Three Bradyrhizobium sp. (Vigna) strains (W-3824, N-169 and 6033) were inoculated on three tropical legumes, namely cowpea (Vigna unguiculata ssp. cylindrica L. Verdc.) cv. MI, peanut (Arachis hypogea L.) cv. 437 and pigeon pea (Cajanus cajan L. Millsp.) cv. MCHP, and plants were grown under greenhouse conditions in Leonard jars under long-term exposure to 0 or 7 mmol/L N-NO3−. When plants were 6 weeks old, cowpeas and pigeon peas inoculated with rhizobia W-3824 were selected for in situ studies of carbon costs of N2 fixation, nitrogenase (EC 1.18.6.1) linked respiration and resistance to O2 diffusion in nodules. Harvest took place 8 weeks after planting, when N2 fixation (acetylene reduction) and nitrate-reductase (EC 1.6.6.1) were determined in nodules, as well as dry matter and total N contents in shoots. Respiratory quotients of roots nodulated by two of the strains (W-3824 and N-169) were also analyzed.
Total N yields of shoots closely paralleled NRA in nodules from plants grown without combined N, and strains possessing NRA produced even higher yields in the presence of nitrate than the NR− strain (6033). Likewise, correlation coefficients were positive and significant between NRA in nodules and most of the N2 fixation parameters measured. A control by the host plant rather than by rhizobia on the efficiency of respiration in nodules under normal or stressed conditions could be concluded. Carbon costs of N2 fixation were also highly dependent on the legume, as it decreased in the nitrate resistant cowpeas, but significantly increased on pigeon peas. Nitrogenase-linked respiration was markedly raised by the addition of nitrate, but mainly the host affected changes of resistance to O2 diffusion in nodules, and the presence of combined N depleted the reaction capability of nodules to adjust to changing O2 levels in the rhizosphere.