Root Nodule Enzymes of Ammonia Assimilation in Alfalfa (Medicago sativa L.)

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Abstract
Nitrogenase-dependent acetylene reduction activity of glasshouse-grown alfalfa (M. sativa L.) [inoculated with Rhizobium meliloti] decreased rapidly in response both to harvesting (80% shoot removal) and applied NO3- at 40 and 80 kg N/ha. Acetylene reduction activity of harvested plants grown on 0 k N/ha began to recover by day 15 as shoot regrowth became significant. In contrast, acetylene reduction activity of all plants treated with 80 kg NO3--N/ha and harvested plants treated with 40 kg NO3--N/ha remained low for the duration of the experiment. Acetylene reduction of unharvested alfalfa treated with 40 kg N/ha declined to an intermediate level and appeared to recover slightly by day 15. Changes in N2-fixing capacity were accompanied by similar changes in levels of nodule soluble protein. After an initial lag of 24 h, specific activities of alfalfa nodule glutamine synthetase [EC 6.3.1.2]. NADH-glutamate synthase [EC 1.4.1.14] and NAD-glutamate dehydrogenase [EC 1.4.1.2] (oxidative amination) decreased similar to but less rapidly than acetylene reduction activity. Increased specific activities of these nodule enzymes occurred as acetylene reduction activity increased and shoot growth resumed. The observed rates of glutamine synthetase and glutamate synthase were sufficient to assimilate NH3 produced via symbiotic N2 fixation. Nodule NADH-dependent glutamate dehydrogenase [EC 1.4.1.4] (reductive amination) specific activity was not associated with changes in acetylene reduction activity. Host plant glutamine synthetase and NADH-glutamate synthase may function to assimilate symbiotically fixed N and NADH-dependent glutamate dehydrogenase may function in ammonia assimilation during senescence in alfalfa nodules.