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Published online 2 October 2006
Published in Crop Sci 46:2396-2402 (2006)
© 2006 Crop Science Society of America
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CROP PHYSIOLOGY & METABOLISM

Genotypic Variation for Shoot N Concentration and Response to Water Deficits in Soybean

C. Andy King and Larry C. Purcell*

Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, 1366 W. Altheimer Dr., Fayetteville, AR 72704

* Corresponding author (lpurcell{at}uark.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
For soybean [Glycine max (L.) Merr.], reports differ as to the degree to which accumulation of shoot biomass and N are inhibited by water deficit (WD). Our first objective was to evaluate responses of shoot biomass and shoot N to WD for soybean genotypes from diverse genetic backgrounds. The second objective was to determine if the severity of WD affected the response of N accumulation relative to biomass accumulation among genotypes. In a 2-yr field study of 15 genotypes, shoot N concentrations ranged from 25 to 33 mg N kg–1 for plants from well-watered (WW) treatments harvested around 60 d after planting, and N2 fixation accounted for 68% of the N accumulated. Following a 16- to 22-d WD period, shoot N concentrations were similar to that of WW plants for genotypes with low WW N concentrations but decreased for genotypes with high WW N. As a result, the shoot N concentration range among genotypes decreased following WD stress. In a greenhouse study where essentially all of the plant N accumulated was from N2 fixation, shoot N concentration increased across a range of WD levels for six genotypes with low WW N concentrations but decreased for two genotypes with high WW N. These results indicate that changes in shoot N concentration in response to WD depend on both the inherent N concentration of a given genotype and the severity of the WD stress.

Abbreviations: FTSW, fraction of transpirable soil water • MG, maturity group • WD, water deficit • WW, well watered


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE REDUCTION of CO2 during photosynthesis and the symbiotic fixation of N2 and subsequent incorporation into amino acids and other N compounds represent two of the most energy-requiring processes in biology. The energy-intensive nature of these processes is also reflective of their importance, and taken together C, H, O, and N compose approximately 95% of the elemental mass found in plant material (Marschner, 1995). For N2–fixing legume crops, such as soybean, differences in relative sensitivity of biomass and N accumulation to WD may reflect primary limitations to yield under WD conditions.

The sensitivity of nitrogenase activity in legumes to WD has long been established (Sprent, 1972). Several reports have concluded that N2 fixation is more sensitive to WD than C assimilation by comparing photosynthesis and nitrogenase activity response to WD (Djekoun and Planchon, 1991) and by evaluating changes in tissue N concentration in response to WD relative to that of WW plants (Sinclair et al., 1987; Sall and Sinclair, 1991; Serraj and Sinclair, 1997). Also supporting the hypothesis that N2 fixation is more sensitive to WD than other physiological processes are reports of soybean yield increases in response to N fertilization in water-limited environments but smaller or no yield response to N fertilizer for high-yielding environments (Lyons and Early, 1952; Sorensen and Penas, 1978; Purcell and King, 1996; Purcell et al., 2004; Ray et al., 2006). In contrast, Streeter (2003) reported increased leaf N concentration in the cv. Flint in response to a 4-wk WD stress. He concluded that there was no evidence of N deficiency and that the negative impact of drought on nodule function was not the cause of the depression of shoot growth.

Conflicting results concerning the sensitivity of biomass and N accumulation to WD may arise from a combination of factors, including genotypic differences in the relative sensitivity of N2 fixation to water deficit. In a direct measurement of nitrogenase activity (acetylene reduction assay) during a progressive WD, Sinclair et al. (2000) reported differences among soybean genotypes in the soil water content at which N2 fixation began to decline. Djekoun and Planchon (1991) found that, for the cv. Hodgson, nitrogenase activity declined earlier than photosynthesis in a progressive WD, but as the deficit became more severe photosynthesis was inhibited more than N2 fixation. Therefore, soybean genotype and the severity of the WD may determine the degree to which photosynthesis or N2 fixation are affected.

