Synthesis and Accumulation of Nitrite in Soybean Nodules Supplied with Nitrate

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Abstract
Nodulated soybean plants (G. max [L.] Merr) were grown in sand culture without combined N or with a continuous supply of nitrate in nutrient solution. Moderate nitrate concentration (30 mg N/l) had little effect on nodule weight/plant while high nitrate concentration (100 ml N/l) depressed nodule weight/plant by 70-80% with harvests 30-60 days after planting and initiation of nitrate treatments. The effect of nitrate supply on ammonium, amino and ureide N concentrations in nodules was small and unconsistent. Nitrate and nitrite concentrations in nodules were directly proportional to nitrate supply and inversely proportional to nodule weight plant. Correlations between nitrate or nitrite concentration in nodules and nodule weight/plant were highly significant. Cytosol from soybean nodules contained NADH-dependent nitrate reductase activity (typical activity was 0.1 .mu.mol/ml protein .times. h). A Rhizobium japonicum mutant (derived from strain 61A76) lacking nitrate reductase showed that the cytosol enzyme activity is of host origin. Growth of nodules formed by the mutant lacking nitrate reductase was inhibited by nitrate. These nodules did contain nitrite although concentrations of nitrite (.apprx. 0.3 .mu. N/g fresh wt) were low relative to nitrite concentrations (.apprx. 1.5 .mu.mol N/g fresh wt) in nodules formed by R. japonicum strain 61A76. Apparently, the depression of legume nodule growth by nitrate is directly related to the metabolism of nitrate in nodules.