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CROP BREEDING, GENETICS & CYTOLOGY

Effect of Stem Termination on Soybean Traits in Southern U.S. Production Systems

^a Dep. of Crop, Soil, and Environmental Science, University of Arkansas, Fayetteville, AR, 72701 USA

csneller{at}comp.uark.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion and conclusions REFERENCES

 
Southern U.S. soybean [Glycine max (L.) Merr.] are primarily determinate and are grown in a diverse array of environments. Understanding the yield response of different plant types to southern environments may lead to improved yield. Our objective was to determine if varying stem termination could increase yield in some environments. Eleven pairs of isolines varying for determinate and indeterminate growth habit were derived from two crosses. All lines and controls were grown at two locations for 2 yr, planted in May and June and with or without irrigation. Seven pairs were grown for a third year. Yield, height, lodging, and maturity data were collected. In all crosses and production systems, determinate lines were shorter, earlier maturing, and had less lodging than indeterminate lines. Yields of determinate and indeterminate lines were equal in all irrigation–planting date combinations. There was a trend for determinate lines to yield better than indeterminate lines with early planting and irrigation. The yield difference between growth habits was conditioned primarily by a complex interaction of random genetic and environment effects. The indeterminate growth habit may confer a yield advantage over the determinate growth habit in environments with limited yield and growth potential. This was due to the lower yield response to increasing environment productivity of indeterminate vs. determinate lines. Perhaps 10 to 15% of southern production environments have conditions where indeterminate types may yield better than determinate types. The yield advantage of indeterminate types in such environments appears small. The success of deploying indeterminate cultivars to low-yield, low-growth southern environments to maximize yield will depend on accurately predicting where these environments will occur.

Abbreviations: WN, Williams 82 x Narow cross • RW, R85-336 x Walters cross • GEI, genotype x environment interaction

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion and conclusions REFERENCES

 
IN THE SOUTHERN USA, soybean is grown in a diverse array of production systems and environments characterized by varied planting dates, soil types, row widths, and environmental stress. In the South, soybean can be planted from April through August. Soybean in late plantings encounters a shorter photoperiod than May plantings, resulting in reduced plant height, branch development, days to maturity, days to flowering, and seed fill period (Board and Hall, 1984; Board, 1985; Dunphy et al., 1979). In addition, sporadic rainfall and high temperatures are common in the South. A midseason (July–August) drought is expected to occur in 9 of 10 yr (Bruce et al., 1985). It is estimated that 13% of the southern soybean acreage is irrigated (U.S. Department of Commerce, 1994), while nearly 40% of the soybean acreage is irrigated in Arkansas (Arkansas Agricultural Statistics Service, 1992).

Different combinations of planting dates, irrigation, and other factors may produce unique environments. While most southern soybean cultivars are determinate, it is possible that indeterminate growth habits are needed to fully exploit the yield potential of the diverse array of southern environments. Determinate (dt₁dt₁) growth habit of soybean is characterized by near cessation of main stem growth at the onset of flowering, a pronounced terminal raceme, and substantially fewer main stem nodes than the indeterminate growth habit (Bernard, 1972). Indeterminate (Dt₁Dt₁) growth habit is characterized by continued main stem growth into the reproductive period, producing a longer stem with more internodes than determinate types (Bernard, 1972). Most soybean cultivars grown in the northern USA are indeterminate.

In the northern USA, there have been extensive yield comparisons of determinate and indeterminate near-isogenic lines, as well as comparisons of large numbers of determinate and indeterminate lines derived from the same cross. Generally, in the northern USA, the yield advantage of one growth habit over another has either been negligible when averaged across environments and genetic backgrounds (Foley et al., 1986; Hartung et al., 1980; Hicks et al., 1969; Pfeiffer and Harris, 1990; Wilcox and Frankenburger, 1987), or indeterminate types have yielded more than determinate types (Ablett et al., 1989; Cooper and Waranyuwat, 1985; McBroom et al., 1981; Robinson and Wilcox, 1998; Shannon et al., 1971; Wilson and Cole, 1968). Several researchers have noted that the yield advantage of one growth habit over another varies by environment (Ablett et al., 1989; Cooper, 1981; Foley et al., 1986; McBroom et al., 1981; Wilson and Cole, 1968), genetic background of the near-isogenic lines (Ablett et al., 1989; Bernard, 1972; Hartung et al., 1980; Hicks et al., 1969; Raymer and Bernard, 1988a, Robinson and Wilcox, 1998), or with specific genetic background and environment combinations (Foley et al., 1986; Hicks et al., 1969; Wilcox and Frankenberger, 1987).

