Effects of Temperature and Oxygen Tension on the Nitrogenase and Respiratory Activities of Turgid and Water-stressed Soybean and French Bean Root Nodules

Abstract

When incubated in air, turgid soybean (Glycine max) and French bean (Phaseolus vulgaris) root nodules exhibited maximum nitrogenase activity between 15 and 30°C. In contrast, moderately water-stressed nodules (apparent nodule water potential equal to−0.8 to −1.0 × 106 Pa) exhibited maximum activity between 10 and 15 °C; their activity at higher temperatures was significantly less than that of the turgid nodules. An Arrhenius plot of the respiration rate of turgid nodules showed a break at 7.5 °C (soybean) and 9 °C (French bean). The Q10 values below these temperatures were 3.38 (soybean) and 2.47 (French bean) compared with values of 1.28 (soybean) and 1.14 (French bean) above these temperatures. Moderately water-stressed nodules showed a break in the Arrhenius plot at slightly higher temperatures, 9 °C (soybean) and 10.5 °C (French bean), but the Q10 values obtained were almost identical to those found for the turgid nodules. The respiratory activity of turgid soybean nodules was fully recoverable after they were exposed to 5.5 °C for 2 h but some nitrogenase activity was lost. A combination of high O2 partial pressure (above 4 × 104 Pa) and low temperature (10 to 15 °C) resulted in gradual inactivation of nitrogenase activity and saturation of respiratory activity in turgid nodules, but not in moderately stressed nodules