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

Abstract

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