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Crop Science 40:1624-1629 (2000)
© 2000 Crop Science Society of America

CROP PHYSIOLOGY & METABOLISM

Agronomic and Phenological Differences of Soybean Isolines Differing in Maturity and Growth Habit

D.F. Curtisa, J.W. Tannerb, B.M. Luzzic and D.J. Humeb

a Eastern Cereal & Oilseed Res. Ctr., Agric. Agri-Food Canada, Ottawa, ON, Canada K1A 0C6
b Crop Science Dep., Univ. of Guelph, Guelph, ON, Canada N1G 2W1
c Pioneer Hi-Bred International, Inc., 7230 N.W. 70th Avenue, P.O. Box 177, Johnston, IA 50131-0177, ECORC contribution No. 001498 USA

curtisd{at}em.agr.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
The effects of maturity genes (E genes) on flowering, maturity, and the seed filling period of soybean [Glycine max (L.) Merr.] have previously been described. However, little work has been done to quantify the effect of maturity and growth habit genes on morphological and phenotypic traits. This study quantifies the effects of maturity and growth habit genes on morphological and phenotypic traits to understand better how these genes may be most agronomically beneficial in northern short-season breeding programs. Four near-isogenic pairs consisting of e1e3e4, e1e3E4, e1E3E4, and E1e3e4 maturity gene combinations containing either indeterminate (Dt1) or determinate (dt1) stem termination type were grown near Woodstock, ON, in 1994 and 1995. When averaged across growth habit, the presence of an additional dominant maturity gene significantly increased the time to the start of the seed filling period (SSFP) and days to maturity. Lines with dominant E genes exhibited increased height, final node number, and lodging, and decreased harvest index. Indeterminate growth habit caused increased days to SSFP in 1995, and increased days to maturity, height, and lodging and decreased harvest index in both years. A positive relationship between whole-plant seed filling rate (SFR) and yield occurred in the Dt1 lines in both years and occurred in the dt1 lines in 1994. There was a positive relationship between whole-plant seed filling period (SFP) and yield in the dt1 lines in 1995. The E1dt1 line consistently had the highest seed filling rate of any isoline. The data indicated that the E1dt1 combination may have unrealized potential in short-season breeding programs.

Abbreviations: CHU, crop heat units • Dt1, Indeterminate growth habit • dt1, determinate growth habit • HI, harvest index • MG, maturity group • LSFP, linear seed filling period • SSFP, start of the seed filling period • SFP, seed filling period • SFR, seed filling rate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
SOYBEAN IS ADAPTED to regions from the equator to approximately 50° north and south latitude. However, any particular genotype is restricted to a narrow zone of latitude in which yield is maximized and maturity is ensured. Genotypes adapted to higher latitude environments will flower early and prematurely ripen, restricting yield when grown at lower latitudes. Conversely, genotypes adapted to lower latitudes will have delayed flowering and will not likely mature at higher latitudes. These narrow bands of adaptation have been attributed to the presence or absence of genes that affect the flower timing and maturity (Bernard, 1971). The first maturity genes, designated E1 and E2, were identified in `Clark' near-isogenic lines (Bernard, 1971). The E1 gene, by itself, delayed time to flowering by 9 d, but caused only a 1-d delay in maturity. The E2 gene, by itself, delayed both flowering and maturity by 14 d. Combined, E1 and E2 caused a 23-d delay in flowering and an 18-d delay in maturity. A third maturity gene, E3, caused a delay in maturity of 8 d (Buzzell, 1971). The E4 gene has been shown to cause a delay in flowering and maturity of 6 and 20 d, respectively, under incandescent long day photoperiods (Saindon et al., 1989). The E4 gene showed little effect under natural daylength. The E5 gene affects flowering and maturity in a manner similar to the E2 gene (McBlain and Bernard, 1987). In addition to known maturity genes, other major or minor loci, or interactions among genes must exist to account for the entire range of maturity and adaptation.

Stem termination, sometimes referred to as growth habit, is classified as indeterminate (Dt1), semi-determinate (Dt2), or determinate (dt1) (Bernard, 1972) with the Dt1 and the dt1 growth habits being most common. Dt1 growth habit is most common in MG 000 to IV and is characterized by overlapping vegetative and reproductive stages (Bernard, 1972). Alternatively, the dt1 growth habit is generally associated with soybean genotypes in MG V to X, and termination of the stem growth occurs at, or shortly after the onset of flowering (Bernard, 1972).