Decreased N concentration due to the sensitivity of N2 fixation to WD stress relative to other physiological processes may lead to reduced soybean yields. A better understanding of phenotypic traits associated with the relative sensitivity to WD of N accumulation among genotypes may be useful in selecting for drought-tolerant plant types. There has been no systematic evaluation of a wide range of soybean germplasm for biomass and N accumulation during WD relative to WW conditions. The first objective of this research was to compare shoot biomass and N accumulation rates and shoot N concentration changes in response to WD stress relative to that under WW conditions among soybean genotypes that represent diverse genetic backgrounds. Our second objective was to determine if changes in N concentration during WD were influenced by the level of WD to which plants were subjected.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Field Evaluation of Major Southern U.S. Soybean Ancestors
Twelve soybean genotypes that are major contributors to the genetic background of modern cultivars in the southern USA (Ziya et al., 1993) were evaluated for differences in shoot biomass, N, and Mn in response to soil dehydration in a 2-yr field study. Cultivars Jackson and KS4895 were also included as tolerant and sensitive controls, respectively, for N2 fixation response to WD (Purcell et al., 2000). A non-nodulating sister line of the cultivar Lee (Hartwig, 1994) was included as an indicator of available soil N. Soybean was sown 2 July 1999 and 11 July 2000 at Keiser, AR (latitude 35°40' N), at a density of 75 seeds m–2. Plots were 6 m long by 1.7 m wide and consisted of nine rows at 19-cm spacings. The soil was a Sharkey silty-clay (very-fine, smectitic, thermic Chromic Epiaquert) with approximately 1% organic matter.

In 1999, the experiment was a split plot with four replications with irrigation (WW and WD) as the main plot and soybean cultivars as subplots. In 2000, main plots were WW, WD, and a WD plus soil-applied Mn (+Mn). For the WD +Mn main plot, Mn was applied to the soil surface before planting as MnSO4 at 75 kg ha–1 in a water carrier volume of 450 L ha–1 using a backpack sprayer. Manganese treatments were included in this experiment because, under low-Mn conditions, supplemental Mn may prolong N2 fixation during WD (Purcell et al., 2000). In these experiments, however, Mn treatment had no effect on shoot biomass or shoot N or Mn concentrations (data not shown). Therefore, the WD +Mn data were pooled with the WD–Mn data to give eight replications of the WD treatment in 2000.

All plots were irrigated with an overhead lateral-move system as needed to maintain a maximum soil water deficit of 50 mm, as determined by the Arkansas Irrigation Scheduling Program (www.aragriculture.org/computer_programs/irrigation_scheduling/default.asp; verified 4 Aug. 2006), to avoid WD stress until plants reached at least 90% canopy closure, at which time the two irrigation regimes were established. All plots received 25 mm of irrigation 7 d before the first biomass harvest (Harvest 1). Rainfall of 8 mm occurred 3 d after Harvest 1 in 1999, and no other rainfall occurred between biomass harvests either year. Well-watered plots received an average of 45 mm of irrigation per week between harvests in both years, while WD plots were not irrigated between harvests. The first harvest was the aboveground biomass from a 1-m2 bordered area from the center five rows of each plot on 10 Aug. 1999 and 22 Aug. 2000 when all cultivars were growing vegetatively. The second 1-m2 harvest (Harvest 2) was taken from a bordered area of the center five rows of each plot on 1 Sept. 1999 and 7 Sept. 2000 (22 and 16 d after Harvest 1, respectively), at which time maturity group (MG) III and IV cultivars were in full bloom and later-maturing genotypes were still vegetative.

Biomass samples were dried, weighed, and ground to pass a 2-mm sieve. A subsample was ground to pass a 1-mm sieve. For finely ground samples from Harvest 2 in 2000, ureide concentration was determined using the colorimetric procedure of Young and Conway (1942). For all sample dates, shoot N concentration was determined by the Dumas method with a Leco FP-428 Determinator (Leco Corporation, St. Joseph, MO) at the Soil Testing and Plant Analysis Laboratory at the University of Arkansas. Total shoot N (g m–2) was calculated by multiplying shoot dry weight by shoot N concentration. Nitrogen and biomass accumulation rates (g m–2 d–1) were determined by dividing the total change in N and biomass (g m–2) from Harvest 1 to Harvest 2 by the number of days between biomass harvests. Nitrogen concentration in the accumulated biomass was determined by dividing the N accumulation rate by the biomass accumulation rate and was converted to grams of N per kilogram of biomass. The N difference method was used to estimate the amount of N derived from N2 fixation for each line at each harvest date by subtracting the total shoot N (g m–2) for the non-nodulating line from that of the N2–fixing lines (Weber, 1966). Patterson and LaRue (1983) reported a good correlation between the N difference method and 15N dilution technique for estimating the fraction of N derived from N2 fixation under field conditions where nodulated lines accumulated significantly more N than a non-nodulating line.

