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

Abstract

The effects of increasing rhizosphere pO2on nitrogenase activity and nodule resistance to O2diffusion 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 mM NO3over 5 days (NO3‐treated), or (c) partial inactivation of nitrogenase activity by an exposure of nodulated roots to 100 kPa O2(O2‐inhibitcd).In control plants and in plants which had been treated with 100 kPa O2, increasing rhizosphere O2concentrations 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 NO3, increasing rhizosphere pO2resulted 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 NO3‐treated soybeans was O2‐limited.For each treatment, theoretical resistance values for O2diffusion into nodules were estimated from measured rates of CO2exchange, assuming a respiratory quotient of 1.1 and 0 kPa of O2in the infected cells. At an external partial pressure of 20 kPa O2, the stem‐girdled and NO3‐‐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 O2‐inhibited plants, increases in pO2from 20 to 70 kPa in 10 kPa increments resulted in a 2.5‐ to 3.9‐fold increase in diffusion resistance to O2, and had little effect on either respiration or nitrogenase activity. In contrast, in stem‐girdled and NO3‐‐treated plants, increases in external pO2had little effect on diffusion resistance to O2, 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 NO3‐‐inhibition treatments limiting phloem supply to nodules causing an increase in diffusion resistance to O2at 20 kPa and an apparent insensitivity of diffusion resistance to increases in external pO2.