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SOYBEAN

Soybean Genetic Improvement in Yield and the Effect of Late-Season Shading and Nitrogen Source and Supply

^a Dep. of Plant and Soil Sciences, Univ. of Kentucky, 1405 Veterans Dr., Lexington, KY 40546-0312
^b Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, Canada N1G 2W1

^* Corresponding author (s.kumudini{at}uky.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Genetic improvement in soybean [Glycine max (L.) Merr.] yield has been associated with both assimilate and N accumulation [especially from dinitrogen (N₂) fixation] during the seed-filling period (SFP). Therefore, the physiological factors associated with genetic improvement may be dependent on abundant assimilate and N supply. The objectives of this study were to quantify genetic improvement in yield under: (i) assimilate limiting conditions, (ii) under low N fertility, and (iii) when either inorganic N fertilizer or N₂ fixation is the main source of N. A randomized complete block experiment was conducted at two locations in 1998 and one in 1999. The main plot factor included no shade or a 63% shade treatment imposed after the R4/R5 developmental stage. The split plot factors were three N treatments: (i) inoculated, (ii) uninoculated with no additional fertilizer, and (iii) uninoculated plus fertilizer N. The split-split plots were four cultivars, representing a pair of older and a pair of newer cultivars. Shading reduced dry matter (DM) accumulation, and both shading and N limitation reduced N accumulation and seed yield. However, the newer cultivars consistently maintained their yield advantage over the older cultivars under both shading and N limiting conditions as well as when the source of N was soil available N or N₂ fixation. The results of the study suggest that the physiological factors that contribute to genetic improvement in yield are not dependent on the level of incident radiation during the SFP, or the source or availability of N during the SFP.

Abbreviations: DM, dry matter • –Fert, uninoculated with no additional fertilizer after R5 • +Fert, uninoculated plus fertilizer N after R5 • HI, harvest index • +Inoc, seeds inoculated at planting • LAD, leaf area duration • NDVI, normalized difference vegetation index • SFP, seed-filling period • –Shade, no shade imposed • +Shade, 63% shade imposed following R4/R5 growth stage

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication July 10, 2006.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
GENETIC YIELD IMPROVEMENT IN SOYBEAN has been reported to be associated with increased assimilate supply during the life cycle. Yield improvement has been linked with increases in leaf carbon exchange rate (Morrison et al., 1999), canopy apparent photosynthesis (Wells et al., 1982; Boerma and Ashley, 1988), leaf greenness as measured by normalized difference vegetation index (NDVI) during reproductive development (Ma et al., 2001), and greater leaf area duration (LAD) and DM accumulation during the SFP (Kumudini et al., 2001).

Nitrogen accumulation has also been proposed as a major yield-limiting factor in soybean (Sinclair and de Wit, 1976). Soybean has a protein rich seed and thus requires relatively large amounts of N to attain high yields (Sinclair and de Wit, 1975). Soybean can accumulate N through either: (i) N₂ fixation in soybean nodules by an association between the soybean plant and Bradyrhizobium bacteria, or (ii) inorganic N uptake. Considering the importance of N to soybean yield, genetic improvement in yield may be associated with improved N accumulation. This may be achieved either by improved N₂ fixation, enhanced N uptake, or both.

Genetic variability in N accumulation is extant in the soybean germplasm (Pazdernik et al., 1997). It was assumed that high-yielding soybean lines would naturally accumulate less N, however, Pazdernik et al. (1997) noted that irrespective of yield, the genetic material tested had both low- and high-N accumulating soybean lines. This suggests a potential for genetic improvement for high-yielding and high-N accumulating traits. In a study of inoculated soybean cultivars, the newer cultivars accumulated more N (g N m^–2) than the older cultivars even though the newer cultivars had lower seed N concentration (Kumudini et al., 2002). This supports the contention that newer cultivars are better accumulators of N (g N m^–2). However, in these studies the mechanism of improved N accumulation (i.e., improved N₂ fixation or uptake of soil N) was unknown. In a Japanese study, genetic improvement of Japanese cultivars was associated with a greater capacity to fix N₂ during the SFP (Shiraiwa et al., 1994), suggesting that genetic improvement maybe related to improved N₂ fixation capacity.

