EFFECT OF POD REMOVAL ON FIXED-N (N-15(2)) EXPORT FROM SOYBEAN (GLYCINE-MAX L) NODULES

To analyze the mechanism underlying the depression of the nodule activity associated with pod removal, an N-15(2) tracer experiment was conducted using a determinate soybean (Glycine max L.) cultivar, Tamahomare. In the depodded plants, where a major sink for fixed-N was removed, the vegetative parts of the host plant acted as an alternative sink, although complete replacement was not observed. The percentage of fixed-N translocated from the nodules was about 65% after 6 h of N-15(2) administration in the control while this value was 53% in the treated plants. Since the nodule activity (ARA) decreased 3-days after the treatment, with the increase of the distribution percentage of labelled-N in the nodules, it is considered that the decrease in the dinitrogen-fixing activity may be partly due to the reduced export rate of fixed-N from the nodules to the host plant. Based on these results, it is suggested that the accumulation of products from fixed-N may be involved in the depression of the root nodule activity.