Respiration and oxygen transport in soybean nodules

The respiration rate of individual soybean (Glycine max Merr.) nodules was measured as a function of pO2 and temperature. At 23°, as the pO2 was increased from 0.1 to 0.9 atm, there was a linear increase in respiration rate. At 13°, similar results were obtained, except that there was an abrupt saturation of respiration at approximately 0.5 atm pO2. When measurements were made on the same nodule, the rate of increase in respiration with pO2 was the same at 13° and 23°. Additional results were that 5% CO in the gas phase had no effect on respiration, except for a small decrease in the pO2 at which respiration became saturated. Also, nodules still attached to the soybean root displayed the same respiratory behavior as detached nodules. A model for oxygen transport in the nodule is presented which explains these results quantitatively. The essence of the model is that the respiration rate of the central tissue of the nodule is almost entirely determined by the rate of oxygen diffusion to the respiratory enzymes. Evidence is given that the nodule cortex is the site of almost all of the resistance to oxygen diffusion within the nodule.