Physiological Studies on Nodule-Nitrate Reductase

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Abstract
The nitrate reduc-tase activity of Rhizobium cells from nodules was studied in relation to factors known to influence N2-fixation by nodules. A definite pattern of enzyme activity, as affected by age of plant, was found. It consisted of an increase of the specific activity of nitrate reductase until an optimum was reached and then a decline of activity when the nodules approached senescence. The change of hemoglobin content of the nodules from soybean and velvet bean plants followed, in general, a similar pattern. The application of nitrate-N and in some cases, ammonium-N to soybean plants resulted in a decreased activity of nitrate reductase in nodules. The hemoglobin content of nodules also was decreased by the nitrate treatments but not by the ammonium treatments. Both Fe and Mo deficiencies resulted in a pronounced reduction in the activity of nodule nitrate reductase and the content of hemoglobin in nodules. The evidence for a possible role of nitrate reductase in N2-fixation is discussed.