Effect of Supra-Ambient Oxygen on Nitrogenase Activity (C2H2) and Root Respiration of Soybeans and Isolated Soybean Bacteroids

Isolated soybean (Glycine max [L.] Merr. cv Wilkin) bacteroids have O(2)-dependent nitrogenase activity which is strongly inhibited by supraoptimal O(2) concentrations. Oxygen-inhibited nitrogenase activity is recovered by addition of 10 millimolar sodium succinate or by lowering the O(2) concentration.Brief treatment of roots of intact soybean plants with 1.0 atmosphere O(2) reduces nitrogenase activity (C(2)H(2)). There is a rapid partial recovery of activity within 2 to 3 hours, and a slower return to near normal levels by 36 hours. The drop and recovery of nitrogenase activity is accompanied by a parallel drop and increase in root respiration. There is a direct relationship between the change in respiration and the change in acetylene reduction following O(2) treatment. The O(2)-mediated changes in nitrogenase activity and root respiration are not affected by the planting medium. The ratio of the change in respiration to the change in nitrogenase activity was the same in 13 soybean cultivars.