SUMMARY: Increased oxygen tension (pO) caused increased respiration by excised soybean nodales of all ages. The increase took place in two steps, the first maximum occurring at about 50% O and the second at 90–100% O for actively nitrogen-fixing nodules. With increasing nodule age the first maximum occurred at decreasing pO until, when fixation ceased at about 6 weeks, this maximum had disappeared. This effect was more marked at 30° than at 23°. The respiration of bacteroids increased with increasing pO with a maximum at 2–3% O; the curve indicated a simple saturation of the terminal respiratory pathway with O. Increased pO raised nitrogen fixation by excised nodules to a maximum which corresponded to the first maximum of the respiratory response to raised pO; higher pO than this decreased nitrogen fixation. Sliced nodules showed the same effect but the stimulation of fixation at the lower pO levels was not as great as with intact nodules. The Michaelis constant () for nitrogen fixation by intact excised nodules was relatively unaffected by pO until this reached the pO for maximum fixation when the rose sharply. At external pO2 of 80%, oxygen was shown to be a competitive inhibitor of nitrogen fixation.

An explanation of these results is offered; it is suggested that the first part of the nodule O consumption/pO curves is due to O consumption by plant tissue and the second part to O consumption by bacteroids. The two components are separated by an O permeability barrier. When this barrier permits a rise in the pO at the bacteroids, nitrogen fixation is inhibited as oxygen competes with nitrogen for the reducing power of the bacteroids.


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