Greenhouse Evaluation of Shoot N Concentration Response to Water Deficit Level
A greenhouse study was conducted to evaluate shoot N concentration response to different soil-watering regimes for eight soybean genotypes. Jackson and KS4895 were included as controls for drought-tolerant and drought-sensitive N2 fixation, respectively. Six MG IV plant introductions were included based on results from a preliminary field screen indicating that two were drought sensitive (PI 28330, PI 578439) and four were drought tolerant (PI 398318, PI 424140, PI 424225, PI 424538). The primary criterion used to determine drought tolerance in this screen was N accumulation (g N m–2) during water deficit (King and Purcell, unpublished data, 2000). The experimental design was a randomized complete block with a factorial arrangement of eight genotypes and four soil-water regimes with four replications.

Seeds were sown on 8 Apr. 2004 in 15-cm pots with an approximate soil volume of 1.9 L. The potting medium was a N-free mixture of peat and perlite (LB2, SunGro Horticulture Inc., Bellevue, WA). Potting medium was saturated and rinsed with excess deionized water, and 750 mL of full-strength N-free nutrient solution (de Silva et al., 1996) was added before sowing. Pots were inoculated with Bradyrhizobium japonicum (USDA 110) after sowing, allowed to drain overnight, and pot-capacity weights recorded. Plants were thinned to one per pot after emergence and maintained WW by adding deionized water as needed. Three weeks after planting, an additional 250 mL of N-free nutrients was added to each pot. Day and night temperatures were approximately 30 and 24°C, respectively. Natural illumination was supplemented with 1000-W metal-halide lamps for a 16-h photoperiod.

Transpirable water at pot capacity was defined as the difference between the pot-capacity weight and the pot weight when daily plant transpiration for WD plants was <10% of WW plants (Ritchie, 1981). Transpirable water was assumed to be zero at 24% of pot capacity weight, based on previous results with the same potting medium (King, unpublished data, 1999). The fraction of transpirable soil water (FTSW) was calculated as described previously (Purcell et al., 1997) and represents the fraction of the total plant-available water left in the pot at a given fraction of pot-capacity weight.

At 28 d after planting, when plants were at V6 to V7 growth stage (Fehr and Caviness, 1977), all pots were watered to 70% of the capacity weight. Well-watered pots were watered to 70% of pot capacity daily for the remainder of the experiment. Pots in the three WD treatments were dried stepwise so that at 32 d after planting pots had reached the desired water levels of 46, 41, and 31% of pot capacity weight corresponding to FTSW values of 0.30, 0.22, and 0.10, respectively. Pots were weighed and watered to these target weights once daily for 10 d until plants were harvested. Previous experiments using the same size pots and similar potting medium have shown that daily watering of WW pots to 70% pot capacity daily does not inhibit growth or transpiration in 28- to 42-d-old plants, and watering once daily to 46, 41, and 31% pot capacity results in moderate to severe reduction in transpiration and biomass accumulation (King, unpublished data, 1999). Plant shoots were harvested at 42 d after planting, when all genotypes were still growing vegetatively. Oven-dry weights were recorded and shoots were ground to pass a 1-mm sieve and assayed for total N and ureides as described previously.

