The understanding of the relationship between root nodule distribution and root architecture and between nitrogen-fixing activity and size of individual root nodules is required for effective utilization of symbiotic nitrogen fixation in leguminous plants. We studied these relationships in peanut. The nitrogen-fixing activity of root nodules was examined with reference to their size in peanut. The nitrogen-fixing activities of individual root nodules are closely related with the size of nodule, and thus classification of root nodules by size is rapid and useful method to evaluate nitrogen-fixing activities. The formation of root nodules was investigated in the field. Many root nodules in peanut are formed at positions on 1st-order lateral roots where 2nd-order lateral roots emerge. The two leading cultivars, Chibahandachi and Nakateyutaka, in Japan were studied in detail. Chibahandachi had fewer, but larger nodules whereas Nakateyutaka had more, but smaller nodules. Nakateyutaka formed more new nodules on the lateral branched from the middle and apical parts of the taproot than Chibahandachi. These facts suggest that root nodules in Chibahandachi may grow for a longer period with plant growth, and that Nakateyutaka may form new nodules at even late stages of plant growth. In addition, the nodule distribution was changed by cutting the root tip of taproot in peanut, suggesting the possibility that the nitrogen-fixing activity can be controlled by the designing of root architecture.

View full abstract