NITRATE AND NITRITE REDUCTION BY ALFALFA ROOT-NODULES - ACCUMULATION OF NITRITE IN RHIZOBIUM-MELILOTI BACTEROIDS AND SENESCENCE OF NODULES

Addition of NO3- rapidly induced senescence of root nodules in alfalfa. Loss of nodule dry matter began at the lowest NO3- concentration (10 mM) but degradation of bacteroid proteins was only detected when nodules were supplied with NO3- concentrations above 20 mM. Bacteroids from R. meliloti contained high specific activites of nitrate reductase (NR) and nitrite reductase (NiR). Both enzymes were presumably substrate-induced although substantial enzyme activities were present in the absence of NO3-. Typical specific activities for soluble NR and NiR of bacteroids under NO3--free conditions were 1.2 and 1.4 .mu.mol (mg protein)-1 h-1, respectively. In the presence of NO3-, the specific activity of NR was considerably greater than that of NiR, thus causing NO2- accumulation in bacteroids. Nitrite levels in the bacteroids were linearly correlated iwth specific activities of NR and NiR, indicating that NO2- is formed by bacteroid NR and that this NO2-, in turn, induces bacteroid NiR. Accumulation of NO2- within bacteroids also indicates that NO2- inhibits nodule activity after feeding plants with NO3-.