A combination of E1 maturity gene and dt1 growth habit produces a tall determinate plant which in some short-season environments yields similar to indeterminate lines (Cober and Tanner, 1995).

Dt1 types had a long, seed-filling period (SFP) with low seed filling rates (SFR) compared with dt1 types (Egli and Leggett, 1973). This may be due to the overlapping of vegetative and reproductive stages in the Dt1 types and the separation of these stages in dt1 types.

The SFP can be represented by a sigmoidal curve. The flat start of the curve represents the initiation of storage organs and is referred to as the lag stage. The linear portion of the curve represents the bulk of the dry matter transfer from the vegetative sources to the storage organs. The period from the end of the lag stage to the end of the linear portion of the curve is defined as the linear seed filling period (LSFP). The SFR is determined during the LSFP. The final flattening of the curve represents senescence in the plant. During the LSFP, SFR has been reported to range from 79 to 98 kg ha-1 d-1, depending on the soybean cultivar (Kaplan and Koller, 1974).

Seed filling rate had shown no correlation with yield in previous studies (McBlain and Hume, 1980; Gay et al., 1980). Significant correlations have been reported between seed yield and the SFP ( r = 0.57 ; Hanson, 1985), as well as yield and the LSFP ( r = 0.93 ; Nelson, 1986).

Previous research in Canada has concentrated on describing the effects of the E genes on phenology without examining the affect these genes have on the morphological characteristics and yield components in the plant. The objective of this study was to evaluate the effect of E1, E3, and E4 maturity genes, and the determinate (dt1) and indeterminate (Dt1) stem types, on phenological and morphological traits, as well as yield components, within similar genetic backgrounds in a short-season Canadian environment.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Eight near-isogenic lines of `Harosoy' differing in maturity and growth habit genes were used in the study (Table 1) . The first factor, E gene combination, consisted of either e1e3e4, e1e3E4, e1E3E4, or E1e3e4. The second factor, growth habit, consisted of either indeterminate (Dt1) or determinate (dt1). This formed a 4 by 2 factorial, which was arranged in a randomized complete block design with four replications. The experiment was grown at the Woodstock Research Station, University of Guelph, Woodstock, ON, during the growing seasons of 1994 and 1995. The research station lies at 43°13' N. Lat. and 250 m above sea level. The tests were planted on 30 May 1994 and 23 May 1995. In 1994, no thinning occurred as final plant stands were approximately 10 to 12 plants m-1 of row. In 1995, the plots were thinned to a final plant stand of 11 to 13 plants m-1 of row. In 1994, e1E3E4dt1 was not sampled as phenotypes indicated seed contamination. However, a pure seed lot was obtained for planting in 1995. The seed for all other lines was obtained from Agriculture and Agri-Food Canada, Central Experimental Farm, Ottawa, Ontario in both years.


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  		Table 1 Harosoy soybean near-isogenic lines used in the study of maturity genes and growth habit loci

 
Each plot consisted of 12 rows, 5.5 m long, spaced 37.5 cm apart. The first four rows and the last four rows in each plot were used for subplot sampling to determine total, vegetative, and reproductive dry matter accumulation rates. The four center rows were used for measuring yield, lodging, final plant height, final node number, days to maturity, and seed size. Each sampling subplot measured 0.5 m long and 1.5 m wide for a total of 10 subplots. Between each subplot 0.5 m was not harvested in order to provide a border for the adjacent subplot. Each subplot represented a different sampling or harvest date and the date of harvest for each subplot was assigned randomly. Since all four rows were harvested in the sampling subplots, an upward bias on the main yield plots may have occurred, but because all plots received the same treatment, the bias was equally applied to all yield plots. For this reason any effect of partial border loss on the yield plots was ignored in our analysis.

