Abstract
Seed productivity of old and modern soybean cultivars was analyzed in relation to accumulation and partitioning of nitrogen during seed filling. Four soybean cultivars were grown on a drained paddy field (silty caly loam). Dry weight and nitrogen content were determined for above ground parts weekly from the beginning of seed filling to maturity. The modern cultivars Enrei and Tachinagaha exhibited greater dry matter and nitrogen accumultaions during seed filling and greater seed yield in comparison with the old cultivars Akazaya and Mizukuguri. Seed nitrogen per top nitrogen (Nseed/Ntop) incresed linearly against days after the beginning of seed filling, while leaf nitrogen per top nitrogen (Nleaf/Ntop) decreased linearly. Cultivar differnces in nitrogen partitioning were quite small. Thus Nleaf was mainly dependent on Ntop. Nitrogen harvest index (Nseed/Ntop at maturity) was not very restrictive to either nitrogen yield or seed yield. It is concluded that the observed difference in seed productivity among old and modern cultivars was associated more with nitrogen accumulation during seed filling than with its partitioning pattern.