The ability of plants to accumulate assimilates and accumulate N are interrelated. Nitrogen accumulation is an energy-requiring process, whether it is achieved by fixation or by nitrate assimilation (Pate et al., 1979) and therefore N accumulation will likely be influenced by assimilate supply. Leaves with higher N concentration have also been shown to improve radiation use efficiency (Sinclair and Horie, 1989). Hence increase in N accumulation may also lead to increased assimilate accumulation. Consequently, genotypes that have a greater inherent capacity to accumulate assimilates may also accumulate more N, and vice versa.

Genetic improvement in yield should involve genetic changes that work to reduce yield-limiting factors. Since assimilate supply and N, either alone, or in combination, is important in soybean yield determination, soybean genetic improvement may well be dependent on factors associated with assimilate and or N accumulation. If genetic improvement is indeed dependent on assimilate or N accumulation, would genetic gain be apparent in environments in which plants are under stress for assimilate supply or N supply by either N₂ fixation or N fertilizer? For example, if genetic improvement in yield is associated with higher maximum leaf photosynthetic rates, genetic gains in yield may only be observed under favorable assimilate accumulation environments. Alternatively, if genetic improvement in yield is associated with increased N₂ fixation rates, genetic gains would be restricted to inoculated soybeans plots. The objectives of the current study were to quantify genetic gains in yield under: (i) assimilate limiting conditions, (ii) under low N fertility, and (iii) when either inorganic N fertilizer or N₂ fixation is the main source of N.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field experiments were conducted in 1998 and 1999. Two field experiments were conducted in 1998 near Elora and Rockwood, ON (43° N, 80° W). Previous crops were barley (Hordeum vulgare L.) and corn (Zea mays L.), respectively. The field in Elora was moldboard plowed in late fall before the spring planting, and the field in Rockwood was disked multiple times in the spring before planting. In 1999, the experiment was conducted near Rockwood, ON, in a field previously planted to corn and the crop residue moldboard plowed in the fall before spring planting. The two locations in 1998, and the single location in 1999 will henceforth be referred to as the three locations and/or years. Fields with no known history of soybean production were selected at each location per year. All locations had loamy soils (Typic Hapludalf) and were well drained or moderately well drained. Weed control was achieved using Pursuit (BASF, Princeton, NJ) {Imidazolinone, 2[4.5-dihydro-4-methyl-4-(1-methyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid}as a postemergent herbicide and hand weeding. The test was machine-planted in 35.6-cm row widths at double the desired plant density and thinned to a population of 500 000 plants ha^–1.

The experiment was laid out as a randomized complete block design in a split-split plot arrangement with four replications in each of the three locations and/or years. The main plots (22.5 by 6 m) were shade (+Shade) or no shade (–Shade). Shade treatments were imposed after the onset of the SFP (after R4/R5) and a 3-m border was planted around each main plot. The shade cloth (Gintec Shade Tech., Windham, ON) reduced incident PPFD (photosynthetic photon flux density) at the top of the canopy by 63% at solar noon. The shade cloth was suspended (1.5–1.3 m) over the soybean canopy using wooden posts to maintain airflow.

The split plots (7.5 by 6 m) were the three N treatments; (i) inoculated (+Inoc), (ii) uninoculated and no additional N (–Fert), and (iii) uninoculated plus additional fertilizer N during the SFP (+Fert). In the early spring of each year, soil samples were tested for presence of Bradyrhizobium japonicum bacteria. Once planted the plants were periodically checked for presence of nodules. All locations were confirmed to be free of Bradyrhizobium bacteria except for the regions of the field under the +Inoc treatment. The +Inoc treatment received a furrow applied granular inoculant (Nitragin, Liphatech Inc, Milwaukee, WI) at planting. Ammonium nitrate fertilizer (34–0–0) was applied on all treatments at a rate of 20 kg N ha^–1 at the V3 stage (Fehr and Caviness, 1977). The purpose of this application was to minimize early season N limitations stress, while not limiting nodulation in the +Inoc treatment due to excess soil available N. The –Fert treatment received no additional N apart from the 20 kg N ha^–1 at the V3 stage. The N management at the V3 stage was expected to minimize N stress during the vegetative and early reproductive stages, but insufficient for N demand by the crop later in the growing season. Growth analysis samples taken before the R4 growth stage and analyzed for N showed that the N content of the three N treatments was not significantly different from one another at R4 (data not shown). The growth analysis samples were again analyzed for N at the R7 growth stage, and +Inoc and the +Fert treatments were found to have accumulated significantly more N and have greater seed yield than the –Fert treatments. Therefore, the –Fert treatment resulted in N limitation only later in reproductive development. The +Fert treatment received an additional 80 kg N ha^–1 of ammonium nitrate fertilizer (34–0–0) after the beginning of the SFP. It was expected that the +Fert treatment would receive sufficient N during the SFP and that this treatment could be related to the –Fert treatment to weigh the impact of N limitation vs. no N limitation during the SFP. Furthermore, since the +Fert treatment received all its N from soil N uptake, this treatment could be compared against the +Inoc treatment to determine the impact of soil N vs. N₂ fixation.

