Effects of planting date on seasonal patterns of nitrogen assimilation and partitioning by two soybean cultivars

Delayed planting consistently reduces grain yields by soybeans [Glycine max (L.) Merr.]. Grain yield of soybeans is thought to be influenced by the duration of seedfill which in turn is influenced by remobilization of N from vegetative tissue. A field study was conducted during two growing seasons in Urbana IL. to evaluate the effect of planting date on N accumulation, partitioning, and remobilization by soybean cultivars ‘Williams 79’ and ‘Corsoy 79’. Soybeans were planted in mid‐May or mid‐June of 1982 and 1983. Tissue samples including abscised material, were collected at R1, R5, R7, and R8 and separated into seeds, pods, stems, leaflets and petioles for N analysis. Kjeldahl plant N prior to seedfill was generally greater in Williams 79 than Corsoy 79; however, the latter stored more N per unit biomass to partially compensate for reductions in total biomass. Cultivar differences in vegetative N were more pronounced with delayed planting. Water stress during 1983 reduced total leaf N levels at R5, and resulted in a reduced rate of N remobilization to the seed. Net N remobilization ranged from 4.5 to 84.0%. Whole plant N accumulation rates during seedfill increased to compensate for reductions in net N remobilization so that total N accumulation by the seed was not greatly reduced by drought. Delayed planting reduced the total N of each vegetative plant part prior to R5 which corresponded to decreases in biomass. Delayed planting also reduced total N transfer to the seed. By R7 treatment differences were reduced.