the effect of pod position on seed elemental concentration in 1992 and 1993. Treatments consisted of 2.0 (Control), 0.2 (Low), 0.1 (Very Low) mM Ca in the nutrient media. Some Control plants were switched to Low or Very Low (Con/L, VL) Ca levels at beginning seed growth stage (R5), and some plants grown at the Very Low Ca level were switched to the Control (VL/Con) Ca level at R5. At harvest, plants were divided into four sections: top-third (T/3), middle-third (M/3), and bottom-third (B/3) of the mainstem and branches (Br). Low and Very Low treatments produced 65 and 10% as much seed mass, respectively, as Control in both years. Low and Very Low treatments retained significantly less total leaf dry matter at R5 in both years. Seed Ca levels were 25 to 300% higher in treatments that included the Control Ca level during any part of reproductive growth compared to other treatments in both years. Seed Ca concentration was highest in the T/3 and Br sections in both years. Germination was reduced in treatments not including the Control solution during part of the reproductive growth period in both years. Decreased Ca levels in the nutrient medium reduced soybean leaf dry matter during seed fill, seed production, seed Ca concentration, and seed germination, and increased the incidence of seedling disorders such as watery hypocotyl and epicotyl necrosis.

Abbreviations: B/3, Bottom-third section: leaves or seeds produced on the lower one-third of the plant (based on node number) • Br, Branch section: leaves or seeds produced on branches • Con/L, Control treatment switched to Low treatment at R5 • Con/VL, Control treatment switched to Very Low treatment at R5 • M/3, Middle-third section: leaves or seeds produced on the middle one-third of the plant (based on node number) • T/3, Top-third section: leaves or seeds produced on the top one-third of the plant (based on node number) • and VL/Con, Very Low treatment switched to the Control treatment at R5

	INTRODUCTION

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LOW CALCIUM LEVELS and soil pH may constrain crop production in highly weathered soils of major soybean producing regions such as the Brazilian cerrados (Smyth and Cravo, 1992). Crops grown in nutrient poor media or in conditions that limit Ca uptake produce lower yields than crops grown with continuous and adequate Ca nutrition (Frost and Kretchman, 1989; Hadidi, 1984; Cox et al., 1976; Smiciklas et al., 1989). In addition, seeds produced by plants grown with inadequate Ca levels in rooting media often have seed quality problems such as reduced germination and vigor (Keiser and Mullen, 1993; Frost and Kretchman, 1989; Smiciklas et al., 1989; Hadidi, 1984; Cox et al., 1976). Shear (1975) indicated the magnitude of crop production problems by compiling a list of more than 30 crop disorders associated with Ca deficiency. These disorders are often found in weakly transpiring organs, including fruits and seeds, and result from inadequate Ca levels in the affected organs rather than from inadequate quantities of Ca in plant, i.e., the distribution of Ca is the problem (Kirkby and Pilbeam, 1984). An improved understanding of Ca and its movement within the plant could help reduce constraints on crop production and aid in the production of high-quality seed in regions with highly weathered soils.

Calcium is usually considered phloem immobile, and its movement in plants generally follows the transpiration stream via the xylem (Hanger, 1979; Hanson, 1983; Kirkby and Pilbeam, 1984; Marschner, 1974, 1986). Thus, the proportion of solute delivered to a developing fruit by the xylem is a factor in determining its Ca content (Kirkby and Pilbeam, 1984). Calcium is believed to be delivered to or moved between developing fruits and seeds in three ways. The first process occurs during the earliest stages of fruit development when rapid cell division takes place and growth rates are slow. During this period, Ca delivery occurs via xylem flow (Bangerth, 1979; Hanger, 1979; Kirkby and Pilbeam, 1984; Marschner, 1986) and is driven, in part, by the creation of binding sites for Ca in newly formed cells (Kirkby and Pilbeam, 1984). The delivery process changes coincident with increasing fruit and seed growth rates. During this period, most of the developing fruit's water needs may be met by increased phloem delivery (Wiersum, 1966). Xylem flow and Ca delivery to the seeds of bean (Phaseolus vulgaris L.) (Mix and Marschner, 1976) and broad bean (Vicia faba L.)(Wiersum, 1966) stops during this period, but continues to the pod walls. Mix and Marschner (1976) and Pate and Hocking (1978) found that Ca can move by yet another process, diffusing apoplastically from pod walls to the seeds of legumes during fruit maturation. Keiser and Mullen (1993) suggested that this type of Ca movement could be responsible for Ca found in seeds produced on soybean plants deprived of Ca from R5 (Fehr and Caviness, 1977) until harvest. The relationship between transpiration and Ca levels in fruit still remains unclear, however.

