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a Plant Science Dep., Rutgers Univ., 125 Lake Oswego Rd., Chatsworth, NJ 08055
b Office of Research, Univ. of Guelph/Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), 1 Stone Road West, Guelph, ON, Canada N1G 4Y2
c Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, Canada N1G 2W1
Corresponding author (kumudini{at}aesop.rutgers.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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Abbreviations: CAP, canopy apparent photosynthetic rate • CER, carbon exchange rate • CGR, crop growth rate • DM, dry matter • HI, harvest index • EEF, effective filling period • LAI, leaf area index • MG, maturity group • SFP, seed-filling period • SGR, seed growth rate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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Grain yield is the product of total DM and HI and can therefore be affected by either change in HI, or a change in DM accumulation, or both. The main breeding challenge in short-season soybean areas is to maximize DM accumulation within the short growing season while allowing for seed maturation to occur before frost.
Most of the early investigations into soybean yield improvement have revealed little evidence for the role of partitioning (HI). In tests on soybeans of different growth habits and yield potential, no evidence was found that HI and improved yield potential were correlated (Schapaugh and Wilcox, 1980; Cregan and Yaklich, 1986). In more recent studies, however, HI has been reported to be a significant contributor to yield improvement (Frederick et al., 1991; Shiraiwa and Hashikawa, 1995; Morrison et al., 1999). The contradictory nature of the reports on HI and yield bring to question the relative contribution of partitioning to soybean yield improvement.
Research on the association between DM accumulation and soybean yield have also reported contradictory results. In early research, no association between DM accumulation and yield were found (Shibles and Weber, 1965; Weber et al., 1966). These researchers used different planting patterns and populations to increase DM accumulation. The treatments they imposed affected DM accumulation predominantly during the vegetative period. A more recent study of four Japanese soybean cultivars reported that difference in DM accumulation between old and modern genotypes was most apparent after the beginning of the SFP (Shiraiwa and Hashikawa, 1995). The contradictory finding in earlier and more recent studies may be evidence of a temporal relationship between DM accumulation and yield improvement.
A number of researchers have attempted to identify critical periods for soybean yield determination (Egli, 1988; Board and Harville, 1993; Hayati et al., 1995; Board et al., 1996). Investigations on the vegetative period have reported no evidence to link either DM accumulation during the vegetative period or the duration of this period with yield (Weber et al., 1966; Egli, 1993). While many researchers have found a correlation between the duration of the SFP and seed yield (Hanway and Weber, 1971; Gay et al., 1980; Smith and Nelson, 1986), the contribution of the duration of the SFP to soybean yield has been contested (Egli et al., 1984). However, assimilate supply after the beginning of the SFP (R4) may affect yield. Board et al. (1996) found that total DM and leaf area index (LAI) at R5 were positively correlated with seed yield. Furthermore, it has been reported that increasing assimilate supply after beginning pod development (by either shade removal or CO2 enrichment) increased seed yield (Hardman and Brun, 1971; Hayati et al., 1995). Therefore, the association between total DM and seed yield may be apparent only if greater DM accumulation occurs after the beginning of the SFP.
The study of yield-determining factors may be conducted through the use of imposed management treatments or through characterization of cultivars known to differ in yield potential. Characterizing cultivars of different eras and yield potential may illustrate genetic traits associated with yield. The objective of the current study was to characterize DM accumulation and HI to determine the traits associated with yield differences between old and modern short-season soybean cultivars. It was hypothesized that newer soybean cultivars are capable of accumulating more DM and are thus less assimilate limited than older cultivars during the SFP.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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Four soybean cultivars were chosen to represent older and newer releases. The "old" cultivars are two genotypes representative of the preliminary cycles of selection in Ontario. The two old cultivars were frequently used as parents for subsequent cycles of selection. The "new" genotypes are representatives of modern genotypes released within 10 yr of the commencement of the current experiment. The two older releases (‘Pagoda’ and ‘Mandarin Ottawa’) and two newer releases (‘OAC Bayfield’ and ‘Maple Glen’) were selected because they represent two pairs with similar seed quality, lodging scores and similar maturities (Table 1). The seeds of the two historical genotypes were obtained from Agriculture and Agrifood Canada (Ottawa, ON, Canada). The two newer cultivars were certified seeds from a local supplier (First Line Seeds, Guelph, ON, Canada). Subplots were harvest dates.
