Submitted to: Plant Physiology Supplement
Publication Type: Abstract Only
Publication Acceptance Date: June 3, 1996
Publication Date: N/A
Technical Abstract: Nitrogenase activity in legume root nodules is closely controlled by the plant. It has been suggested that a hypothetical control mechanism could involve sensing of current N2 fixation possibly through monitoring concentrations of free amino acid in the nodule. We investigated this hypothesis in two experiments by modifying nitrogenase activity and N2 fixation through (i) varying pN2 surrounding the nodules and (ii) defoliation. In the first experiment we exposed nodulated alfalfa (Medicago sativa L.) root systems to a gaseous atmosphere containing 20% O2 and either 80%, 7%, 2%, or 0% N2 with the balance being Ar for 96 h. Total nitrogenase activity (TNA) was not different among the treatments that received 80%, 2%, and 0% N2, respectively. Exposure to 7% N2 resulted in a 70% increase of TNA. During this period, N2 fixation, as calculated from H2 evolution data, was absent with 0% N2, reduced to 13% with 2% N2 and to 57% with 7% N2, as compared to the control. In the second experiment we followed nitrogenase activity in alfalfa after defoliation. Five hours after defoliation TNA was reduced by 63%. In both experiments differences in TNA and N2 fixation were not reflected in differences in free amino acid concentrations within the nodules. We conclude from these experiments that the regulation of nitrogenase activity is not mediated through nodule amino acid pools and is independent of N2 fixation.