Wiersum (1966) found that restricting the transpiration of tomato (Lycopersicon esculentum Mill.) fruits by enclosing them in plastic bags decreased Ca concentration and increased Ca deficiency disorders. Keiser and Mullen (1993), however, found no differences in seed Ca concentration or seed quality among soybean plants grown in 55 and 95% relative humidity environments. Whether differences exist in the Ca concentration in soybean seeds that develop on different sections of the plant (e.g., top-third vs. bottom-third of the mainstem) has not been considered.

Hypocotyl collapse (watery hypocotyl or hypocotyl necrosis) has been associated with seed Ca deficiency in peanut (Arachis hypogaea L.) (Cox et al., 1976), cowpea [Vigna unguiculata (L.) Walp.] (Helms and Myers, 1972), and Phaseolus spp. (Helms, 1971; Helms and Myers, 1972; Shannon et al., 1967) seedlings. Peanut is also susceptible to epicotyl necrosis (dark plumule) in addition to hypocotyl collapse as a consequence of Ca deficiency (Cox et al., 1976). Calcium deficiency symptoms in soybean seedlings have not been characterized.

These investigations demonstrated that Ca affects seed growth and development, yet the effects of this essential element on soybean seed and seedling performance are only beginning to be clarified (Keiser and Mullen, 1993; Keiser et al., 1995). The objectives of this study were to determine (i) the effects of low Ca levels in plant rooting medium on soybean seed production, elemental concentration, and seed quality in an indeterminate soybean cultivar; (ii) whether seed elemental concentration is affected by position of the pod on the plant; and (iii) the Ca deficiency symptoms in soybean seedlings germinated from low Ca seed.

	Materials and methods

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Plant Material
Indeterminate Williams 82 soybean (Maturity Group III) was grown in a modified hydroponic culture in a greenhouse at Columbus, OH, during the summer and fall of 1992 and 1993. The nutrient solution (similar to that described by Lauer et al., 1989) was prepared with distilled water (>200 k resistance) and contained: 1.25 mM K₂SO₄, 0.5 mM MgSO₄-7H₂O, 0.5 mM K₂HPO₄, 26.0 µM Fe citrate, 2.3 µM H₃BO₃, 0.9 µM MnSO₄-H₂O, 0.6 µM ZnSO₄-7H₂O, 0.15 µM CuSO₄-5H₂O, 0.14 µM (NH₄)₆Mo₇O₂₄-4H₂O, 0.01 µM CoCl₂-6H₂O, 0.11 µM NiCl₂-6H₂O, and either 2.0 (Control), 0.2 (Low) or 0.1 (Very Low) mM CaCl₂.

Treatments consisted of the three different Ca levels described above plus additional treatments in which the Ca level was changed at R5 as follows: Con/Low, 2.0 mM Ca switched to 0.2 mM Ca (1992 only); Con/VL, 2.0 mM Ca switched to 0.1 mM Ca; VL/Con, 0.1 mM Ca switched to 2.0 mM Ca. The above nutrient solution was modified [0.5 mM (NH₄)₂HPO₄ was substituted for 0.5 mM K₂HPO₄, 3.5 mM KNO₃ was substituted for 1.25 mM K₂SO₄, and 3.0 mM NH₄NO₃ was added] about 30 d after planting in 1992 to minimize N deficiency, which may result from reduced nodulation in the Very Low and Low Ca treatments. This modified solution was used throughout 1993.

