Root Nodule Enzymes of Ammonia Metabolism from Medicago sativa L. as Influenced by Nitrate Levels

Plants of alfalfa (Medicago sativa L.) were subjected to six levels of nitrate ranging from 0 to 200 mM. Then, nodule content of soluble protein and activities of the nodule enzymes, nitrogenase (C(2)H(2) reduction activity), glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.1.14), and NADH/NAD(+)-glutamate dehydrogenases (EC 1.4.1.2) were determined. Decline of glutamate synthase activity closely paralleled that of nitrogenase. In contrast, glutamine synthetase activity reached a minimum at 20mM nitrate but began to recover at higher nitrate concentrations. Patterns of NADH-(aminating) and NAD(+)-(deaminating) glutamate dehydrogenase activities were opposed with a great increase (2- to 5-fold) in the aminating direction at highest levels of nitrate. Results indicate a main role of glutamine synthetase/glutamate synthase pathway in the assimilation of fixed N. In addition, increase of the activities of glutamine synthetase and NADH-glutamate dehydrogenase concomitant with an almost fully suppressed nitrogenase and decreased NAD(+)-glutamate dehydrogenase activities at above 20mM nitrate reflect: (a) rapid induced senescence of nodules and (b) the production of ammonia by alternative means as nitrate reduction or oxidative deamination of amino acids.