Nitrate Metabolism in Alfalfa Root Nodules under Water Stress

Three-month-old plants (vegetative stage) of alfalfa (Medicago saliva L cv. Aragon) were supplied for one week with 1.0dm³ (uniformly distributed) nutrient solutions containing 0 or 20 mol m⁻³$NO3-$. One week after initiation of $NO3-$ treatment, the plants were subjected to drought by withholding water. Bacteroids and cytosol of nodules were obtained at different stages of stress, and used for enzyme assays and for determination of $NH4+$, $NO2-$ and $NO3-$. Proteins of bacteroids were more stable than cytosolic proteins with respect to the detrimental effects of water stress and $NO2-$. Protein contents of bacteroids and cytosol were inversely related to proteolytic activities against azocasein in both nodule fractions. Specific nitrate reductase activity (NRA) and nitrite reductase activity (NiRA) of bacteroids from $NO3-$-treated plants were inhibited by c. 70% and 45%, respectively, as leaf water potential (ψ_w) declined from −0.5 MPa (control) to −1.8 MPa. At still lower ψ_w both activities began to increase: NRA was doubled, whereas NiRA only returned to its control level. Cytosolic NRA was strongly inhibited by drought, but the corresponding NiRA remained constant. Ammonia concentration in bacteroids and nodule cytosol kept basically constant, whereas $NO2-$ accumulated in the cytosol at severe stress, due to the activation of bacteroid nitrate reductase. Results indicate that nitrate and nitrite reductases of the bacteroids and the nodule cytosol act in different form: assimilatory, the cytosolic enzymes; and dissimilatory, the enzymes of bacteroids at low ψ_w The possibility that assimilation of $NO3-$ also occurs in bacteroids at control or mild water stress conditions is suggested.