Regulation of o(2) concentration in soybean nodules observed by in situ spectroscopic measurement of leghemoglobin oxygenation

A fiber optic spectrophotometric system was used to monitor the in vivo oxygenation of leghemoglobin in intact, attached soybean root nodules (Glycine max L. Merr. × USDA 16 Bradyrhizobium japonicum) which were flattened during development by growth in narrow, glass-walled cuvettes. When equilibrated at an external pO₂ of 20 kilopascals, leghemoglobin was 36.6 ± 5.4% oxygenated, a value estimated to represent an infected cell O₂ concentration of 21.5 nanomolar. Increasing the external pO₂ from 20 to 25 kilopascals caused a rapid increase in leghemoglobin oxygenation, followed by a recovery to the initial level, all within 7.5 minutes. At 25 kilopascals O₂, the rates of H₂ and CO₂ evolution were similar to those at 20 kilopascals. Since respiration had not increased, the results support the proposal that nodules adapt to increased external pO₂ by regulating their resistance to O₂ diffusion.