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Data Collection and Measurements
The samples were taken when all the cultivars were at approximately vegetative V5 growth stage (1996 only), flowering (R1-2), pod elongation (R4), start of seed (R5), full seed (R6), mid-seed maturity (R6.5, 1997 only), physiological maturity (R7), and harvest maturity (R8) (Fehr and Caviness, 1977). The later-maturing group were usually a little advanced (after R1) of the early maturing group when the samples were taken. The comparisons were designed to be made within the maturity grouping and therefore, the small differences in the phenological stages were tolerated. Sample areas (0.5 m2) were dug with a spade to a 15-cm depth. Plants were counted and a representative subsample of 10 plants was taken from the harvested area and separated into leaflets (referred to hereafter as leaves), stems (+ petioles), roots, pod walls, and seeds. Roots of the 10 subsample plants were placed in screen boxes, washed, and nodules removed before drying. The green leaves were used for measurements of leaf area (model LI-3000, LI-COR, Inc., Lincoln NE), and the separated plant materials were dried for at least 72 h in a forced-air oven (80°C). Dry weights of the plants parts were measured as well as the DM of the total harvested area. The DM of plant parts over an area was calculated by the total harvested area dry weight and the DM percentage of the plant part of interest. Leaf area index was calculated from specific leaf weight of the 10 subplants and the leaf dry weight of total sample. Senesced leaves were not collected.
Many alternative estimates of the duration of the SFP have been proposed, each have advantages and disadvantages. In identifying the beginning of the SFP, both Nelson (1986) and Egli et al. (1984) supported the use of either R4 or R5 as estimates of the beginning of the SFP. The effective filling period (EFP), as calculated by Daynard et al. (1971) was used to estimate the duration of the SFP.
Data Analysis
The experiment was laid out as a split-plot arrangement of treatments in a randomized complete block design with four replications (blocks). The main plots were the four genotypes, with seven sampling dates as the sub units. The data were analyzed by Proc Mixed (Windows SAS v. 6.12, SAS Institute Inc., Cary, NC). Analyses were done on individual year data and then on the 2 yr combined. In the combined year analysis year was the main unit with replication nested within year, the cultivars were the sub units and the sampling dates were the sub-subunits. Year and replication were considered random effects, and all other treatments were considered fixed effects. The sample dates were combined across years on the basis of phenological staging at sample time. When no significant year interactions were present, the combined year data are presented, when significant year x treatment interactions occurred, the data are presented for each year. When significant treatment effects (P < 0.1) were found, contrast statements (95% confidence limits) were constructed to compare old versus new cultivars utilizing the correct error terms for the specific split-plot comparison.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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The seed yield of newer cultivars was significantly greater than that of old cultivars over both years (Fig. 1 and Table 2 and 3). The annual yield improvement per hectare was 12 kg ha-1 (calculated by averaging the groups of old and newer cultivars for date of release and yield). This value corresponds to a 0.55% increase in yield per year, which is comparable to values obtained by Voldeng et al. (1997) (11 kg ha-1 representing a 0.5% annual yield improvement) in a study of 41 soybean cultivars.
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Weight of pods increased during the SFP to reach a maximum value and then declined slightly by harvest maturity (Fig. 4a and 4b). The newer cultivars accumulated significantly greater DM in the pods than the older cultivars as early as the R4 growth stage and continued this trend to maturity such that final pod weight was significantly greater in the newer cultivars.
The HI at maturity was significantly greater in the newer cultivars than the old ones, when averaged over both maturity groups and years (Table 4). In the later-maturing group (OAC Bayfield and Mandarin), the newer cultivar had a higher seed number but similar 100 seed weight. In the earlier-maturing group (Maple Glen and Pagoda), the newer cultivar had a higher 100 seed weight but not a significant difference in seed number. There was also no obvious relationship between seed growth rate (SGR) or SFP (as estimated by effective filling period) and final yield (Table 4).
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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Y) = ln(HI x DM) was used (modified equation from Tollenaar et al., 1994) to separate the contribution of increased DM and HI to higher yield (Y). The natural log of the percentage increase in the yield component (HI or DM) over the natural log of the percentage increase in yield was calculated as the contribution of the yield component (e.g, contribution due to HI = ln HI/ ln Y). On the basis of this relationship, total DM accounted for 78% and apparent harvest index accounted for 22% of the difference in yield between high and low yielding cultivars.
Dry Matter Accumulation
It was evident from the DM data that all cultivars tested have similar abilities to accumulate DM until the R4 stage (Fig. 2a and 2b). After R4, the newer cultivars showed a greater accumulation of DM than the old cultivars. Therefore, among the cultivars tested, genetic improvement in yield has resulted in continued carbon assimilation further into the SFP. These results are consistent with those of Shiraiwa and Hashikawa (1995). They observed that newer Japanese cultivars (grown in widely spaced rows) accumulated more above ground DM than older cultivars during the SFP.
