Shading and thinning effects on seed and shoot dry matter increase in determinate soybean during the seed-filling period

The relationship between seed and shoot dry matter increase (R _S/W) during the seed-filling period (SFP) can reflect the balance between the assimilate supply and the sink capacity of harvest organs. In a 2-yr experiment, the R_S/W in determinate soybean [Glycine max (L.) Merr.] during SFP was investigated under various growing conditions induced by shading and thinning for plants grown under both standard and reduced densities. The R_S/W approximated a positive linear regression 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. The shoot dry matter increase (W) hence seemed to directly determine seed dry matter increase (S). A positive relationship between total pod number and W or S was observed in both years. Only minimal changes in seed numbers per pod and individual seed weight were observed, regardless of the treatment applied. A positive relationship also existed between total pod number and the number of branch nodes, and hence W increased S via increase in the number of branch node bearing pods. Half the current assimilate product during SFP appeared to be used for increase of vegetative plant parts. This determined total pod number and hence potential S. Determinate soybean is thus very different from cereal crops, In which almost all the current assimilate during the SFP is appropriated to S. The stability of the harvest index in soybean reflects the important effect of the W on pod establishment during SFP.