Nitrogen fixation in nitrate reductase-deficient mutants of cultured rhizobia

Abstract

Mutants (48) unable to reduce nitrate were isolated from cowpea Rhizobium sp. strain 32Hl and examined for nitrogenase activity in culture. All but 2 of the mutants had nitrogenase activity comparable with the parental strain, and 2 nitrogenase-defective strains showed alterations in their symbiotic properties. One strain was unable to nodulate Macroptilium atropurpureum or Vigna unguiculata, and with the other, nodules appeared promptly, but effective N2 fixation was delayed. These results and the relatively low proportion of nitrate reductase mutants with impaired nitrogenase activity do not support the proposed commonality between nitrogenase and nitrate reductase in cowpea rhizobia. Inhibition studies of the effect of nitrate and its reduction products on the nitrogenase activity in cultured strains 32Hl and the nitrate reductase-deficient, Nif+ strains indicated that nitrogenase activity was sensitive to nitrite rather than to nitrate.