Both Cregan and Yaklich (1986) and Board et al. (1996) have reported on the importance of DM accumulation to soybean yield. The greater DM accumulation of the higher yielding cultivars during the SFP as observed in the current study, may help explain why researchers who have generally focused on DM accumulation during the vegetative period (Shibles and Weber, 1965; Weber et al., 1966) found no correlation between total DM and yield.
Other researchers interested in yield-limiting factors in soybean have also confirmed the importance of assimilate (source) supply during the SFP to seed yield. Gay et al. (1980) found no correlation between CO2 uptake and seed yield, among the four old and new cultivars they tested, when measured during vegetative or early reproductive development, whereas Hayati et al. (1995) were able to show that increasing photosynthesis at R5 increased yields. Other studies which modified source supply during the SFP using shade treatments or CO2-enhancement treatments indicated that assimilate supply has a significant effect on yield (Hardman and Brun, 1971; Egli, 1988; Hayati et al., 1995).
The ability of the newer cultivars to accumulate more DM during the SFP is due to increased photosynthetic capacity either from higher rates of photosynthesis or increased light interception. Morrison et al. (1999) reported a significant relationship between leaf carbon exchange rate (CER) and genetic improvement in yield in 14 short-season cultivars tested. However, the higher CER did not result in increased total DM accumulation (reported as total DM at R8 + leaf DM at R6). Furthermore, the CER measurements reported in the study were averages of measurements taken during vegetative and reproductive periods. Therefore, the performance of the cultivars during the SFP were not distinguishable.
The value of leaf CER measurement has limitations as an estimate of plant productivity. Leaf CER can vary considerably because of the time of day, stage of leaf development, and previous and current environmental conditions (Frederick and Hesketh, 1994). Canopy apparent photosynthesis (CAP) is considered a better estimator of cultivar productivity because it also considers leaf area. Leaf area and leaf CER have been reported to be negatively correlated (Bhagsari and Brown, 1986; Morrison et al., 1999) which may undermine the role of increased leaf CER on total plant productivity. Larson et al. (1981) studied the seasonal and diurnal CAP of 13 old and new soybean cultivars. They concluded that, in almost all cases, there was a strong relationship between CAP and date of cultivar release. Their data summarized measurements from senescing and non-senescing canopies, reflective of both leaf CER and light interception. These studies on leaf CER and CAP did not attempt to identify the developmental phase during which these differences are most closely related to yield improvement.
The current study measured total plant productivity as measured by DM accumulation at various stages during the growing season and concluded that total plant productivity was greater in the new than the old cultivars following the R4 stage of development. Although, no measure of leaf CER was taken, the data on higher LAI in the new cultivars following the R4 stage indicates a strong role of light interception in the greater DM accumulation ability of the newer cultivars.
Leaf Area Index
In the current study, there was no indication that the onset of senescence was delayed. However, the greater LAI of the newer cultivars during the SFP indicates a delay in the rate at which the leaves senesced resulting in a longer "stay green" period. Greater LAI of the newer cultivars would enable greater radiation absorption during the SFP especially when LAI values are below the critical value for 95% radiation interception (approx. 3–4 LAI). The greater LAI during the SFP is consistent with the maintenance of DM accumulation later into the SFP of the newer cultivars (Fig. 3a and 3b). Therefore, increased radiation interception by photosynthetically active leaves of the newer cultivars is postulated to have contributed to the greater continued DM accumulation observed in the new cultivars. In the order of 78% of yield improvement was found to be due to greater dry matter accumulation. Therefore, longer leaf area duration plays an important role in yield improvement.
Genetic improvement in yield of maize (Zea mays L.) hybrids has been associated with increased "stay green" characteristics (Duvick, 1992). However, studies on soybeans have mostly discounted LAI as an important component of genetic yield improvement (Shibles and Sundberg, 1998; Morrison et al., 1999). Shibles and Sundberg (1998) compared 63 soybean genotypes; however, they reported LAI at only one growth stage (R5) which is more representative of maximum LAI than leaf area duration. Morrison et al. (1999) presented averaged LAI values (eight samples from V3 to R8). Under conditions of very high LAI, as reported in their paper, averaged LAI values may not be good representations of leaf area duration. Furthermore, considering the importance of phenology on LAI, pairing of old and new cultivars by maturity dates is important to ensure fair comparisons.
