Comparative studies on the distribution of nitrogen in soybean plants supplied with N2and No3-at the pod filling stage

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Abstract
Well nodulated soybean plants were subjected to 15N2 or 15NO8 - pulse label experiments at the pod filling stage, when the vegetative organs and the pod shells were fully developed. and the seeds were growing rapidly. Immediately after 10 hr of 15N2 treatment, only 36% of the fixed 15N remained in the nodules, and the rest was located as follows; roots 9%, sterns 17%, leaves 18%, pod shells 10%, seeds 10%. During the 5 consecutive days of cultivation under non-labelled conditions, 15N content decreased in the nodules, roots and sterns, and the corresponding amount of 15N was translocated to the seeds. At the end of 10 hr of NO3 - feeding, 36% of the total 15N was distributed in the roots, and 0.4% in the nodules, 17% in the sterns, 36% in the leaves, 5% in the pod shells, and 5% in the seeds, respectively. During 5 days of cold chase period, uN was redistributed to seeds mainly from the insoluble fraction of roots, stems and leaves. Finally 44% of the total 15NO3 ---N absorbed was located in the seeds. and the proportion was rather higher than the percentage of total fixed 15N2-N in seeds (36%). The results of 15N2 and 15NO3 - treatments were compared and it was concluded that although both N2-N and NO2 --N can be utilized for either the vegetative or the reproductive growth of soybean plants, the characteristics of the pathways of N metabolism are markedly different. Some portion of N originating from N2 was translocated directly to the pod shells and seeds, while another portion was derived from the protein degradation products of nodules or other vegetative parts. On the other hand, most of the NO3 --N absorbed was immediately incorporated into the proteins of roots and leaves, and subsequently N was resolubilized and redistributed to the reproductive parts.