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

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.

Key Words: Glycine max (L.) Merr • Acetylene reduction • Nodule effeciency

² Postdoctoral research scientist, research scientists and associaten director, respectively.

Received for publication May 13, 1976.