Oxygen limitation of N₂ fixation in stem‐girdled and nitrate‐treated soybean

The effects of increasing rhizosphere pO 2 on nitrogenase activity and nodule resistance to O 2 diffusion were investigated in soybean plants [ Glycine max (L.) Merr. cv. Harosoy 63] in which nitrogenase (EC 1.7.99.2) activities were inhibited by (a) removal of the phloem tissue at the base of the stem (stem girdling), (b) exposure of roots to 10 m M NO 3 over 5 days (NO 3 ‐treated), or (c) partial inactivation of nitrogenase activity by an exposure of nodulated roots to 100 kPa O 2 (O 2 ‐inhibitcd). In control plants and in plants which had been treated with 100 kPa O 2 , increasing rhizosphere O 2 concentrations in 10 kPa increments from 20 to 70 kPa did not alter the steady‐state nitrogenase activity. In contrast, in plants in which nitrogenase activities were depressed by stem girdling or by exposure to NO 3 , increasing rhizosphere pO 2 resulted in a recovery of 57 or 67%, respectively, of the initial, depressed rates of nitrogenase activity. This suggests that the nitrogenase activity of stem‐girdled and NO 3 ‐treated soybeans was O 2 ‐limited. For each treatment, theoretical resistance values for O 2 diffusion into nodules were estimated from measured rates of CO 2 exchange, assuming a respiratory quotient of 1.1 and 0 kPa of O 2 in the infected cells. At an external partial pressure of 20 kPa O 2 , the stem‐girdled and NO 3 ‐ ‐treated plants displayed resistance values which were 4 to 8.6 times higher than those in the nodules of the control plants. In control and O 2 ‐inhibited plants, increases in pO 2 from 20 to 70 kPa in 10 kPa increments resulted in a 2.5‐ to 3.9‐fold increase in diffusion resistance to O 2 , and had little effect on either respiration or nitrogenase activity. In contrast, in stem‐girdled and NO 3 ‐ ‐treated plants, increases in external pO 2 had little effect on diffusion resistance to O 2 , but resulted in a 2.3‐ to 3.2‐fold increase in nodule respiration and nitrogenase activity. These results are consistent with stem‐girdling and NO 3 ‐ ‐inhibition treatments limiting phloem supply to nodules causing an increase in diffusion resistance to O 2 at 20 kPa and an apparent insensitivity of diffusion resistance to increases in external pO 2 .