Changing Activities of Nitrogen-assimilating Enzymes During Growth and Senescence of Dwarf Beans (Phaseolus vulgaris L.)

Nitrate reductase, glutamine synthetase, glutamate synthase and glutamate dehydrogenase were measured in extracts of leaves, pods and seeds of Phaseolus vulgaris L. grown hydroponically with 16 mM nitrate as nitrogen source. Nitrate reductase activity per leaf was highest in young expanding and well illuminated leaves, and decreased rapidly in the lower leaves after full expansion. Low nitrate reductase activity was found in senescing leaves and pods. No nitrate reductase activity was detectable in seeds. Glutamine synthetase and methylviologen dependent glutamate synthase both increased in activity with leaf expansion, remained active longer than nitrate reductase and decreased during leaf nitrogen mobilization. Glutamine synthetase and glutamate synthase activities were low in pods, and in seeds, only glutamine synthetase was measurable with the methods used. Glutamate dehydrogenase was present in all organs analyzed. In leaves, glutamate dehydrogenase activity increased in fully expanded leaves and remained fully active during leaf senescence. Nitrate reductase activity was always lowest of the enzymes measured. The time courses of extractable nitrate reductase activity suggest that nitrate assimilation within the shoot was most active in developing and well illuminated leaves. Since glutamine synthetase and glutamate synthase remained active longer than nitrate reductase in the individual leaves, these two enzymes could continue to reassimilate ammonia released from mitochondria during photo-respiration. The high glutamate dehydrogenase activity in senescing leaves is consistent with its possible function in the oxidation of glutamate