Seeds were imbibed for 12 h prior to planting between sheets of germination paper moistened with distilled or deionized water. Five seeds were planted 2.5 cm deep in 21.5-cm pots in perlite rinsed three times with distilled water. Each pot was fitted with a fine-weave polyester wick inserted into the nutrient reservoir. The nutrient reservoirs were 5.7-L plastic containers painted (outside only) with two coats of black latex paint covered by one coat of white paint to reduce algal growth and insolation. After planting, each pot received 200 to 300 mL of the appropriate nutrient solution each day for the first week. Three weeks after planting, plants were thinned to three plants per pot and placed in a randomized complete block design with 0.45-m row spacing. Pots served as the experimental units.

Nutrient solutions and reservoirs were changed once per week. Distilled or deionized water was added to each pot at mid-week, or as needed, to wash accumulated salts from the perlite and provide additional water. Additional nutrient solution was added if the solution level was depleted prior to the weekly solution change. Maintaining reservoir fluid levels at the bottom of the pot for the first few weeks after planting allowed roots to grow into the nutrient reservoir. Reservoirs were rinsed twice with a stream of distilled or deionized water, once with 0.1 M HCl, again with distilled or deionized water, and allowed to air-dry before being reused.

Plants were harvested at R5 for leaf dry matter and at R7 for seed. Plants harvested at R7 were removed from nutrient solution and allowed to dry for a few days in the greenhouse. These plants were not allowed to mature to R8 (full maturity) to reduce the likelihood of precocious seed germination (John Keiser, 1992, personal communication). At harvest, each plant was sectioned into thirds based on mainstem node number (top-third, T/3; middle-third, M/3; bottom-third, B/3). Branches (Br) were removed and considered a separate section. Pods <1 cm in length were not retained. Pods were air-dried in paper bags before shelling to ensure optimal seed quality (Fjerstad et al., 1981). Leaf material was air-dried prior to oven-drying at 60°C for 24 h. Three replicates of each treatment were analyzed for seed production and four replicates were analyzed for leaf dry matter.

Elemental Analysis
Complete elemental analysis was conducted on all treatments and sections for which sufficient seed material existed. Analysis was performed by inductively-coupled plasma spectroscopy. Seed material from 1992 was analyzed by Kevin Jewell, Dept. of Agronomy, Ohio Agriculture Research and Development Center (OARDC), Wooster, OH. In 1993, seed material was analyzed by the Research Extension Analytical Laboratory, OARDC. Oven-dried 1-g samples were analyzed for each treatment and section. A split-plot design with three replications was used for statistical analysis with treatments as main plots and plant sections as sub-plots. Treatment and section mean comparisons were performed with the Least Significant Difference (LSD) method ( = 0.05).

Seed Quality
Harvested seeds were subjected to the standard germination test by the rolled-towel method according to the Rules for Testing Seeds (AOSA, 1993) except that 25 seeds were used for each of three replicates and the test was evaluated after 7 d. Only seeds that were normal in appearance and were retained on a 5.4-mm round-hole sieve were used. Seedlings were sorted into normal and abnormal categories according to the Seedling Evaluation Handbook (AOSA, 1992), then normal seedlings were further separated into strong and weak categories according to the seedling vigor classification test described in the Seed Vigor Testing Handbook (AOSA, 1983). In addition to the deficiencies described in the Seedling Evaluation Handbook, watery hypocotyl was considered abnormal. Seeds from the Very Low and VL/Con treatments were bulked (within treatment) to provide sufficient seed for seedling vigor classification.

	Results

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Plant Productivity
Seed production was significantly affected by nutrient solution Ca concentration in both years (Fig. 1) . The control plants produced 50% more seed mass than the Low treatment in both years. The Very Low treatment seed yield was near 10% of the Control treatment seed yield in both years. Calcium deficiency, particularly for the Very Low treatment, also reduced leaf dry matter retained on the plant at R5 (Fig. 2) . The Control and Low treatments had more leaf mass at R5 in all three sections of the mainstem than the Very Low treatment. The mass of leaves retained in the Br section and the Total leaf dry matter present at R5 in each treatment decreased significantly with Ca level.