Four indeterminate genotypes (Table 1 ) were arranged as split-split plots. These genotypes were paired into two groups based on similarities in maturity (early vs. later maturing), and lodging scores but differed in date of cultivar release. Each pair consisted of an old (released before 1950) and a newer (released post-1980) cultivar. These cultivars had been previously tested and described in prior publications (Kumudini et al., 2001, 2002; Specht et al., 1999). The objective of using pairs was to reduce the impact of confounding factors such as lodging, and maturity on yield gains. Seeds for the two historical genotypes (cv. Pagoda and cv. Mandarin Ottawa) were obtained from Agriculture and Agri-Food Canada (Ottawa, Canada). Certified seed was obtained from a local supplier (First Line Seeds, ON) for the two newer cultivars (cv. Maple Glen and cv. OAC Bayfield).

The PROC MIXED procedure of SAS (v. 8, SAS Institute, Cary, NC) was used for data analysis and for generating all reported means. The three locations and/or years and the four replications were considered random effects. The shade, N and genotype treatments were all considered fixed effects. The objective of the study was to determine the role of the treatments (assimilate supply and N source and supply) on genetic improvement (difference due to old vs. newer cultivars), hence, if genotype was found to be significant, an "Estimate" statement was written to contrast old vs. newer soybean genotypes. If a genotype by treatment interaction was found to be significant, the interaction term was tested for validation of differences in response due to genetic improvement, that is, "Estimate" statements were written to determine if the difference between old and newer cultivars changed due to the treatments imposed. Therefore, only those genotype by treatment interaction comparisons that were related to genetic improvement (i.e., when the estimate statement was significant) were considered of interest in this study.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
There was a significant location/year effect on seed yield and HI. However, neither effects of the imposed treatments on the measured variables, nor differences between older and newer genotypes were differentially affected by the location/year effect. Consequently, all data reported are means across the three locations and/or years. It should be noted, however, that seed yield, HI, and N accumulation were lower than normal in 1998. Precipitation was 222 mm during the 1998 growing season and 473 mm during the 1999 growing season, whereas the 20-yr mean seasonal precipitation was 423 mm (data from the Elora experiment station weather station). Seed yields were significantly lower in 1998 than in 1999 (P < 0.0001), likely due to the lower than average precipitation that year. Yield, HI, and N accumulation in 1999 were consistent with those obtained in a normal growing season for the region.

In both years of the test, experimental yields were equivalent to mean county commercial yields: Yield of OAC Bayfield (cultivar adapted to the region) grown under normal growing conditions (i.e., +Fert or +Inoc and –Shade) was 1.9 Mg ha^–1 (which is equivalent to 28 bu acre^–1) in Elora and 2.2 Mg ha^–1 in Rockwood, which was consistent with the mean commercial yield in the county of 2.2 Mg ha^–1 (32 bu acre^–1) in 1998. In 1999, the yield of OAC Bayfield was 2.8 Mg ha^–1 (42 bu acre^–1), which was consistent with the mean commercial yield in the county of 2.7 Mg ha (40 bu acre^–1) (http://www.omafra.gov.on.ca/english/stats/crops/index.html). Means for seed yield, HI, and N accumulation across the three locations and/or years reported herein were relatively low because two-thirds of the data generated in this study were collected in 1998 (see above) and, in addition, means depicted in tables represent means across N or assimilate limited treatments.

