Effect of Rhizosphere pO2 on Nitrogen Fixation by Excised and Intact Nodulated Soybean Roots1

Abstract

The N2‐fixing legume nodule has an O2 requirement for ATP production to support nitrogenase (ECI.7.99.2) activity, however, the internal partial pressure of O2 (pO2) in the nodule must be maintained at low level because of the O2 sensitivity of nitrogenase. Effects on N2 fixation of short‐term exposures of the rhizosphere to altered external pO2's using both excised nodulated soybean [Glycine max (L.) Merr.] roots and intact plants were investigated utilizing acetylene (C2‐H2) reduction. Exposures of 30 min to supra‐ambient pO2's of 0.26 to 0.41 atm stimulated N2[C2H2] fixation by excised roots, but not intact plants while short exposures to a sub‐ambient pO2 of 0.05 atm reduced N2[C2H2] fixation by excised roots more than by intact plants. Short exposures of 30 rain to a supra‐optimum pO2 of 0.65 to 0.90 atm reduced N2[C2H2] fixation in both excised roots and intact plants, and in contrast to exposure to other O2 concentrations, there was only partial recovery of N2[C2H2] fixation by the intact plants and a further decline by the excised roots on reincubation in ambient O2. With excised roots, altered pO2 effects were age‐dependent with a decline in the optimum pO2 from 0.41 to 0.26 atm and intensification of the detrimental effects of high pO2. It is concluded that; 1) the excised root is more sensitive to altered pO2 than intact plants, 2) measurements of the effects of stress, such as altered pO2, on N2 fixation should be made on intact plants, and 3) ambient pO2 is not limiting for N2 fixation but short‐term exposures to sub‐ambient pO2 will limit N2 fixation.