To establish soil N availability in our greenhouse experiment, the cultivar Lee and a non-nodulating sister line to Lee were evaluated in a separate experiment for biomass and N accumulation. Average seed weight and seed N concentrations were determined for each genotype to estimate the approximate seed N available to each plant. Six replications of each genotype were planted in 15-cm pots using the same potting medium and N-free nutrient solution used in the previous greenhouse experiment. Pots were watered to 70% of saturated soil weight daily until plants were harvested at 35 d after planting. After harvest, plants were dried at 65°C in a forced air oven and dry weights and total N concentrations were determined for each plant as described previously.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Field Evaluation of Major Southern U.S. Soybean Ancestors
There was no effect of year, irrigation treatment, or genotype on the fraction of shoot N that was derived from N2 fixation. Averaged over years, irrigation treatment, and genotypes, the fraction of shoot N derived from N2 fixation, as determined by the N difference method, was 0.51 at Harvest 1, 0.61 at Harvest 2, and 0.68 for N that accumulated between Harvests 1 and 2.

Data for shoot biomass and N for Harvest 1 were analyzed as a split plot with year as main plot and genotype as subplot. Biomass and N data from Harvest 2 and data for biomass and N accumulated between harvests were analyzed as a split-split plot design with year as main plot, irrigation as subplot, and genotype as sub-subplot. There was no interacting effect of year with genotype or irrigation treatment for shoot N concentration. There was a main effect of genotype on biomass at Harvest 1, which ranged from 88 g m–2 for Peking to 120 g m–2 for PI 417234 (Table 1). Main effects of genotype and irrigation treatment were significant for shoot N and biomass accumulation rates and for N concentration in shoot mass accumulated between harvests (Tables 1 and 2). Averaged over genotypes, N and biomass accumulation rates were 0.30 and 11.2 g m–2 d–1, respectively, in the WW treatment (Table 2). Nitrogen concentration in the accumulated shoot mass averaged 26.2 g N kg–1 (Table 2). Averaged over genotypes for the WD treatment, N and biomass accumulation rates were 0.13 and 6.5 g m–2 d–1, respectively. The average N concentration in shoot biomass accumulated between harvests for the WD treatment was 20 g N kg–1 (Table 2). The WD treatment decreased the accumulation rate of shoot biomass by 42% and decreased the N accumulation rate by 56% relative to the WW treatment (Table 2), indicating that N accumulation was more sensitive to WD than biomass accumulation when averaged across genotypes.


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  		Table 1. Main effect of genotype averaged over year (NS) and irrigation treatment on shoot N and biomass for the 2-yr field experiment. The genotype x irrigation interaction was NS.

 

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  		Table 2. Main effect of irrigation treatment averaged over year (NS) and genotype on N and shoot biomass accumulated between Harvest 1 and Harvest 2 for the 2-yr field experiment. The genotype by irrigation interaction was NS.

 
Averaged over years and irrigation treatments, N concentration in shoot biomass accumulated between harvests ranged from 18.4 g N kg–1 for Jackson to 28.2 g N kg–1 for KS4895 (Table 1). Excluding PI 88788, which had low accumulation rates for both N and biomass, shoot biomass accumulation rates between Harvest 1 and Harvest 2 ranged from 7.94 to 9.23 g m–2 d–1 for a difference of 14% among genotypes. Shoot N accumulation rates between biomass harvests ranged from 0.170 to 0.255 g m–2 d–1 for a 35% difference between genotypic extremes for N accumulation. These results indicate that differences among genotypes for N concentration in accumulated shoot mass (Table 1) were affected more by differences in N accumulation rates than differences in biomass accumulation rates.

For shoot N concentration, there was a main effect of genotype at Harvest 1 and an interaction of genotype x water treatment at Harvest 2 (Fig. 1). Shoot N concentration at Harvest 1 ranged from 35 to 39 g N kg–1 (Fig. 1), and by Harvest 2 shoot N concentration for WW treatments had declined to 26 to 33 g N kg–1. Biomass samples for Harvests 1 and 2 were taken at approximately 40 and 60 d after planting, respectively, both years. Declines in shoot N concentration of 5 to 10 g N kg–1 between 40 d after planting and the early seed-filling period were previously reported for field-grown soybean (Hanway and Weber, 1971; Overman and Scholtz, 2003).