Effect of Nitrogen Source and Nitrogen Supply During the Seed-Filling Period
The N treatments had a significant effect on seed yield, N accumulation and HI (Table 2 ). The additional late season N made available due to the +Fert and +Inoc treatments, resulted in greater accumulation of N during the SFP (Table 3 ) and improved seed yield (Table 4 ). Sinclair and de Wit (1975) postulated that soybean yield was dependent on N availability, and a number of reports have confirmed the importance of N accumulation to seed yield, especially during reproductive development. Vasilas et al. (1995) reported that in a test of eight soybean lines, yield was positively correlated to N₂ fixation during the SFP and both Brevedan et al. (1978) and Afza et al. (1987) reported that application of fertilizer N during reproductive development can increase soybean yields. The results of the current study confirm that late-season N availability, whether through increased fertilizer N availability or N₂ fixation, is important for yield determination in soybean.

Although late-season N availability was important in yield determination, the source of the available N did not appear to impact yield determination. Soybean yields under the +Fert and the +Inoc treatments were equivalent and these yields were significantly greater than those under the –Fert treatment (Table 4). Based on both greenhouse and field studies, Imsande (1989, 1998) speculated that inoculated soybean plants have an advantage over those that acquire N through soil available N because rapid N₂ fixation during the SFP increases net photosynthesis and this in turn may increase seed yield. In a report by Vasilas et al. (1995) they also observed a relationship between N₂ fixation and yield. In their study of eight inoculated soybean lines, it was found that seed yield was positively correlated to the amount of N₂ fixed during the SFP. Although changes in levels of N₂ fixation may well improve seed yields within a line or in lines with variable ability for N₂ fixation, the results of the current study show that seed yields were the same irrespective of whether the plant acquired N by either N₂ fixation or uptake of soil available N and consequently, current results do not support the contention that the source of the available N has an important impact on the determination of soybean yield in field grown plants.

Genetic improvement in yield was neither dependent on source of N nor the quantity of N supplied during the SFP (Tables 2 and 4). Although N supply is important for yield determination, physiological factors associated with the greater yielding capacity of the newer cultivars, were not thwarted by low N supply, or by the source of the available N (Table 4). The physiological factors associated with genetic improvement in yield, must therefore not be dependent on either a specific source of N, or the level of available N to the plant. To the best of the authors' knowledge, the current study is the first in which genetic improvement in yield has been tested under non-N₂ fixation conditions (field trials on Bradyrhizobium-free soils). A previous report by Shiraiwa et al. (1994), suggested that genetic improvement may be associated with improved N₂ fixation late in reproductive development. Their study was conducted on four cultivars grown over 2 yr and N₂ fixation was estimated using the difference method (comparison of nodulating and non-nodulating cultivars). Although N₂ fixation capacity may be improved in newer genotypes, the data from the current study does not support the hypothesis that N₂ fixation is critical in genetic improvement in yield. If improved N₂ fixation capacity was critical, then genetic improvement in yield would only be observed when plants obtained N through N₂ fixation, and not when plant N supply was limited to soil available N. In the current study, N₂ fixation was not found to result in higher yields, and genetic improvement in yield was apparent under all N treatments (Table 4): under limited soil available N supply (–Fert), under sole-source, soil available N supply (+Fert) and under a combination with N₂ fixation (+Inoc). The current study was based on four cultivars over six treatments repeated across three environments (locations and years). Ideally a study with a large number of cultivars and direct measurement of N₂ fixation rates would give a more definitive determination of the role of improved N₂ fixation in genetic improvement in yield of soybean.

The physiological mechanisms by which genetic improvement in yield can function under low or different sources of N is not yet understood. The results of the current study suggest that future research into genetic improvement in yield should focus on factors in addition to improved N₂ fixation. A possible future direction may be to study factors that improve seed yield irrespective of N source and supply. Since N acquisition through both N₂ fixation and soil N uptake are energy requiring processes, one possible area of interest maybe to look at assimilate accumulation process that are independent of a requirement for high incident radiation during the SFP.

Effect of Incident Radiation Levels During the Seed-Filling Period
The +Shade treatment reduced DM accumulation during the SFP (Table 2), by 40% (–Shade accumulated 423 g m^–2 and +Shade accumulated 252 g m^–2 DM), confirming that the +Shade treatment effectively reduced assimilate availability during the SFP. A number of researchers have reported that DM accumulation during the reproductive phase is critical for yield determination (Hardman and Brun, 1971; Hayati et al., 1995), and the results from the current study are consistent with these earlier reports. The +Shade treatment had a significant impact on the final seed yield across all cultivars tested (Table 6 ). Since the +Shade treatment reduced DM accumulation during the SFP and yield but did not affect HI, the reduction in yield due to the +Shade treatment was related to the reduction in DM accumulation and not to reduced partitioning to the grain.

