Functions of nitrogen accumulation and redistribution in leaves and its contribution to seed yield in soybean

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It is well known that large amount of nitrogen are redistributed from vegetative organs to the seeds during seed filling in soybean (Glycine max [L.] Merrill). However, the role of distributed nitrogen from leaves on the seed yield production are not clear. The functions of nitrogen accumulation and redistribution in leaves and its contribution to the seed yield were evaluated. The redistributed nitrogen was estimated by the difference in the nitrogen contents between R5 and R7 stage. The nitrogen content in leaves began decreasing after R5 stage in all cultivars, indicating the start of nitrogen redistribution. About 13.8% to 37.9% of the total nitrogen in the seeds was estimated being redistributed from the leaf tissues among ten cultivars. The seed yield was correlated positively with the amount of redistributed nitrogen from leaves but with neither the nitrogen concentration in the leaves at R5 nor the proportion of redistributed nitrogen in the seeds. However, in high seed yielding year, the seed yield is not likely associated with nitrogen redistribution, even the lowest nitrogen redistribution was associated with relatively higher seed yield in Tamahomare. Furthermore, enhanced nitrogen concentration increased the seed yield before seed filling in Sachiyutaka, which is dependent on the nitrogen redistribution, whereas after seed filling in Tamahomare, which is independent of the nitrogen redistribution. The results indicated that large nitrogen redistribution does not always contribute to high seed yielding, implying the direct nitrogen uptake during seed filling could be more important factor for high seed yielding depend on the cultivars.