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

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Abstract
Isolated soybean (Glycine max [L.] Merr. cv Wilkin) bacteroids have O2-dependent nitrogenase activity which is strongly inhibited by supraoptimal O2 concentrations. Oxygen-inhibited nitrogenase activity is recovered by addition of 10 millimolar sodium succinate or by lowering the O2 concentration. Brief treatment of roots of intact soybean plants with 1.0 atmosphere O2 reduces nitrogenase activity (C2H2). 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 O2 treatment. The O2-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.