In the northern USA, some researchers report that indeterminate lines flower for a longer period of time than determinate lines (Bernard, 1972; Hicks et al., 1969; Rode, 1979). Others have not observed an effect of growth habit on phenology (Ablett et al., 1989; Foley et al., 1986; Ouattara and Weaver, 1994; Robinson and Wilcox, 1998). If it exists, the longer flowering period of the indeterminate types may result in more stable yield and better stress tolerance than the determinate growth habit. Several researchers have noted that the yield of determinate lines is more variable than the yield of indeterminate lines (Ablett et al., 1989; Beaver and Johnson, 1981b). Beaver and Johnson (1981a) suggested that determinate lines should not be grown in drought-prone or yield-limited environments, a view shared by Cooper (1981). Shannon et al. (1971) and Wilson and Cole (1968) noted that the yield advantage of indeterminate lines vs. determinate lines was greatest when plant populations were low.

Comparing different planting dates in the northern USA, Wilcox and Frankenburger (1987) reported that indeterminate types yielded better than determinate types, especially in earlier planting dates. Determinate types had less yield and height reductions from delayed planting than indeterminate types (McBroom et al., 1981; Raymer and Bernard, 1988b; Wilcox and Frankenburger, 1987). Beaver and Johnson (1981a) reported similar responses to planting dates with nonisogenic lines. Pfeiffer and Harris (1990) reported that indeterminate and determinate isolines had equal yield in May and June plantings.

There are few tests of the effect of stem termination on agronomic traits in the southern USA. Hartwig and Edwards (1970) noted that indeterminate isolines had 20% less yield than determinate isolines but did not describe the testing environments. It has been suggested (Boerma et al., 1982) that indeterminate cultivars may be advantageous in late plantings due to their continued growth after flowering. In a study involving 23 pairs of isolines and late plantings (July–August), Ouatara and Weaver (1994) found that determinate and indeterminate lines had equal yield when averaged across late-planted environments. They also noted considerable stem termination x environment interaction, with the indeterminate types yielding better than the determinate types in the most growth-limited environment. Panter and Allen (1989) compared the yield of 25 indeterminate and 25 determinate lines of varying maturity derived from the same crosses at two planting dates. They reported that the determinate lines had greater yield than the indeterminate lines at the late planting dates, but not at the early planting dates; high-yielding lines of both types were found at each date. The best determinate lines suffered no height reduction from delayed planting, while the height of the best indeterminate lines averaged 10.5 cm lower in late vs. early plantings.

Using a few nonisogenic lines, others have also noted that determinate types show less height reduction to delayed planting than indeterminate types, but have reported that indeterminate types had less yield loss than determinate types when planting was delayed (Parvez et al., 1989; Weaver et al., 1991). Indeterminate and determinate Group IV lines derived from the same population had the same yield from both April and June plantings, but planting date had a greater effect on the morphology of the indeterminate lines than the determinate lines (Akhter and Sneller, 1994). The stress tolerance of different growth habits has not been compared in the southern USA.

Our objective was to compare the effect of different stem termination genes on yield and other agronomic traits in production systems defined by different planting dates and irrigation levels in the southern USA.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion and conclusions REFERENCES

 
Eleven pairs of near-isogenic soybean lines that contrasted in growth habit were derived from two crosses. Six pairs of indeterminate and determinate lines were derived from the cross `Williams 82' x `Narow' (WN cross). Five pairs of nondeterminate and determinate lines were derived from the cross `R85-336' x `Walters' (RW cross). Based on genetics, field appearance, and behavior, the nondeterminate lines in the RW cross were probably indeterminate, even though R85-336 was derived from an isoline of `Clark' that possessed the Dt₂ allele for semideterminacy, as well as a parent that was indeterminate. These 11 pairs of lines were used in the field study.

