Variation in Nitrogenase and Hydrogenase Activity of Alaska Pea Root Nodules 1

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Hydrogenase activity of root nodules in the symbiotic association between Pisum sativum L. and Rhizobium leguminosarum was determined by incubating unexcised nodules with tritiated H2 and measuring tissue HTO. Hydrogenase activity saturated at 0.50 millimolar H2 and was not inhibited by the presence of 0.10 atmosphere C2H2, which prevented H2 evolution from nitrogenase. Total H2 production from nitogenase was estimated as net H2 evolution in air plus H2 exchange in 0.10 atmosphere C2H2. Although such an estimate of nitrogenase function may not be quantitatively exact, due to uncertain relationships between H2 exchange and H2 uptake activity of hydrogenase, differences observed in H2 exchange under various conditions represent an indication of changes in hydrogenase activity. Hydrogenase activity was lower in associations grown under higher photosynthetic photon flux densities and decreased relative to total H2 production by nitrogenase. Total H2 production and hydrogenase activity were maximum 28 days after planting. Thereafter, hydrogenase activity and H2 production declined, but the potential proportion of nitrogenase-produced H2 recovered by the uptake hydrogenase system increased. Of five R. leguminosarum strains tested two possessed hydrogenase activity. Strains which had the potential to reassimilate H2 had significantly higher rates of N2 reduction than those which did not exhibit hydrogenase activity.

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