Seasonal Patterns of Nitrate Reductase and Nitrogenase Activities in Phaseolus vulgaris L.

The patterns of nitrate reductase activity (NRA) in the leaves ( in vivo assay) and root nodule nitrogenase activity (C 2 H 2 reduction) were investigated throughout the season in field-grown Phaseolus vulgaris plants. Maximal NRA (per g fresh weight) occurred at early stages of leaf development but total activity (per leaf) was maximal when the leaf reached full size. In mature plants, most NRA was associated with the upper leaves. Nitrogenase activity was initiated about 2 weeks after sowing, reached a maximum at flowering (5 weeks after sowing) and declined rapidly thereafter. Nitrogenase activity followed the pattern of nodule development. After flowering, P. vulgaris was apparently able to take up and assimilate NO − 3 as evidenced by the increase in NO − 3 content of the stem and the high levels of NRA in the leaves. Total plant NRA was maximal after flowering and addition of NH 4 NO 3 to the soil at flowering resulted in even higher levels of NRA through most of the pod-filling period, thus resulting in higher seed yields (59% over control). It is proposed that P. vulgaris can benefit from both N 2 fixation and NO − 3 assimilation and that nitrate reductase plays an important role in the assimilation of nitrogen after flowering.