Changes in Nitrogenase Activity and Nodule Diffusion Resistance of Subterranean Clover in Response topO2

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Abstract
Diffusion resistance to oxygen within nodules was calculated using the respiratory quotient (RQ) of nodules from intact plants of subterranean clover (Trifolium subterraneum L.) cv. Seaton Park nodulated by Rhizobiun trifolii WU95. From 21 to 52% O2, the RQ remained between 0.94 and 1.04, whereas at 10% O2, the RQ was 1.65. When nodulated roots of intact plants were exposed to sub-ambient pO2 in a continuous flow-through system, respiration declined immediately, followed by a partial recovery within 30 min. The magnitude of the final respiration rate was dependent upon the pO2 in the gas stream. Initial rates of respiration were re-established after 24 h at sub-ambient pO2 as a result of changes in the resistance of the variable barrier to oxygen diffusion within the nodules. Nitrogenase activity also decreased linearly with decreasing pO2 in the gas stream, but partial recovery occurred after 24 h incubation at sub-ambient pO2. Maximum rates of nitrogenase activity occurred at rhizosphere oxygen concentrations between 21% and 36% O2. Resistance to the diffusion of oxygen within the nodules increased at supra-ambient pO2 and at oxygen concentrations above 36% O2, resulted in a decrease in both nitrogenase activity and nodulated root respiration. The diffusion resistance of nodules to oxygen increased rapidly in the presence of either supra-ambient pO2 or saturating pC2H2. Reductions in nodule diffusion resistance either during recovery from exposure to 10% acetylene or to sub-ambient pO2 occurred more slowly. It is concluded that subterranean clover is well adapted for maximum nitrogen fixation at ambient pO2.