Figure 1
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  		Fig. 1. Shoot N concentration (g N kg–1 dry weight) of soybean ancestors, Jackson, and KS4895 from the 2-yr field study at Keiser, AR. Harvest 1 was made when soybean reached 95% full-canopy coverage and Harvest 2 was made 22 and 16 d later in 1999 and 2000, respectively. Main effect of genotypes was significant at Harvest 1, and there was a genotype x irrigation treatment interaction at Harvest 2. There was no interacting effect of year with genotype or irrigation treatment on shoot N concentration at Harvests 1 or 2. LSD (P = 0.05) for Harvest 1 is 1.3 and for Harvest 2 comparing genotypes within irrigation is 1.5 and between irrigation is 1.9. An asterisk (*) indicates a significant difference between well-watered (WW) and water-deficit (WD) shoot N concentration within a genotype at Harvest 2. Maturity group is given below the genotype name.

 
In Fig. 1, genotypes are arranged from left to right on the x axis in order of increasing WW N concentration at Harvest 2. For genotypes with higher WW shoot N concentrations (≥29 mg kg–1) at Harvest 2, WD treatments tended to cause a decrease in shoot N concentration. Seven of the nine genotypes with the highest WW shoot N concentration had significantly lower shoot N for the WD treatment than for the WW treatment (Fig. 1). Shoot N concentrations were the same in WW and WD treatments for the five genotypes with the lowest WW shoot N concentrations (≤28 mg kg–1) (Fig. 1). Exceptions to these general trends were Arksoy and CNS, which both had relatively high (29–31 mg kg–1) shoot N concentrations under WW and WD conditions. CNS was previously shown to maintain high N accumulation rates during WDs under field-grown conditions (Sall and Sinclair, 1991). There is no indication that differences in soil N availability or degree of WD encountered by these genotypes would have contributed to the lack of response to WD for shoot N concentration. Plant size at Harvest 1 and growth rate between harvests of these two genotypes do not indicate that small plant sizes contributed to a different response to WD treatment for Arksoy and CNS than for other genotypes. Although PI 88788 had the smallest shoot biomass at Harvest 1, the slowest growth rate between harvests of all genotypes, and had a similar WW N concentration to Arksoy and CNS, it had a significant decrease in shoot N in response to WD.

There was no clear influence of maturity on the responses of shoot N to water treatment, although the three genotypes with the highest WW shoot N at Harvest 2 were MG IV and were at R2 growth stage at Harvest 2. Previous research evaluating shoot N concentration in eight soybean genotypes indicated that, for well-watered plants, shoot N declined during mid to late vegetative growth until the R4 growth stage and then increased until maturity (Hanway and Weber, 1971). Because none of these three genotypes were beyond R4 growth stage, it is unlikely that maturity was the controlling factor in determining WW N concentration at Harvest 2.

Shoot ureide concentration was linearly related (r2 = 0.59) to shoot N concentration in the WW treatment at Harvest 2 in 2000 (Fig. 2). There was some genotypic variation in the linear relationship between shoot ureides and N (Fig. 2). For example, CNS had relatively high shoot N yet had low shoot ureides under WW conditions. Interestingly, CNS was one of two genotypes with moderately high shoot N under WW conditions that maintained a similar N concentration under WD conditions (Fig. 1). Arksoy, the other genotype with moderately high shoot N under WW and WD conditions, had a shoot ureide concentration very close to the value predicted from the overall regression presented in Fig. 2.


Figure 2
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  		Fig. 2. Genotypic averages for well-watered (WW) shoot ureide concentration versus WW shoot N concentration at Harvest 2 for the 2000 field experiment at Keiser, AR. Only those genotypes that are specifically discussed in the text of this manuscript are identified.

 
Under WD conditions, there was no significant relationship between shoot ureides and shoot N concentration (P = 0.23). Shoot ureide concentrations in soybean reflect a balance between ureide production, transport, and catabolism (de Silva et al., 1996; Purcell et al., 2000). Therefore, due to the varying degree that WD stress interrupts these processes among soybean genotypes, it is not surprising that there was no strong relationship between shoot ureides and shoot N concentration following WD stress.

