The effects of a short-term supply of combined nitrogen (N) on the Rhizobium–legume symbiosis were studied in faba bean (Vicia faba L.) and pea (Pisum sativum L.) nodulated by R. leguminosarum biovar viceae isolate 175F5 or 175F19 and subjected to increasing levels of [Formula: see text] (0, 5, 10, or 20 mol∙m−3) from 28 to 36 days after planting. Trends in N2-fixing activity (acetylene reduction) showed that faba bean was more tolerant to [Formula: see text] than pea with isolate 175F5 but not with isolate 175F19. Nitrite reductase activities in the leaf, root, and nodule cytosol fractions were severalfold higher than nitrate reductase activities for both hosts. The levels of these enzymes in the nodule cytosol, the absence of bacteroid nitrate reductase and the lack of induction of bacteroid nitrite reductase in response to [Formula: see text] addition are consistent with the lack of [Formula: see text] accumulation in nodules of both hosts. Therefore, it is unlikely that the inhibitory effects of [Formula: see text] on N2 fixation are due to [Formula: see text] inhibition of nitrogenase. The relative levels of nitrate reductase activity in the root and nodule cytosol fractions were, respectively, higher and lower in the two faba bean symbioses ([Formula: see text] tolerant) and in the pea–175F5 symbiosis ([Formula: see text] sensitive) than in the pea–175F19 symbiosis ([Formula: see text] tolerant).
Decrease of Nitrate Reductase Activity in Spinach Leaves during a Light-Dark Transition