Sodium stimulation of uptake hydrogenase activity in symbiotic Rhizobium

Initial observations showed a 100% increase in H₂-uptake (Hup) activity of Rhizobium leguminosarum strain 3855 in pea root nodules (Pisum sativum L. cv Alaska) on plants growing in a baked clay substrate relative to those growing in vermiculite, and an investigation of nutrient factors responsible for the phenomenon was initiated. Significantly greater Hup activity was first measured in the clay-grown plants 24 days after germination, and higher activity was maintained relative to the vermiculite treatment until experiments were terminated at day 32. The increase in Hup activity was associated with a decrease in H₂ evolution for plants with comparable rates of acetylene reduction. Analyses of the clay showed that it contained more Na⁺ (29 versus 9 milligrams per kilogram) and less K⁺ (6 versus 74 milligrams per kilogram) than the vermiculite. Analyses of plants, however, showed a large increase in Na⁺ concentration of clay-grown plants with a much smaller reduction in K⁺ concentration. In tests with the same organisms in a hydroponic system with controlled pH, 40 millimolar NaCl increased Hup activity more than 100% over plants grown in solutions lacking NaCl. Plants with increased Hup activity, however, did not have greater net carbon or total nitrogen assimilation. KCl treatments from 5 to 80 millimolar produced slight increased in Hup activity at 10 millimolar KCl, and tests with other salts in the hydroponic system indicated that only Na⁺ strongly promoted Hup activity. Treating vermiculite with 50 millimolar NaCl increased Na⁺ concentration in pea plant tissue and greatly promoted Hup activity of root nodules in a manner analogous to the original observation with the clay rooting medium. A wider generality of the phenomenon was suggested by demonstrating that exogenous Na⁺ increased Hup activity of other R. leguminosarum strains and promoted Hup activity of R. meliloti strain B300 in alfalfa (Medicago sativa L.).