DENITRIFICATION AND RESPIRATION IN RHIZOBIUM-MELILOTI BACTEROIDS AND LUCERNE NODULES AS AFFECTED BY NITRATE SUPPLY

Nodulated lucerne plants (Medicago sativa L. cv. Aragon) at the late vegetative stage were supplied with several nitrate concentrations (0 - 20 mM) for 1 week; then denitrification, respiration and activities related to inorganic nitrogen metabolism from both whole nodules and anaerobically isolated bacteroids of Rhizobium meliloti were studied. Denitrification showed a non-linear dependence on nitrate supply in both whole nodules and bacteroids. Bacteroid nitrate reduction activity accounted for more than 90% of the total nitrate reduction in nodules when low nitrate concentrations were supplied, but nitrate reduction in the plant fraction showed increasing importance with higher nitrate concentrations. In vivo bacteroid nitrate reduction enhancement was further supported by in vitro nitrate reductase measurements; however, nitrite reductase was absent in control bacteroids, suggesting that this enzyme was responsible for the restriction in denitrification. As a consequence of the increase in activities of inorganic nitrogen metabolism enzymes of bacteroids, bacteroid respiration was enhanced in the presence of nitrate, supporting the view that bacteroids are able to benefit from both aerobic and anaerobic respiration. However, nodule gross respiration was inhibited by nitrate. This result is discussed in relation to nitrate entry and changes in the oxygen diffusion barrier of the nodules.