Nitrate and nitrite reduction by alfalfa root nodules: Accumulation of nitrite in Rhizobium melioti bacteroids and senescence of nodules

Abstract

Addition of NO−3 rapidly induced senescence of root nodules in alfalfa (Medicago sativa L. cv. Aragon). Loss of nodule dry matter began at the lowest NO−3 concentration (10 mM) but degradation of bacteroid proteins was only detected when nodules were supplied with NO−3 concentrations above 20 mM.Bacteroids from Rhizobium meliloti contained high specific activities of nitrate reductase (NR) and nitrite reductase (NiR). Both enzymes were presumably substrate‐induced although substantial enzyme activities were present in the absence of NO−3 Typical specific activities for soluble NR and NiR of bacteroids under NO−3 free conditions were 1.2 and 1.4 μmol (mg protein)−1h−1, respectively. In the presence of NO−3, the specific activity of NR was considerably greater than that of NiR, thus causing NO−2 accumulation in bacteroids. Nitrite levels in the bacteroids were linearly correlated with specific activities of NR and NiR, indicating that NO−2 is formed by bacteroid NR and that this NO−2 in turn, induces bacteroid NiR. Accumulation of NO−2 within bacteroids also indicates that NO−2 inhibits nodule activity after feeding plants with NO−3