Greenhouse Evaluation of Shoot N Concentration Response to Water Deficit Level
Nitrogen available for plant uptake was determined for the potting mix used in our greenhouse experiment by analyzing N accumulation for nodulating and non-nodulating lines of the cv. Lee. Seed N before planting for Lee and Lee non-nodulating lines averaged 8.3 and 6.6 mg N per seed, respectively. When plants were harvested at 35 d after planting, total N for Lee averaged 75 mg N per plant, while for Lee non-nodulating the total N averaged 6.3 mg N per plant. These results indicate that practically all of the N accumulated by nodulated soybean growing in this plant-growth medium comes from N2 fixation.

In the greenhouse experiment, which was designed to evaluate genotypic changes in shoot N concentration in response to severity of WD, average WW N concentrations ranged from 30 g N kg–1 for Jackson to 39 g N kg–1 for KS4895 (Fig. 3). Averaged over the WD treatment period, daily evapotranspiration of WD treatments relative to the WW treatment differed significantly (P = 0.0001, LSD 0.05 = 0.02) among WD treatments and was 26% (FTSW 0.10), 50% (FTSW = 0.22), and 61% (FTSW = 0.3) of the WW treatment for all genotypes.


Figure 3
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  		Fig. 3. Change in shoot N concentration of eight soybean genotypes in response to the fraction of transpirable soil water (FTSW) to which pots were watered daily during a 10-d water deficit in the greenhouse experiment. Fraction of transpirable soil water of 0.6 was considered well watered (WW) and decreasing FTSW values represent an increasing water deficit (WD). Bars represent LSD (0.05) for treatment separation within a FTSW level. Numbers in parentheses to the right of a genotype indicate the shoot N content (g N kg–1 dry weight) for plants in the WW (FTSW = 0.60) treatment. There was a significant genotype effect for WW shoot N (LSD = 3.9 g kg–1, P = 0.05). Data points above the zero reference line on the y axis represent an increase in shoot N in response to WD and those below the line represent decrease in shoot N in response to WD.

 
Figure 3 shows the change in shoot N concentration in response to the 10-d water treatments. The y axis for Fig. 3 represents the difference in shoot N concentration between WW plants (daily watering target of FTSW = 0.60) and those plants grown at varying degrees of WD for each genotype. Data above the zero-reference line indicate an increase in shoot N concentration and data below zero indicate a decrease in shoot N concentration in response to WD relative to that of WW plants. Shoot N concentration responded similarly to WD as in the field experiment. In response to the moderate (FTSW = 0.30) and intermediate (FTSW = 0.22) WD treatments, shoot N concentration increased for the six genotypes with the lowest WW shoot N concentration and decreased for the two genotypes with the highest WW shoot N concentration (Fig. 3). At the most severe WD (FTSW = 0.1), there was generally no additional increase in shoot N concentration for genotypes with low WW shoot N. For the high-N genotypes, KS4895 and PI283330, N concentrations for plants subjected to the most severe WD were similar to WW control plants. It is important to note that shoot N concentration increased in response to WD for the four PIs that were selected from the preliminary field results as potentially drought tolerant (Fig. 3). Only PI 578439, which appeared drought sensitive in the field, responded differently than expected in the greenhouse study and had an increase in shoot N in response to WD.

Well-watered shoot N concentrations ranged from 30 to 39 g kg–1 and shoot ureide concentrations ranged from 10 to 22 µmol g–1 dry weight, but unlike the field data from Harvest 2 at Keiser in 2000, there was no linear relationship between WW ureide and WW N concentrations (r2 = 0.03) (data not shown). These results indicate that WW shoot N concentration is more likely a better indicator of genotypic response of shoot N concentration to WD than WW shoot ureides.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Our results indicate that differences in shoot N concentration exist among soybean genotypes under WW conditions, and changes in shoot N concentration in response to WD stress are likely related to WW shoot N concentration. Shoot N concentration generally declined in response to WD for genotypes with high WW shoot N concentration but not for those with lower shoot N concentration. The fact that there was no interacting effect of year with genotype on shoot N concentration at Harvest 2 in the field study indicates that genotypic differences for WW shoot N were repeatable. Also in support of this statement, Jackson and KS4895, the two genotypes included in both field and greenhouse studies, were consistently at the low and high extremes, respectively, for WW shoot N.

