Influence of Nitrogen Enrichment during Reproductive Growth Stage on Leaf Nitrogen Accumulation and Seed Yield in Soybean

Nitrogen assimilation during seed filling limits the seed yield in soybean. Seed nitrogen dependence on either redistributed nitrogen or absorbed nitrogen from soil during seed filling shows varietal differences. The objective of this study was to investigate the timing of nitrogen enrichment for effective nitrogen assimilation. Two soybean cultivars Sachiyutaka and Tamahomare were sown in the pots filled with well-washed fine sand. The plants were well watered with nutrient solution containing 100 ppm nitrogen and other nutrient elements before and after the treatment. The treatments were conducted from reproductive stage R1 to 115 or from 115 to R7 by applying the nutrient solution with different nitrogen concentrations. The high nitrogen concentration from R1 to R5 delayed the decline in SPAD value and leaf nitrogen concentration and improved the seed yield performance in Sachiyutaka, whereas stimulated the decline in SPAD value and leaf nitrogen concentration and had no effect on seed yield in Tamahomare. However, high nitrogen concenntration during 115 to R7 delayed the decline in SPAD value and leaf nitrogen concentration and improved the yield performance more significantly in Tamahomare than in Sachiyutaka. The large seed yield increase by nitrogen enrichment during R5 to R7 in Tamahomare could be caused by both the high photosynthetic rate and vigorous nitrogen uptake during seed filling. These results suggested that the most effective timing of nitrogen enrichment during the reproductive growth period to increase seed yield varies with the cultivar due to the difference in the pattern of nitrogen assimilation.