Adaptation of Nitrogen Fixation by Intact Soybean Nodules to Altered Rhizosphere pO2

Abstract

The N2-fixing legume nodule [Rhizobium japonicum] requires O2 for ATP production; the O2 sensitivity of nitrogenase dictates a requirement for a low pO2 [partial pressure of O2] inside the nodule. The effects of long term exposures to various pO2 on N2[C2H2] fixation were evaluated with intact soybean (Glycine max [L.] Merr., cv. ''Wye'') plants. Continuous exposure of their rhizosphere to a pO2 of 0.06 atm initially reduced nitrogenase activity by 37-45%, with restoration of original activity in 4-24 h and no further change in tests up to 95 h; continuous exposure to 0.02 atm of O2 initially reduced nitrogenase activity 72%, with only partial recovery by 95 h. Similar exposures to a pO2 of 0.32 atm had little effect on N2[C2H2] fixation; a pO2 of 0.89 atm initially reduced nitrogenase activity by 98% with restoration to only 14-24% of that of the ambient O2 controls by 95 h. Reexposure to ambient pO2 of plants adapted to nonambient pO2 reduced N2[C2H2] fixation to similar magnitudes as the reductions which occurred upon initial exposure to variant pO2 conditions, and a time period was required to readapt to ambient O2. Apparently the N2[C2H2]-fixing system of intact soybean plants is able to adapt to a wide range of external pO2 as probably occur in soil. This occurs through an undefined mechanism which enables the nodule to maintain an internal pO2 optimal for nitrogenase activity.