Hydrogen Reactions of Nodulated Leguminous Plants

The ATP-dependent evolution of H(2) catalyzed by nitrogenase and the hydrogenase-catalyzed oxidation of H(2) have been implicated as factors influencing the efficiency of energy utilization in the N(2) fixation process. The effects of rhizobial strain and plant age on the H(2)-evolving and H(2)-utilizing activity of leguminous root nodules are described in this manuscript. Two classes of legume-Rhizobium combinations were observed in studies with soybeans (Glycine max L. Merr.) and cowpeas (Vigna unguiculata L. Walp.). One group evolved H(2) in air; the other group did not exhibit net evolution of H(2). The latter group metabolized H(2) formed within the nodule through the action of a hydrogenase. The capacity to oxidize H(2) was strongly linked to the strain of Rhizobium used to inoculate cowpeas and soybeans. Although the magnitude of H(2) evolution in air changed during vegetative growth of a given symbiont, the ratio of H(2) evolved in air to total nitrogenase activity was not appreciably altered during this period. No consistent difference in nitrogenase activity as measured by the C(2)H(2) reduction assay was observed between symbionts with an active hydrogenase and those that apparently lack the enzyme and evolve H(2). The effects of the two reactions of H(2) on total N(2) fixation and yield must now be established.