Isolines were developed by inbreeding to the F₅ generation via single seed descent and then selecting for heterozygosity at the Dt₁ locus. F₅-derived families that segregated for stem termination were identified from each cross. Each family was derived from a different F₂ plant. The segregating families from the RW cross only had two classes of plants: determinate and uniformly tall indeterminate plants. This strongly indicates that the nondeterminate individuals have the Dt₁ and dt₂ genes for indeterminacy. To obtain determinate and semideterminate plants from the same F₅ plant, the F₅ plant would have to have been Dt₁dt₁Dt₂dt₂ and when selfed would have produced determinate, semideterminate, and indeterminate individuals. The occurrence of three classes of individuals in the same family was not noted. Approximately 25 F₆-derived families (F_6:7) were harvested from each segregating F₅ family. One F_6:7 family that was homogeneous for the determinate trait and one that was homogeneous for the indeterminate trait were identified from each F₅ family. Seed of these two families were bulked separately to form an isogenic pair. As the two selected F_6:7 families derived from the same F₅ plant, they should be identical at more than 93.6% of their loci.

Trials were conducted in 1994, 1995, and 1996 as a split-split-split plot design with three replications for nonirrigated treatments and two replications for irrigated treatments. The first split was an irrigated vs. nonirrigated treatment. Irrigation was applied to the irrigated plots according to a computer program that predicts soil moisture deficits on the basis of rainfall and crop growth. The second split was late May vs. late June planting dates (Table 1) . The third split was genotype where each pair of isolines was considered a genotype. The two members of an isoline pair were then randomly assigned to adjacent plots. All 11 pairs were tested in 1994 and 1995. In 1996 only three isoline pairs from the RW and four pairs from the WN cross were tested. These sets were selected based on their superior yield potential. The control cultivars Hutcheson and Asgrow A6297 were included as a separate genotype pair in all treatments, but data from these controls were excluded from the analysis.

Maturity was scored as the number of days from 31 August until 95% of the pods attained their mature color. Lodging was scored at maturity on a scale of one (all plants erect) to five (almost all plants prostrate). Plant height was measured at maturity as the distance from the soil line at the base of the plants to the stem apex.

In the initial analyses of variance of the lines tested in 1994 and 1995, data from each cross were analyzed separately. Irrigation, planting dates, stem termination, and location were considered fixed factors, while year and genotype (different F₅ lines within a cross) effects were considered random. Additional analyses of variance were conducted for each irrigation treatment, for each planting date, and for each irrigation–planting date treatment combination to assess the significance of stem termination effect within each production system. All analyses used the GLM procedure of SAS (SAS Institute, 1987). Similar analyses were conducted considering each year–location combination as a random environment. Satterthwaite's approximate F test (Snedecor and Cochran, 1967) was used to test the significance of stem termination and some other effects due to the combination of fixed and random effects in the model. Results from the WN and RW crosses were very similar. Thus, we also performed an analysis combining data from both crosses where each isoline pair was considered a random genotype.

Data from the seven isoline pairs tested in 1996 were combined with the 1994 and 1995 data, and the above analyses were rerun. In addition, we performed a paired t test of the yield difference between growth habits (yield of determinate type vs. yield of indeterminate type) within each environment–production system combination. We also assessed the association of the average yield difference between growth habits in each environment–production system with environment yield and growth potential by determining the correlation of these values using the CORR procedure of SAS. Environment yield and growth potential were estimated three ways: (i) as the average yield and height of all lines, (ii) as the average yield and height of all determinate lines, or (iii) as the average yield and height of all indeterminate lines. The average yield difference between the determinate and indeterminate types in an environment–production system combination was also regressed on the average yield and height of Hutcheson in that environment using the REG procedure of SAS (SAS Institute, 1987).

