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

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Abstract
The patterns of nitrate reductase activity (NRA) in the leaves (in vivo assay) and root nodule nitrogenase activity (C2H2 reduction) were investigated throughout the season in field-grown P. 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 wk after sowing, reached a maximum at flowering (5 wk 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 .**GRAPHIC**. as evidenced by the increase in .**GRAPHIC**. content of the stem and the high levels of NRA in the leaves. Total plant NRA was maximal after flowering and addition of NH4NO3 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). Apparently P. vulgaris can benefit from both N2 fixation and .**GRAPHIC**. assimilation and nitrate reductase plays an important role in the assimilation of N after flowering.