On each sampling date, all plants within the sampling sub-plot were cut at ground level and the total number of plants recorded. The material was then dried to a constant weight at 80°C and weighed. At appropriate sampling dates seed material were removed from the pod walls after drying to determine seed dry matter accumulation. In 1994, each genotype was harvested every 7 to 10 d beginning at the V5 growth stage (Fehr and Caviness, 1977) until maturity. In 1995, sampling also began at the V5 stage, but the harvest frequency was adjusted to the particular maturity of each line in order that the majority of the sampling dates occurred during the seed filling period.

The SSFP for each genotype was determined by linear regression analysis of the seed sampling data in each replication after excluding the non-linear data points (Johnson and Tanner, 1972). The regression line was generated by setting days after planting as the independent variable and seed yield as the dependent variable. The intersection of the regression line on the days after planting x-axis represented the SSFP. The SFP was then calculated by subtracting the SSFP value from the physiological maturity date. Day of physiological maturity, R7, was assessed based on the definition given by Fehr and Caviness (1977). SFR was determined by dividing plot yield (kg ha-1) by the duration of the SFP in each individual plot. Harvest index (HI) was calculated using the last sampling date at maturity by dividing seed dry matter by total above ground vegetative and reproductive dry matter present at maturity. Lodging was rated from 1 to 5 on the four center rows of each plot, where 1 represented erect and 5 represented prostrate plants. The four center rows were harvested with a small plot combine. Yield and seed size were determined on the harvested seed after the seed was cleaned using small plot cleaners to remove only non-seed material.

The E genes and stem termination factors were analyzed as fixed effects. An ANOVA was run on all data and where appropriate a protected LSD was used to determine differences among individual genotypes, E gene combinations and growth habit.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
The 1994 growing season accumulated only 2620 Crop Heat Units (CHU) (Brown and Bootsma, 1993), whereas the 1995 growing season was more typical of the Woodstock location and accumulated 2830 CHU. In both years, precipitation during the growing season was similar and normal for the area. In 1994, emergence was poor and variable across genotypes, however stands of 10 to 12 plants m-1 row were obtained without thinning. In 1995, only the E1e3e4Dt1 genotype had poor emergence and a slightly lower stand of 10 plants m-1 row, as opposed to all other plots which were thinned to 11 to 13 plants m-1 row. The e1E3E4dt1 genotype exhibited a mixture of phenotypes in 1994 and therefore was not sampled.

There were significant differences (P < 0.05) between years in yield, days to SSFP, days to maturity, seed size, SFP, SFR, HI, and final plant height. The difference between years may have been a result of an earlier planting date and slightly warmer season in 1995. In both years, there was a significant E gene x growth habit interaction for almost all traits (Table 2) . However, in all instances, except SFP, the main effect of E gene or growth habit had substantially larger mean square values than the interaction effects indicating the importance of individual main effects.


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  		Table 2 Estimates of degrees of freedom, mean squares, and significance of effects for given yield components of Harosoy isolines grown at Woodstock, ON in 1994 and 1995

 
Across years and growth habit, genotypes with the E1e3e4 gene combination significantly delayed the SSFP by 15 d, and maturity by 13 d compared with the genotypes with the e1e3e4 combination (Table 3) . These results differ from previously reported values of Bernard (1971) who used lines which had additional dominant E genes and Cober and Tanner (1995) who only reported values for Dt1 lines. Increased lodging also occurred, likely associated with the increase in height and node number as was also suggested by Wilcox and Sediyama (1981). The seed size and HI were significantly reduced in the E1e3e4 genotypes compared with the e1e3e4 combinations. The reduction in seed size has been shown to occur in lines with an E1 background (Cober et al., 1996). Lower HI is expected with later maturing cultivars (Wallace, 1985).


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  		Table 3 Effects of the E1, E4 and E3E4 maturity genes relative to corresponding recessive genes in a Harosoy soybean background in 1994 and 1995 at Woodstock, ON{ddagger}

 
Across years and growth habit, the E4 gene compared with e4 gene delayed the SSFP and maturity by 12 and 11 d, respectively, and decreased HI (Table 3). The additional days to SSFP and maturity were similar to those reported by Saindon et al. (1989). The reduced HI was expected and has been shown to be associated with lateness (Wallace, 1985).

