Effects of boron on nodule development and symbiotic nitrogen fixation in soybean plants

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Abstract
Effects of boron (B) on soybean growth, nodule development, and nitrogen fixation were studied. When soybean plants were grown in B-free medium for 35,40, or 72 d, B deficiency symptoms appeared. Nodules of B-deficient soybeans were damaged and showed low acetylene reduction activities. Soybean seed production was seriously depressed. Production of whole plant, nodules and pods was maximum at concentrations of 15–180 μg B L-1 (1.4–16 μm). At concentrations of 29–88 μg B L-1 nitrogen fixation on a plant and nodule weight basis was markedly enhanced. Element absorption into soybean plants grown without boron and at 22, 44, 440, and 1,300 p.g B L-1 was examined. At 22–44 μg B L-1, the total amount of K, Ca, and Mg absorbed into soybean plants was relatively high, although the concentrations of these elements on a dry weight basis were almost the same. Boron concentration in the whole soybean plant increased when the amount of B in the culture media increased. The B exogenously supplied accumulated in the leaves of soybean plants. At a B concentration of 1,300 μg L-1, soybean plants were damaged, followed by a decrease in nitrogen fixation. To study the primary action of B, B was removed from the culture medium during the last 2 or 3 weeks of the 6 week culture. Symptoms associated with B removal appeared at the root tips. The dry weight of the whole plant and the total nitrogen fixation activity per plant were also reduced as compared with those of control plants. However, since the large-sized nodules detached from the basal portions of primary roots already contained a sufficient level of B for nitrogen fixation, this activity was not reduced by B removal for the last 2 weeks of culture. On the contrary, the young nodules which were still small, showed a decrease in nitrogen fixation activity after B removal. These results clearly indicate that B removal influenced more severely the developing nodules than the developed mature nodules and that B is important for the development and nitrogen fixation of nodules.