View larger version (25K): [in this window] [in a new window]   Fig. 1 Seed yield per plant for three main Ca treatments in 1992 and 1993. Values followed by the same letter within a year are not significantly different by LSD at = 0.05

View larger version (27K): [in this window] [in a new window]   Fig. 2 Dry mass of leaves for 1992 and 1993 (data pooled, year effect not significant) from the top- (T/3), middle- (M/3), and bottom- (B/3) third of the mainstem; the Branch (Br) section; and the Total per soybean plant at R5. Within a plant section (e.g., T/3), bars labeled with the same letters are not significantly different by LSD at = 0.05

The percentage of seedlings exhibiting Ca deficiency symptoms such as watery hypocotyl and epicotyl necrosis increased when the Ca level of the treatment decreased in both years (Table 3). In 1992, 7% of seeds from the Low and 41% of seeds from the Very Low treatments produced seedlings exhibiting Ca deficiency disorders. The same pattern of increasing frequency of Ca deficiency disorders with decreasing Ca occurred in 1993, but with less severity (i.e., less than 5% of the seedlings germinated from Low treatment seeds and 10% of seeds from the Very Low treatment expressed a Ca deficiency disorder).

	Discussion

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Plant Productivity
The decrease in seed production in the Low and Very Low treatments was associated with large reductions in leaf dry matter by R5 (Fig. 1 and 2). Egli and Leggett (1976) found that soybean seed yield was significantly depressed by partial defoliation while complete defoliation at the end of flowering reduced seed yield over 90% (mostly because of fruit abortion following defoliation). The reductions in seed yield in both years might be related to leaf abscission in the Low and Very Low Ca treatments. Environmental differences (shading) between 1992 and 1993 may also have influenced seed yield. Shading effects were more prevalent in early 1992 because of whitewash on the greenhouse roof and walls and cloud cover (NOAA, 1992 and 1993). Shading increases soybean flower and pod abortion and decreases yield in soybean (Jiang and Egli, 1993).

The increased abscission of leaves in the Low and especially the Very Low treatment is attributed to the low Ca levels in their nutrient solutions. High Ca levels delay tissue degradation via inhibition of polygalacturonase (Marschner, 1986). Poovaiah and Leopold (1973a, b) confirmed the role of Ca in maintaining leaf membrane integrity and delaying leaf senescence and abscission. Data presented here confirm that decreasing Ca in the rooting medium leads to decreased retention of leaf dry matter, which reduces photosynthetic capacity and soybean yield.

Seed Elemental Concentration
Low Ca levels also influenced the levels of Ca and other elements in normal seeds. The seeds produced on Control plants, or any of the treatments including the Control level of Ca either before or after R5, contained higher concentrations of Ca than found in the Low and Very Low treatments from either year (Table 1). This indicates that the seeds in the Con/Low and Con/VL were supplied with ample Ca prior to the R5 switch or that Ca was remobilized to the seeds apoplastically from the pod walls as found by Wiersum (1966) and Mix and Marschner (1976) and suggested by Keiser and Mullen (1993). Mauk and Nooden (1992) also found that Ca could be remobilized from the leaves of pod-bearing soybean explants. The VL/Con treatment may have provided seeds with high levels of Ca by the same apoplastic pathway or via the transpiration stream. Plants subjected to the Very Low and the VL/Con treatments produced seeds with high Ca concentrations in 1992. However, plants from these treatments produced seeds only on the Br or T/3 sections (data not shown). Because of their position in the canopy, the pods produced might have had higher levels of transpiration and, consequently, Ca delivery. Most of the leaves and pods (Fig. 2) in the lower sections of the plant abscised by R5, which likely increased the relative transpirational flow to the remaining pods. Since soybean flowers occur at the top of plant and on branches last, the seeds from the T/3 and Br sections could have had more of their phenological development under the Control level of Ca in the VL/Con treatment than seeds produced elsewhere on the plant.

The increased concentration of Mg, Mn, Zn, and B in both years, and of Cu and K in 1992, in treatments receiving low levels of Ca throughout growth or until R5 (Table 1) may indicate a compensatory uptake of these ions or, more likely, a decrease in competition for uptake and binding sites. Competition with high Ca levels for uptake in other treatments may have reduced plant uptake of Mg, Mn, and Zn, but not of B (Marschner, 1986). Boron is unique in the above group of elements. While the others are divalent cations, B exists as an anion in aqueous solution. However, B moves in the xylem and functions to stabilize cell walls and membranes in much the same way as Ca (Marschner, 1986). Hanson et al. (1985) indicated that B may also be remobilized from tissues containing high concentrations of the borate ion. Phosphorous concentrations in seeds produced in either year followed no clear pattern.