The Eberhart and Russell (1966) model for analyzing genotype x environment interactions (GEI) was used to evaluate the heterogeneity of genotype yield responses to increased environment productivity. In this analysis, the yield of each line in each environment–production system combination was regressed on an index of environment productivity. We used four indices of productivity: (i) the average yield of all lines, (ii) the average height of all lines, (iii) the average yield of Hutcheson, and (iv) the average height of Hutcheson. Data were analyzed for the seven isoline pairs tested from 1994 to 1996, where the 24 year–location–production system combinations were each considered an environment and each member of an isoline pair was considered a unique genotype thus providing 14 genotypes. These analyses were performed with the GLM and REG procedures of SAS.

	Results

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion and conclusions REFERENCES

 
The analyses of variance of data from 1994 and 1995 (results not shown) indicated that locations and years had little effect on any of the measured traits. Most of the significant interactions of date, irrigation, genotype, or stem termination with environment factors involved the random effects year or year–location, and not the fixed factor of location alone. Thus, despite different soil types and row widths used at the two locations, location effect did not appear to be an important factor in the study. Subsequent analyses considered each year–location combination to be a unique environment. Each combination of irrigation and planting date treatments is termed a production system.

Comparisons of growth habits were very similar in both crosses. For each cross, indeterminate lines were taller, more prone to lodging, and later maturing than the determinate lines (Table 1) in each production system when averaged across environments. Yield was not significantly affected by stem termination, either across all environments or within each production system (Tables 1 and 2) . For all traits, the sign and magnitude of the difference between determinate and indeterminate types was very consistent between the two crosses within each production system (Table 1). Yield and height were affected by irrigation and planting date for both crosses, while planting date affected maturity in both crosses (Table 2). In addition, the significance of interactions involving stem termination effects was quite consistent in each cross: 49 of 64 interaction effects involving stem termination were either simultaneously significant or nonsignificant in the two crosses.

In 1994 and 1995, the 11 isoline pairs were grown in four environments, each with four production systems, providing a total of 176 contrasts of growth habits (data not shown). In each genotype–environment–production system contrast, the indeterminate line was taller, had greater lodging, and in 85% of the contrasts was later maturing than the determinate line. Thus for height, maturity, and lodging, interactions of stem termination with environment or production systems rarely involved rank changes.

When averaged across environments and genotypes, the yield of determinate and indeterminate types was statistically equal in each production system (Table 1). This may have been due in part to considerable variation in yield and growth potential among environments within a production system (Table 3) . There was a trend for determinate lines to yield better than indeterminate lines with early planting and irrigation and for indeterminate lines to yield better than determinate lines with late planting and nonirrigation (Table 1). The interactions of stem termination with irrigation, planting date, environment, and genotype effects indicate that the yield value of one stem termination type vs. the other is conditioned by a complex mix of factors (Table 2). In particular, interactions with random environment and genotype effects were important.

Stem termination had a significant effect on yield in some environment–production system combinations (Table 3) but not others. Indeterminate types had significantly greater yield than determinate types in seven of 24 environment–production system combinations, while determinate types had significantly greater yield than indeterminate types in eight environment–production system combinations. The sign of the yield difference of the different growth habits often varied by genotype (Table 3), such that consistent yield differences were rarely observed across genetic backgrounds within a single environment–production system combination. This inconsistency may be due to the effect of different genetic backgrounds, though the stem termination x genotype interaction was not significant for yield in either the 1994 to 1995 (Table 2) or 1994 to 1996 analyses. The inconsistency may also be due to imprecise estimates of yield for one or both members of an isoline pair within a single environment–production system combination because of the use of only two or three replications.

There was a significant (P < 0.05) correlation of the average yield difference between growth habits within an environment–production system combination with the average yield (r = 0.60) and height (r = 0.58) of the determinate lines within that environment–production system combination. These correlations indicated a trend for the yield advantage of the determinate growth habit to be greater as environment yield and growth potential increased. The average yield difference between growth habits within an environment–production system was regressed on the average yield and height of the control cultivar Hutcheson. The linear effect of yield and height, as well as the quadratic effect of yield were significant. The combined regression accounted for 52.9% of the variation among the yield difference values. The predicted yield difference between the determinate and indeterminate growth habits in an environment was 25.4 - (Y0.354 + (Y²0.00011) + (H4.37), where Y and H are the average yield (kg ha^-1) and height (cm), respectively, of Hutcheson.

