Uptake and distribution of combined nitrogen and its incorporation into seeds of nodulated soybean plants as revealed by 15N studies

Pot experiments were carried out with soybean plants grown with N-free solution, then supplied with NO3- or NH4+ before 15N administration, in order to study the uptake, distribution and redistribution of absorbed nitrogen. To determine the utilization and incorporation of combined nitrogen to the yield of seeds, as influenced by the form of N and application time, another series of experiments was carried out, in which the plants were grown with N-free solution, then 15N was fed as 15NO3- or 15NH4+ at pod setting stage (PS) or initial pod filling stage (IPF). By comparing the nitrate-treated plants (Nt plants) with the ammonia-treated ones (Am plants), and the treatment at PS with that at IPF, the following results were obtained. Nt plants accumulated larger amount of dry matters in respective plant parts than Am plants, particularly in the pods, and it was proved that the pods were the active sinks for newly metabolized nitrate. Both 15N abundance and 15N incorporation in the whole plants were much higher in Nt plants, and those in respective plant parts except nodules were also higher in Nt plants. Large part of the 15N in roots was redistributed to the top parts in Nt plants, while large part of the absorbed N was distributed to the nodules in Am plants. Some more results are reported. (Kako, I.)