ROLE OF OXYGEN LIMITATION AND NITRATE METABOLISM IN THE NITRATE INHIBITION OF NITROGEN-FIXATION BY PEA

The impact of nitrate (5-15 mM, 2 to 7 days) on nitrogenase activity a nd nodule-oxygen limitation was investigated in nodulated, 21-day-old plants of a near-isogenic nitrate reductase-deficient pea mutant (A317 I) and its wild-type parent (Pisum sativum L. cv. Juneau). Within 2 da ys, 10 or 15 nuM nitrate, but not 5 mM nitrate, inhibited the apparent nitrogenase activity (measured as in situ hydrogen evolution from nod ules of intact plants) of wild-type plants; none of these nitrate leve ls inhibited the apparent nitrogenase activity of A317I plants. Nodule -oxygen limitation, measured as the ratio of total nitrogenase activit y to potential nitrogenase activity, was increased in both wild-type a nd A317I plants by all nitrate treatments. By 3 to 4 days the apparent nitrogenase activity of A317I and wild-type plants supplied with 5 mM nitrate declined to 53 to 69% of control plants not receiving nitrate . By 6 to 7 days the apparent nitrogenase activity of A317I plants was similar to the control value whereas that of the wild-type plants con tinued to decline. From 3 to 7 days, no significant differences in nod ule-oxygen limitation were observed between the nitrate (5 mM) and con trol treatments. The results are interpreted as evidence for separate mechanisms in the initial (O-2 limitation) and longer-term (nitrate me tabolism) effects of nitrate on nitrogen fixation by effectively nodul ated pea.