Effect of Increases in Oxygen Concentration during the Argon-Induced Decline in Nitrogenase Activity in Root Nodules of Soybean

When intact nodulated roots of soybean (Glycine max L. Merr. nodulated with Bradyrhizobium japonicum strain USDA 16) were exposed to an atmosphere lacking N₂ gas (Ar:O₂ 80:20), total nitrogenase activity (measured as H₂ evolution) and respiration (CO₂ evolution) declined with time of exposure. In Ar-inhibited nodules, when the O₂ concentration in the rhizosphere was increased in a linear `ramp' of 2.7% per minute, 93% of the original H₂ evolution and 99% of the CO₂ evolution could be recovered. The internal nodule O₂ concentration (estimated from leghemoglobin oxygenation) declined to 56% of its initial value after 60 minutes of Ar:O₂ exposure and could be partially recovered by the linear increases in O₂ concentration. Nodule gas permeability, as estimated from the lag in ethylene production following exposure of nodules to acetylene, decreased to 26% of its initial value during the Ar-induced decline. Collectively, the results provide direct evidence that the Ar-induced decline results from decreased nodule gas permeability and indicate that the decline in permeability, rather than being immediate, occurs gradually over the period of Ar:O₂ exposure.