Genetic improvement in yield was not affected by the shade treatments. In other words, the difference in yield between the old and the newer cultivars were the same, irrespective of whether they were exposed to normal (–Shade) or reduced incident radiation (+Shade) levels during the SFP. For the four cultivars tested in this study, yield was significantly affected by a genotype x Shade interaction (Table 2). However when this response was further tested to determine whether differences in seed yield between older and newer cultivars were influenced by the shade treatments, (i.e., whether genetic improvement in yield was influenced by shade) it was found to be nonsignificant (estimate statement nonsignificant at P > 0.50). Although incident radiation levels during the SFP had an obvious impact on seed yield (Table 6), the newer cultivars were able to yield significantly more than the older cultivars under either shade treatments. Consequently genetic improvement in yield was unaltered by the availability of incident radiation during the SFP. The four genotypes tested differed in their response to shading during the SFP, but genetic improvement in yield was maintained despite the reduction in incident radiation during this phase. This suggests that physiological factors associated with genetic improvement in yield were not impeded by reduced incident radiation during the SFP. Therefore, the physiological changes associated with genetic improvement in yield must be independent of a requirement for high incident radiation levels during the SFP.

Studies on the physiological factors associated with genetic improvement in yield have reported on a number of different plausible mechanisms. Increases in leaf photosynthetic rate, canopy apparent photosynthesis, NDVI during reproductive development, and LAD during the SFP have all been reported to be associated with genetic improvement in yield (Wells et al., 1982; Boerma and Ashley, 1988; Morrison et al., 1999; Kumudini et al., 2001; Ma et al., 2001). The question therefore, is what plausible physiological mechanisms can best explain how genetic improvement in yield in soybean can be maintained under both low and high incident radiation levels during the SFP. Longer leaf area duration is one of a possible number of different mechanisms by which newer genotypes could maintain higher yields than older cultivars and be independent of a requirement for high incident radiation levels during the SFP. During the SFP, when total leaf area index declines and falls below the critical level, genotypes that maintain leaf area (increased LAD) would have greater advantage since this would allow for continued light interception and therefore higher canopy apparent photosynthesis and assimilate accumulation even when daily incident radiation was low (e.g., +Shade). In a previous report, Kumudini (2002) suggested that the longer LAD may have led to greater source/sink ratio in newer soybean cultivars. In support of this contention, when the older cultivars were exposed to the –Shade treatment (greater source/sink ratio), they had similar yields to the newer cultivars exposed to the +Shade treatment (lower source/sink ratio) (Table 6).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The results of this study showed that treatments that impacted N accumulation and late-season DM accumulation have a significant impact on seed yield. Increasing late-season N accumulation, by either providing additional fertilizer N during the SFP or by allowing N₂ fixation, had a positive impact on seed yield irrespective of assimilate supply (+Shade or –Shade). Similarly, increasing DM accumulation during the SFP (–Shade) increased seed yield irrespective of the N treatments imposed. Although it was shown that late-season N and DM accumulation are critical for yield determination, the differences between older and newer cultivars in seed yield, i.e., genetic improvement in seed yield, was not dependent on late-season N or DM accumulation. Seed yield of the newer cultivars was also significantly greater than that of the older cultivars irrespective of the N treatments imposed and, consequently, results indicate that genetic improvement in yield is independent of the source and supply of available N. Yields of the newer cultivars were significantly greater than that of the older cultivars irrespective of the shade treatments imposed and, consequently, results indicate that genetic improvement in yield is also independent of incident radiation levels during the SFP.

Previous reports on genetic improvement in yield have attributed a number of physiological factors as associated with genetic improvement. The results of the current study may help focus the list of possible physiological mechanisms of genetic improvement and suggests that physiological factors associated with genetic improvement in yield in soybean are independent of a requirement for high levels of incident radiation during the SFP, are not dependent on a specific source of N, nor require high levels of N during the SFP.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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