In 1994, the e1E3E4dt1 genotype was not sampled and therefore comparisons were only made on 1995 data (Table 3). The E3E4 combination compared with the e1e3e4 gene combination significantly increased days to SSFP by 9 d and maturity by 15 d. These results are similar to those by Buzzell and Bernard (1975) and Saindon et al. (1989). The E3E4 combination also significantly increased lodging likely due to greater plant height. Similar results by Wilcox and Sediyama (1981) reported increased plant height related increased lodging. The E3E4 gene combinations decreased HI and seed size in 1995. Lateness has been associated with a decreased HI (Wallace, 1985), but the reduction in seed size was unexpected as lateness is generally associated with increased seed size (Swank et al., 1987).

The E3E4 gene combination, when compared with the E4 gene alone, increased time to maturity by 5 d, but not days to SSFP (Table 3). However, because of absence of sampling in 1994 of the e1E3E4dt1 line, these comparisons were only valid for 1995. This result may indicate that the E4 gene in the presence of the E3 gene shortens the time to the SSFP and increases the days to maturity which is opposite to the effect of the E1 gene. The E3E4 gene combination increased height compared with the E4 gene alone which likely resulted in the increased lodging. The E3E4 combination decreased seed size compared with the E4 gene alone which is a contrast to a report that longer seed-filling periods resulted in higher seed size (Swank et al., 1987).

Because of missing plots in the e1E3E4dt1 lines in 1994, only data from 1995 could be used to compare Dt1 and dt1 genotypes across E gene combinations. Dt1 growth habit increased the days to SSFP, time to maturity, final plant height, node number, and lodging, and decreased HI compared with the dt1 growth habit (Table 3). Increased days to SSFP and maturity due to the Dt1 are in agreement with previously reported data by Metz et al. (1985). Increased final plant height, node number, and lodging are in agreement with previous reports (Ablett et al., 1989; Wilcox and Sediyama, 1981). The reduction in HI shown in the Dt1 lines was likely a result of dry matter partitioning to vegetative growth and away from seed dry matter during the SFP.

There were significant differences among lines for height, final node number, lodging, and seed size in both years (Table 4) . Each E gene dt1 combination was significantly shorter and had a lower final node number than the corresponding E gene Dt1 combination. In both years, the maturity genes had no effect on increasing height within the dt1 growth habit, with the exception of the E1 gene. Within the Dt1 growth habit, the addition of dominant maturity genes increased final height, with the exception of the E4 gene in 1994. However, increases in plant height due to the presence of additional dominant E genes may not be a direct effect of these E genes, but rather a result of a greater time spent in the vegetative growth phase as dominant E genes delayed the onset of the reproductive phase. In both years the E1dt1 combination was similar in height to the e1Dt1 combination which indicated that the stem lengthening effect of the E1 gene in a dt1 background was approximately equal to the stem lengthening effect of the Dt1 growth habit with no dominant maturity genes.


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  		Table 4 Effects of maturity genes and determinate (dt1) or indeterminate (Dt1) growth habit on final height, final node number, lodging, seed size, and harvest index of Harosoy soybean isolines grown in 1994 and 1995 at Woodstock, ON

 
In 1994, Dt1 lines had greater seed size than their dt1 counterparts (Table 4). In 1995, seed size tended to decrease with lateness of the genotypes. This result conflicts with previously reported studies in which lateness was associated with greater seed size (Swank et al., 1987).

Harvest index differed among lines only in 1995. Later Dt1 genotypes had lower harvest indices than either early Dt1 or all dt1 genotypes, with the exception of the E1dt1 genotype. This result was expected as both earliness and dt1 growth habit have been shown to increase HI (Wallace, 1985; Wilcox and Frankenberger, 1987).

Averaged across genotypes, seed yield was significantly higher (P < 0.05) in 1995 than in 1994, with dt1 lines showing greater increases than their Dt1 counterparts comparing 1994 to 1995 (Table 5) . Every Dt1 line, with the exception of the E1e3e4 gene combination, significantly outyielded its dt1 isoline counterpart in 1994; however in 1995, E1e3e4dt1 and e1E3E4dt1 combinations significantly outyielded their Dt1 counterparts. In both years, the E1e3e4dt1 genotype produced relatively high yields indicating that the E1 gene may ensure yield uniformity under varying conditions. The yields of the Dt1 genotypes with any dominant E genes were consistent in both years. The results from 1995 may indicate that dt1 types with dominant E genes may have unrealized yield potential in short-season environments.


