Nitrate Inhibition of Legume Nodule Growth and Activity

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Abstract
The synthesis and accumulation of nitrite was suggested as a causative factor in the inhibition of legume nodules supplied with nitrate. Plants were grown in sand culture with a moderate level of nitrate (2.1 to 6.4 mM) supplied continuously from seed germination to 30-50 days after planting. In a comparison of nitrate treatments, a highly significant negative correlation between nitrite concentration in soybean (Glycine max [L.] Merr.) nodules and nodule fresh weight per shoot dry weight was found even when bacteroids lacked nitrate reductase (NR). In a comparison of 2 Rhizobium japonicum strains, there was only 12% as much nitrite in nodules formed by NR- R. japonicum as in nodules formed by NR+ R. japonicum, and growth and acetylene reduction activity of both types of nodules was about equally inhibited. In a comparison of 8 other NR+ and NR- R. japonicum strains, and a comparison of G. max, Phaseolus vulgaris and Pisum sativum, the concentration of nitrite in nodules was unrelated to nodule weight per plant or to specific acetylene reduction activity. The very small concentration of nitrite found in P. vulgaris nodules (0.05 .mu.g NO2--N/g fresh wt) was probably below that required for the inhibition of nitrogenase based on published in vitro experiments, and yet the specific acetylene reduction activity was inhibited 83% by nitrate. The overall results do not support the idea that nitrite plays a role in the inhibition of nodule growth and nitrogenase activity by nitrate.