A new model for the rapid effects of non-invasive treatments on nitrogenase and respiratory activity in legume nodules

Effects of non-invasive treatments of legume nodules with argon, acetylene and oxygen have led others to the conclusion that there is a variable barrier to gas diffusion in the nodule cortex which responds to treatments after time periods as short as 15 min. A new model is presented here in which the energy-dependent transport of protons from host cytoplasm to bacterioids, the consumption of oxygen by mitochondria to maintain the proton gradient, and the balance between oxygen consumption in bacterioids and mitochondria are emphasized. The point of impact of the non-invasive argon or acetylene treatments is placed at the enzyme nitrogenase in bacteroids. Because nitrogenase consumes eight protons in each catalytic cycle, depression of the enzyme activity may also perturb the proton gradient. Restoration of nitrogenase activity would be accomplished by increasing the oxygen-dependent proton gradient across the bacteroid cytoplasmic membrane. Effects of argon, acetylene and oxygen may be due to disturbance of a delicate balance in the supply and demand for protons in bacteroids and/or a disturbance of the balance between the consumption of oxygen in mitochondria and bacteroids.