Variation in Nitrogenase and Hydrogenase Activity of Alaska Pea Root Nodules

Hydrogenase activity of root nodules in the symbiotic association between Pisum sativum L. and Rhizobium leguminosarum was determined by incubating unexcised nodules with tritiated H(2) and measuring tissue HTO. Hydrogenase activity saturated at 0.50 millimolar H(2) and was not inhibited by the presence of 0.10 atmosphere C(2)H(2), which prevented H(2) evolution from nitrogenase. Total H(2) production from nitogenase was estimated as net H(2) evolution in air plus H(2) exchange in 0.10 atmosphere C(2)H(2). Although such an estimate of nitrogenase function may not be quantitatively exact, due to uncertain relationships between H(2) exchange and H(2) uptake activity of hydrogenase, differences observed in H(2) 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 H(2) production by nitrogenase. Total H(2) production and hydrogenase activity were maximum 28 days after planting. Thereafter, hydrogenase activity and H(2) production declined, but the potential proportion of nitrogenase-produced H(2) recovered by the uptake hydrogenase system increased. Of five R. leguminosarum strains tested two possessed hydrogenase activity. Strains which had the potential to reassimilate H(2) had significantly higher rates of N(2) reduction than those which did not exhibit hydrogenase activity.