Hydrogen Recycling by Rhizobium leguminosarum Isolates and Growth and Nitrogen Contents of Pea Plants (Pisum sativum L.).

The ability to recycle H(2) evolved by nitrogenase is thought to be of importance in increasing the efficiency of N(2) fixation and to be a factor in increasing plant yield in symbiotic systems. To determine whether this ability is a significant factor in the Rhizobium leguminosarum-Pisum sativum L. system, plants were inoculated with R. leguminosarum isolates which differed in their ability to oxidize H(2) and in their relative efficiency of N(2) fixation. These plants were grown at three levels of irradiance and harvested after 3, 4, and 5 weeks of growth for determination of C(2)H(2) reduction, H(2) evolution and uptake, plant dry weight, and N content. Plants inoculated with uptake hydrogenase-positive (Hup) isolates did not exhibit higher dry weight or N content than those inoculated with Hup isolates under any of the growth conditions studied. The efficiency of the nitrogenase system of Hup isolates increased at a low irradiance, a factor which may allow them to compete successfully with Hup isolates. In some HupR. leguminosarum isolates, H(2) oxidation is coupled to ATP formation, whereas in others, it is not. There were no differences in plant dry weight and N content in plants inoculated with the two types and grown for 5 weeks at three irradiance levels. The addition of H(2) to Hup nodules whose supply of photosynthate had been removed by stem excision did not increase C(2)H(2) reduction in either coupled or uncoupled types.