under drought conditions (Kobata et al., 1992). Both cereal crops clearly change their growth phase fromThe relationship between seed and shoot dry matter increase (R S/W ) during the seed-filling period (SFP) can reflect the balance between vegetative to reproductive stages, and the potential S the assimilate supply and the sink capacity of harvest organs. In a barely changes once the SFP begins (Evans, 1996). In 2-yr experiment, the R S/W in determinate soybean [Glycine max (L.) soybean, however, active stem and leaf growth continue Merr.] during SFP was investigated under various growing conditions after onset of SFP because flowering starts with vegetainduced by shading and thinning for plants grown under both standard tive growth earlier in the growing season than in most and reduced densities. The R S/W approximated a positive linear regrescereal crops (Ojima and Fukui, 1966). Hence, in soysion in both experiments. Slopes of the lines were less than 0.5 and did not vary within the same year even if plant density was reduced.bean, the potential S may change with the W. If the The shoot dry matter increase (W) hence seemed to directly determine potential S changes with W, the S should not reach a seed dry matter increase (S). A positive relationship between total limit such as the plateau observed in Case 1. If so, the pod number and W or S was observed in both years. Only minimal R S/W may be typed as three cases (Fig. 1, Cases 2-4). In changes in seed numbers per pod and individual seed weight wereCase 2, S during the SFP is equal to W (S ϭ W), and observed, regardless of the treatment applied. A positive relationshipthere is no net transfer of assimilate reserves to the also existed between total pod number and the number of branch nodes, and hence W increased S via increase in the number of branch seed. In Case 3, S is greater than W due to net transfer node bearing pods. Half the current assimilate product during SFP of assimilate reserves from the stem to the seed. Conappeared to be used for increase of vegetative plant parts. This deterversely, in Case 4, S is less than W because excess dry mined total pod number and hence potential S. Determinate soybean matter is stored in the stem rather than in the seed. is thus very different from cereal crops, in which almost all the currentThe soybean harvest index (HI), the ratio of seed assimilate during the SFP is appropriated to S. The stability of the yield to total biomass at maturity, has been considered harvest index in soybean reflects the important effect of the W on pod establishment during SFP.

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Seed yield in soybeans [Glycine max (L.) Merr.] strongly depends on nitrogen (N) assimilation (NA). The NA in nodulated soybeans is based on two N sources, soil and N fixation. Hence, the controlling NA in soybean to increase seed yield is more difficult than in many other crops that depend on fertilizer. The objective of this study was to establish a quantitative relationship between NA by plants and N applied via fertilizer (NF) to increase soybean seed production. The NA was modelled as a function of NF with two parameters: amount of available soil N (NS) and maximum N assimilation (NAmax), both to be determined experimentally, and one equation to describe fixed N (NFIX), would be maximum under nonfertilizer conditions (NFIXmax). It is assumed that NA is equal to NAmax when NF ≥ NAmax − NS and that, below this threshold, NA decreases in proportion with NFIX + NS. To obtain variable datasets, different rates of N fertilizer were applied to nodulated and non‐nodulated isolines under pot and field conditions from full bloom to beginning of maturity. Seed yields were found to be highly dependent on NA regardless of nodulation type or genotype. In non‐nodulated isolines, NA − NF was a good estimate of NS. In nodulated soybeans, NFIX could be modelled as a liner function of NF. These results indicate that the relationship between NA and NF with NS and NFIX can be modelled in a simplified way. The relationships between NFIX, NF, and NFIXmax indicate key traits for determining NA for seed production in nodulated soybeans.

under drought conditions (Kobata et al., 1992). Both cereal crops clearly change their growth phase from The relationship between seed and shoot dry matter increase (R S/W ) during the seed-filling period (SFP) can reflect the balance between vegetative to reproductive stages, and the potential S the assimilate supply and the sink capacity of harvest organs. In a barely changes once the SFP begins (Evans, 1996). In 2-yr experiment, the R S/W in determinate soybean [Glycine max (L.) soybean, however, active stem and leaf growth continue Merr.] during SFP was investigated under various growing conditions after onset of SFP because flowering starts with vegetainduced by shading and thinning for plants grown under both standard tive growth earlier in the growing season than in most and reduced densities. The R S/W approximated a positive linear regrescereal crops (Ojima and Fukui, 1966). Hence, in soysion in both experiments. Slopes of the lines were less than 0.5 and did not vary within the same year even if plant density was reduced. bean, the potential S may change with the W. If the The shoot dry matter increase (W) hence seemed to directly determine potential S changes with W, the S should not reach a seed dry matter increase (S). A positive relationship between total limit such as the plateau observed in Case 1. If so, the pod number and W or S was observed in both years. Only minimal R S/W may be typed as three cases (Fig. 1, Cases 2-4). In changes in seed numbers per pod and individual seed weight were Case 2, S during the SFP is equal to W (S ϭ W), and observed, regardless of the treatment applied. A positive relationshipthere is no net transfer of assimilate reserves to the also existed between total pod number and the number of branch nodes, and hence W increased S via increase in the number of branch seed. In Case 3, S is greater than W due to net transfer node bearing pods. Half the current assimilate product during SFP of assimilate reserves from the stem to the seed. Conappeared to be used for increase of vegetative plant parts. This deterversely, in Case 4, S is less than W because excess dry mined total pod number and hence potential S. Determinate soybean matter is stored in the stem rather than in the seed. is thus very different from cereal crops, in which almost all the currentThe soybean harvest index (HI), the ratio of seed assimilate during the SFP is appropriated to S. The stability of the yield to total biomass at maturity, has been considered harvest index in soybean reflects the important effect of the W on pod establishment during SFP.