Nitrate reductase activity in alfalfa plants supplied with increasing rates of mineral nitrogen and inoculated with different Rhizobium meliloti strains

Nitrate reductase (NRA) and nitrogenase (NGA) activities were studied in alfalfa (Medicago varia Mart.) plants inoculated with Rhizobium meliloti, strains 425a and M4. Sand cultures of alfalfa supplied with various rates of mineral nitrogen (rates of 0, 0.25, 0.5, and 1 according to Hellriegel) were used in the experiments. Strains 425a and M4 significantly differed in NGA and markedly affected NRA in alfalfa. In plants inoculated with the standard strain 425a and cultivated without mineral nitrogen, a higher NGA level was found than in plants grown in the presence of mineral nitrogen. In contrast, nodules formed with the new strain M4 were more active when plants were supplied with mineral nitrogen at rates of 0.25-1. In the presence of mineral nitrogen at rates of 0.5 and 1, high NRA in stems was a physiological feature of alfalfa plants inoculated with strain M4, whereas plants inoculated with strain 425a possessed high enzyme activity in leaves. In leaves and stems of plants grown without mineral nitrogen or in the presence of nitrogen at a rate of 0.25 and inoculated with strain 425a, higher NRA activity was observed than in plants inoculated with strain M4.