Abstract
In soybeans, increase in the dry weight of seed (IWS) during the seed-filling period (SFP) is nearly half that of the whole plant (IWP). Hence, the contributions of assimilate during SFP to seed growth in soybeans is apparently lower than that in other cereal crops. However, analysis of the assimilate budget for seed production based on dry matter could be misleading in soybeans because the carbon (C) contents of seed and that of other organs are different due to the differences in lipid and protein content and respiration loss in each organ results in underestimation of the C contribution to seed production. In field soybeans, irrespective of shading and thinning, IWS/IWP and harvest index (HIW) for dry weight were slightly lower than ICS/ICP and HIC for C, respectively. Of the 13C fed at the vegetative stage and early SFP, 3.4% and 16.4%, respectively were accumulated in the seed, 0.9% and 28.2%, respectively, in the pod shell and 26.8% and 37.8%, respectively were respired before maturity. Ninety-six percent of seed C was assimilated during the SFP. Respiratory loss of 13C fed at the early SFP was larger than that from 13C at the vegetative stage, showing that seed production requires much more C than leaf or stem growth. These results suggest that the difference in C contents between plant organs has little effect on the estimation of assimilate partition into soybean seeds based on dry matter, and high respiration loss for seed growth reduces the partition of dry matter into seeds.
