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

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The ability to recycle H2 evolved by nitrogenase is thought to be of importance in increasing the efficiency of N2 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 H2 and in their relative efficiency of N2 fixation. These plants were grown at three levels of irradiance and harvested after 3, 4, and 5 weeks of growth for determination of C2H2 reduction, H2 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 Hup+R. leguminosarum isolates, H2 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 H2 to Hup+ nodules whose supply of photosynthate had been removed by stem excision did not increase C2H2 reduction in either coupled or uncoupled types.

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