Abstract
An experiment was conducted to test the hypothesis that, when nitrogenase and nitrate reductase both contribute to the nitrogen nutrition of a nodulated legume, nitrogenase activity is inversely proportional to the rate of accumulation of organic nitrogen derived from the reduction of nitrate. Trifolium subterraneum L. plants, inoculated with Rhizobium trifolii and sown as small swards, were allowed to establish a closed canopy and steady rates of growth, dinitrogen fixation, and nitrogen accumulation. Swards were then supplied with nutrient solutions of 0, 0.5, 1.0, or 2.5 mm NO3− with a 29.69% enrichment of 15N and allowed to grow for a further 33 days. Harvests were made to measure dry weight, nitrogen accumulation, 15N accumulation, NO3− content and nitrogenase activity by acetylene reduction assay. Since the 15N of the plant organic matter could have been derived only from the NO3− of the nutrient solution, its rate of accumulation provided a measure of the rate of NO3− reduction. It was found that as this rate increased in response to external NO3− concentration the rate of nitrogenase activity decreased proportionately. It is concluded that the reduction of nitrate and the reduction of dinitrogen act in a complementary manner to supply a plant with organic nitrogen for growth.
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