Although only a limited number of cultivars were tested in the current study, the data suggest that incorporation of the "stay green" characteristic may be a means of promoting yield in soybeans. Future research with larger sets of genotypes which utilizes pairs of cultivars on the basis of the number of days to maturity (within a maturity group), as well as the individual analysis of multiple sampling dates within the SFP will enable a more precise estimate of difference in leaf area duration.
Harvest Index and Yield Components
In the current study, although improved HI contributed (22%) to increased yield potential, it was not the greatest contributor to yield improvement. Reports on the association between HI and seed yield have been contradictory (Schapaugh and Wilcox, 1980; Cregan and Yaklich, 1986; Frederick et al., 1991; Shiraiwa and Hashikawa, 1995; Morrison et al., 1999). Schapaugh and Wilcox (1980) reported no correlation between HI and soybean yield. However, they did note that HI was highly correlated with maturity and that the limited sample of genotypes within a maturity group in their study prevented a good evaluation of the relationship between HI and seed yield. Cregan and Yaklich (1986) found that, of the MG II material tested, the modern and ancestral cultivars had greater HI than the plant introductions. More recent research has confirmed the relationship between increased HI and improved yield potential (Frederick et al., 1991; Shiraiwa and Hashikawa, 1995; Morrison et al., 1999). The contradictory nature of the literature in reporting the relationship between HI and seed yield may be an indication that HI is not the predominant means by which yield improvement has been achieved in soybean.
In the current study, no obvious correlation was found between either SGR or SFP and yield. The duration of SFP has been frequently cited as being associated with yield (Hanway and Weber, 1971; Gay et al., 1980; Hanson, 1985; Smith and Nelson, 1986). However, other reports have suggested that the significant genotype x environment interactions limit the usefulness of the reproductive period durations as a selection criteria for yield improvement (Egli et al., 1984; Salado-Navarro et al., 1985; Smith and Nelson, 1986).
Considering that seed yield is the product of SGR and the SFP, and the conflicting reports on the importance of the duration of the SFP to seed yield, it may be speculated that improved genetic yield potential may not be a consequence of either increased SGR or duration of the SFP alone but the optimization of both—possibly to maintain a balanced source-sink relationship.
In the current study, there was no association between seed size and yield. The newer cultivars had similar or higher seed numbers than the ancestral cultivars (Table 4). Egli and Crafts-Bradner (1996), in their comprehensive review of soybean source-sink relationships, suggested that most variation in yield components is related to variation in seeds per unit area and not seed size.
Seed number seems to be far more flexible than seed size in adapting to normal environmental changes (Shibles et al., 1975). Studies on maize have indicated a significant positive association between kernel number and crop growth rate (CGR) during early reproductive development (Edmeades and Daynard, 1979; Tollenaar et al., 1992; Uhart and Andrade, 1995). There appears to be an association between DM accumulation and seed number in soybeans similar to that seen in maize. Bruening and Egli (1999) used stem girdling to isolate individual nodes to study the relationship between photosynthesis and seed number at a single node. They found a curvilinear response of seed number to increasing nodal carbon input with a peak around 0.10 µmol CO2 node-1 s-2. Egli and Yu (1991) were able to show a similar response at the whole canopy level. They reported a linear relationships between CGR (R1–R5) and seed number per square meter within individual genotypes. In maize, this response is mainly associated with CGR during silking (Uhart and Andrade, 1995). In an indeterminate soybean cultivar, seed number may continue to rise past the R5 stage. Therefore, the potential exists for increased DM accumulation after the R4 stage to increase seed number and thereby improve yield. In previous studies, increasing the source-sink ratio during flowering and fruit set enhanced yield primarily through increase in seed number (Hardman and Brun, 1971; Egli, 1988; Egli and Yu, 1991; Board and Harville, 1993), although in some instances seed size was also increased (Egli, 1993; Hayati et al., 1995). In the current study, cultivars which accumulated the most DM during the reproductive period, had seed number (m-2) and seed mass (g seed-1) which were similar or greater than cultivars which accumulated less DM (the older cultivars).
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SUMMARY |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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The ability of the new cultivars to accumulate greater DM occurred after the onset of the R4 growth stage. The improved "stay green" characteristic as evidenced by the greater LAI during the SFP was consistent with the greater DM accumulation of the newer cultivars. Genetic improvement in yield, within the limits of the small group of old and new short-season soybean cultivars tested, was related to greater LAI during the SFP, and the subsequent greater DM accumulation during the SFP.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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Received for publication April 15, 2000.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARYREFERENCES |
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) or two new (
) soybean cultivars (
,