The differences in elemental concentration in seeds from different plant sections (Table 2) are attributed to differences in the relative transpiration of pods produced in a given section. Boron moves almost exclusively in the xylem and its translocation and uptake are not mediated by exchange processes or competition with other mineral nutrients (Marschner, 1986). Consequently, the elevated B concentrations in the T/3 and Br sections in both years and in the M/3 section in 1993 could indicate higher rates of transpiration by these fruits because of their position in the canopy. This also may explain the markedly higher B concentrations in seeds from the sparsely foliated VL and VL/Con (at R5, VL/Con is equivalent to VL in Fig. 2) treatments. Results for Ca were similar except that the M/3 section was not higher in Ca in 1993. These observations suggest that transpiration may be different among pods developing in different plant sections (and at different times in the case of indeterminate cultivars). This conclusion differs from the findings of Keiser and Mullen (1993) who found no differences in seed Ca concentration between determinate soybean plants grown in 55 and 95% RH environments. The higher seed Ca and B concentrations found in 1993 might have been due to environmental differences caused by the presence of shading material on the greenhouse and cloud cover in the spring of 1992.

Seed Quality
The observed reductions in germination of low Ca seeds in the Low and Very Low treatments in both years have been reported by others (Cox et al., 1976; Keiser and Mullen, 1993; Smiciklas et al., 1989; Keiser et al., 1995). Treatments involving Control levels of Ca (Control, Con/Low, Con/VL, and VL/Con) had percentage germination that equaled or exceeded the germination of the Control treatment in each year. Seed quality in terms of germination was not reduced regardless of the timing of increased Ca delivery to the fruit. As with seed elemental concentration, the high germination rate of VL/Con should be evaluated with the understanding that all seeds in this treatment were produced on the T/3 and Br sections in 1992.

The number of strong seedlings was lowest in the Very Low treatment in both years, which supports the observation of reduced seedling axis dry weight of seeds from low Ca soybean plants (Keiser and Mullen, 1993), and reduced mean fresh weight and hypocotyl elongation in low Ca bean and cowpea seedlings (Helms and Myers, 1972). The increased frequency of abnormal seedlings in the low Ca treatments in both years (Table 3) was largely attributed to the increased incidence of watery hypocotyl and epicotyl necrosis. Watery hypocotyl and epicotyl necrosis are symptoms of Ca deficiency in soybean and have been thoroughly characterized in other legumes (Cox et al., 1989; Helms, 1971; Helms and Myers, 1972). When Ca deficiency disorders appeared in more than approximately 1% of the seedlings, a minimum of half the abnormal seedlings possessed a Ca deficiency disorder. Differences in seed quality between years might be explained by differences in shading and transpiration that occurred during plant growth (Smiciklas et al., 1989).

This study confirms that Ca levels in the nutrient medium of soybean plants grown in modified hydroponic culture influence seed Ca concentration. Apoplastic movement of Ca to seeds is implicated when sufficient levels are present in the fruit wall. Differences in seed nutrient concentration due to the position of the fruit on the plant also occur during development and can be related to transpirational or phenological effects on the delivery of Ca to the fruit. When deficient Ca levels were found in seeds, seed quality was reduced and watery hypocotyl and epicotyl necrosis disorders were present. Conditions of inadequate Ca supply may lead to low seed production and reduced seed quality in soybean.National Oceanic and Atmospheric Administration. 1992; National Oceanic and Atmospheric Administration. 1993

	ACKNOWLEDGMENTS

 
The authors extend their appreciation to Dr. Don Eckert of the School of Natural Resources, The Ohio State University, for his comments on an early version of this manuscript. The authors also thank Mr. Bert Bishop for statistical consulting and Mr. Harold Brown for assistance in maintaining greenhouse space and resources.

	NOTES

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Salaries and research support provided by state and federal funds appropriated to the Ohio Agric. Res. and Development Ctr., Ohio State Univ. Journal article no. 53-95.

Received for publication June 23, 1999.

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