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  		Table 5 Effects of maturity genes and determinate (dt1) or indeterminate (Dt1) growth habit on seed yield, days to start of seed filling (SSFP), seed filling period (SFP), days to maturity, and seed filling rate (SFR) of Harosoy soybean isolines grown in 1994 and 1995

 
There was a significant genotype x year interaction for days to SSFP. Both the e1e3E4dt1 and E1e3e4dt1 lines had significantly longer vegetative periods in 1994 than in 1995, whereas the e1e3e4 genotypes had a longer period of vegetative growth in 1995 than in 1994. Across growth habits, the E1e3e4 gene combination had the longest time from planting to SSFP in both years. This was in agreement with previous work which has shown a large delay in the onset of the reproductive period when the E1 gene is present (Bernard, 1971).

There was a significant genotype x year interaction for SFP. E1e3e4dt1 and e1e3E4dt1 lines had a shorter time to SSFP in 1995 resulting in an increase in SFP as time to maturity was constant in both years. The e1E3E4dt1 line had a large increase in yield from 1994 to 1995 likely due to an increase in SFP similar to other dominant E gene dt1 lines.

The yield of the dt1 lines, in 1994, was determined by the SFR as indicated by the significant (P < 0.001) relationship between SFR and yield (Table 6) . In 1995, increased yield in dt1 lines was likely a result of relatively high SFRs and longer SFPs as indicated by the significant (P < 0.001) relationship between SFP and yield. The E1e3e4dt1 line seemed to ensure consistent yields across years through an increased SFR which compensated for a shortened SFP and adverse conditions, such as later planting and lower CHU in 1994. The E1e3e4dt1 line possibly maintained yield under adverse conditions by a combination of responses. By delaying the onset of flowering the line is able to accumulate adequate vegetative dry matter necessary to ensure consistent yields. Also, the E1 gene in combination with the dt1 background may be more sensitive to shortening daylengths later in the season than other E genes and respond by producing very high SFR values. This response may compensate for the shortened SFP as a result of the delayed flowering by ensuring the maximum amount of vegetative dry matter is converted into seed dry matter. This may be important in short season areas where maturity, completion of seed filling, and adequate yields are considered ongoing challenges for breeders.


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  		Table 6 Regression of yield on seed filling period (SFP) and seed filling rate (SFR) for determinate (dt1) and indeterminate (Dt1) Harosoy soybean isolines grown in 1994 and 1995

 
No relationship between SFP and yield existed in the Dt1 genotypes in either year (Table 6). However, the Dt1 genotypes did have a linear relationship between SFR and yield in both years. This result indicates that the SFP was not a limiting factor for Dt1 genotypes and the SFR of individual genotypes during the SFP was a more important aspect of yield. This is in contrast with the work of others which has described SFP as the most important determinant of yield in both Dt1 and dt1 genotypes (Gay et al., 1980; Hanson, 1985; Nelson, 1986). The contrasting results may be due to various factors including environmental conditions and varieties used in the previous studies. It should also be noted that only one genetic background was used in this study and extrapolation to all backgrounds should be done with caution.

The linear regression line slopes were not significantly different (P < 0.05) between years indicating that the SFR, in both years, of the Dt1 genotypes was similar. SFR values in this study compare well to previously reported values (Kaplan and Koller, 1974).

In conclusion, the data indicated that yields of Dt1 genotypes were more consistent over years. The data also indicated SFR, and not SFP, was the more important component of yield in the Dt1 types. The dt1 genotypes exhibited very good yield potential, given the earlier planting in 1995 and yield limitations with the later planting in 1994. Within the dt1 genotypes, the presence of E1 appeared to confer the ability to ensure relatively high and stable yields. This would indicate that an E1dt1 combination in northern environments may be useful in a breeding program. These results suggest that further research into the use of determinate, early-maturing cultivars may be warranted for high-yielding Canadian environments. Studies should include the ability of the E1dt1 combination to maintain the high SFR under delayed planting to ensure high and stable yields.

Received for publication January 18, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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