Reversible O₂ Inhibition of Nitrogenase Activity in Attached Soybean Nodules

Various forms of stress result in decreased O(2) permeability or decreased capacity to consume O(2) in legume root nodules. These changes alter the nodule interior O(2) concentration (O(i)). To determine the relationship between O(i) and nitrogenase activity in attached soybean (Glycine max) nodules, we controlled O(i) by varying external pO(2) while monitoring internal H(2) concentration (H(i)) with microelectrodes. O(i) was monitored by noninvasive leghemoglobin spectrophotometry (nodule oximetry). After each step-change in O(i), H(i) approached a new steady state, with a time constant averaging 23 s. The rate of H(2) production by nitrogenase was calculated as the product of H(i), nodule surface area, and nodule H(2) permeability. H(2) permeability was estimated from O(2) permeability (measured by nodule oximetry) by assuming diffusion through air-filled pores; support for this assumption is presented. O(i) was nearly optimal for nitrogenase activity (H(2) production) between 15 and 150 nm. A 1- to 2-min exposure to elevated external pO(2) (40-100 kPa) reduced H(i) to zero, but nitrogenase activity recovered quickly under air, often in <20 min. This rapid recovery contrasts with previous reports of much slower recovery with longer exposures to elevated pO(2). The mechanism of nitrogenase inhibition may differ between brief and prolonged O(2) exposures.