Nitrate reductase activities of rhizobia and the correlation between nitrate reduction and nitrogen fixation

Abstract

All Rhizobium spp. except R. lupini had nitrate reductase activity. Only R. lupini was incapable of growth with nitrate as the sole N source. The conditions necessary for the induction of nitrate reductase varied among Rhizobium spp. R. japonicum and some Rhizobium spp. of the cowpea strains expressed nitrate reductase activities in the root nodules of appropriate leguminous hosts and when grown in the presence of nitrate. R. trifolii, R. phaseoli and R. leguminosarum did not express nitrate reductase activities in the root nodules, but they did express them when grown in the presence of nitrate. In bacteroids of R. japonicum and some strains of cowpea Rhizobium, high N2 fixation activities were accompanied by high nitrate reductase activities. In bacteroids of R. trifolii, R. leguminosarum and R. phaseoli, high N2 fixation activities were not accompanied by high nitrate reductase activities.