Although the range of WW N concentrations was markedly different between our field and greenhouse studies, the response of shoot N to WD was similar. In the field, shoot N did not change in response to WD for genotypes with relatively low shoot N concentrations. In the greenhouse, shoot N increased in response to WD for genotypes with low shoot N. For genotypes with high WW shoot N, WD generally caused a decline in shoot N concentration under field and greenhouse conditions. As a result, the range in shoot N concentration among genotypes was less extreme and concentrations tended to be more intermediate following WD than for WW plants. Results from the greenhouse experiment indicate that the severity of WD stress affected the absolute change in N concentration but not the general trends associated with WW N concentrations.

Streeter (2003) reported a 65% reduction in shoot growth and a concurrent increase in leaf (blade plus petiole) and pod N concentration for greenhouse-grown plants of cv. Flint in response to a 4-wk WD lasting from early pod set until plants were harvested at mid R5. Streeter (2003) concluded that N deficiency was not responsible for the observed depression of shoot growth, but rather N2 fixation was depressed because demand for fixed N was lower. Streeter's data represent one genotype and shoot N concentrations are not given, making it difficult to compare his results with ours. It is possible that the increase in shoot N concentration reported by Streeter for Flint following WD are in response to inherently low N concentrations. It is also possible that reproductively growing plants, such a those in Streeter's research, may respond differently to WD than those growing vegetatively as presented in this research.

Sall and Sinclair (1991) reported differences among genotypes for sensitivity of N2 fixation to WD based on N accumulation rates during a drought and by evaluating relative rates of N2 fixation (acetylene-reduction assay) during WD stress. In that report, there was no mention of plant N concentrations or other phenotypic traits possibly associated with the genotypic differences for drought sensitivity of N2 fixation. Subsequently, it has been reported that genotypes with drought-tolerant N2 fixation generally maintain lower shoot ureide concentrations than drought-sensitive genotypes (Serraj and Sinclair, 1997; Sinclair et al., 2000; Purcell et al., 2000). Our results from Harvest 2 in the 2000 field study indicate that genotypic differences for WW shoot N were linearly related to WW shoot ureide concentrations at that sample date.

Our data provide evidence that genotypic differences for sensitivity of N accumulation during WD are apparently associated with WW shoot N concentrations. Since 30% of the N accumulated between harvests in the field study apparently came from mineral N in the soil, we cannot determine the absolute relative importance of WD on mineral N uptake versus N2 fixation in the field experiment. It should be noted, however, that 70% of the shoot N accumulated between harvests in our field studies and 100% of the total-plant N accumulated in the greenhouse were derived from N2 fixation and that the response of N concentration to WD was similar for the two experiments. These facts indicate that genotypic differences in WW shoot N concentration are associated with genotypic differences in sensitivity of N2 fixation to WD. Well-watered ureide concentrations were not consistently related to shoot N concentrations but cannot be ruled out as a potentially important phenotypic trait associated with genotypic differences in sensitivity of N2 fixation to WD.

It is unknown whether drought-tolerant N2 fixation can lead to increased yields under WD conditions or whether the possible link between drought-tolerant N2 fixation and low WW shoot N may limit yield potential in high-yield environments. Previous research indicates a positive correlation between soybean yields and shoot N concentration (Pal and Saxena, 1976). However, Jeppson et al. (1978) noted differences in total shoot N among similar yielding soybean genotypes that resulted from differences in N harvest index. If, however, sensitivity of N2 fixation to WD is associated with WW shoot ureides rather than WW N concentrations, a genotype with high shoot N that maintains lower shoot ureides might provide a source of germplasm for drought-tolerant N2 fixation and high yield potential. More research is needed evaluating the possible links between relative sensitivity of N2 fixation to WD among genotypes with differing shoot N and/or shoot ureide concentrations. Especially useful would the identification of genotypes that are exceptions to the trend for high WW N and drought sensitive N2 fixation.


   ACKNOWLEDGMENTS
 
We thank Bob Glover for his excellent assistance in collecting field samples at Keiser, AR, in 1999 and 2000 and Marilynn Davies for her assistance in sample preparation and tissue ureide analysis.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Research supported in part by the United Soybean Board, project 5213.

Received for publication March 13, 2006.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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