Each year, the USDA coordinates a cooperative trial of Maturity Group V breeding lines at many locations throughout the South with Hutcheson as a control. Yield and height data for Hutcheson was collected at 107 test sites from 1993 to 1996. Using the above equation, we would predict a yield advantage of determinate types over indeterminate types in 90.5% of the USDA sites, with the advantage ranging from 14.8 to 1505 kg ha^-1. The predicted yield advantage of the indeterminate types in the remaining sites ranged from 0.7 to 71 kg ha^-1. The USDA data may overestimate the percentage of southern environments where determinate types would yield better than indeterminate types. The USDA sites tend to experience better soil and cultural conditions and thus better yield and growth than many southern production fields. Late-planted and nonirrigated sites are also underrepresented among the USDA test sites.

The regression results suggest that determinate and indeterminate genotypes vary in their response to increasing environment growth and yield potential. This trend was further investigated using the Eberhart and Russell (1966) model for analyzing GEI. In this analysis, the average yield of a line within an environment is regressed on an index of environment productivity. The GEI is then partitioned into a component that is attributed to heterogeneity among line responses to increased environment productivity and a residual. The analysis provides a measure of a line's yield response to increased environment productivity. We used four indices of environment productivity: (i) average yield of all isolines, (ii) average yield of Hutcheson, (iii) average height of all isolines, and (iv) average height of Hutcheson.

Genotype x environment interaction was significant in the 1994 to 1996 analysis and accounted for 19.9% of the treatment (genotype + environment + GEI) sum of squares. Interestingly, GEI accounted for only 11 and 14% of the treatment sum of squares when only determinate or indeterminate genotypes, respectively, were included in the analysis. Heterogeneity among genotype responses was a significant source of variation for all indices when both determinate and indeterminate genotypes were included (Table 4) . This was not true when the analysis included only determinate or only indeterminate genotypes. Thus, introducing morphological variation increased GEI for yield and heterogeneity among genotypes for yield response to increasing environment productivity.

	Discussion and conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion and conclusions REFERENCES

The yield difference between determinate and indeterminate types was conditioned primarily by a complex interaction of random genetic and environment effects (Table 2). Several other researchers have noted that the relative value of stem termination types depends on genetic background (Ablett et al., 1989; Bernard, 1972; Hartung et al., 1980; Hicks et al., 1969; Raymer and Bernard, 1988a; Robinson and Wilcox, 1998). There were environment–production system combinations where one stem termination type had better yield than the other in most genetic backgrounds (Table 3). Using regression techniques, it appears that indeterminate growth habit is likely to confer a yield advantage over the determinate growth habit in environments with limited yield and growth potential. This is due to the lower yield response to increasing environment productivity of indeterminate lines vs. determinate lines (Table 5). Wilcox and Sediyama (1981) and Robinson and Wilcox (1998) also noted that indeterminate lines had lower seed yield increase with increased height than did determinate lines, indicating that their yield response to increasing productivity may be less than that found in determinate lines. Ouatara and Weaver (1994) found that the yield advantage of determinate and indeterminate isolines in the South varied by location and reported similar results. Their data showed that determinate lines had better yield than indeterminate lines in a high-yield, high-growth environment, while indeterminate lines had better yield than determinate lines in a low-yield, low-growth environment. Similar results have been reported in the northern USA. Beaver and Johnson (1981a) recommended that determinate lines not be grown in drought environments, while Cooper (1981) promotes determinate types for high-yield, high-growth environments and indeterminate types for all other northern environments.

Perhaps only 10 to 15% of southern production environments present conditions where indeterminate types would yield better than determinate types, and the yield advantage of the indeterminate types in such environments appeared small. Certainly breeders of full-season southern soybean should continue to devote the majority of their resources to developing improved determinate types. Still, indeterminate cultivars may be useful in fully exploiting the yield potential of low-yield, low-growth environments in the southern USA. The success of deploying indeterminate cultivars to these environments to maximize yield will depend on our ability to predict where these environments will occur. Factors such as late-planting, nonirrigation, and past history may be useful in predicting if a field will have limited yield and growth potential and thus be suited for an improved indeterminate cultivar.

Received for publication February 12, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion and conclusions REFERENCES

 

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