Oxygen-induced membrane depolarizations in legume root nodules: Possible evidence for an osmoelectrical mechanism controlling nodule gas permeability

Various stresses trigger rapid and reversible decreases in the O₂ permeability (P_O) of legume root nodules. Several possible mechanisms have been proposed, but no supporting data have previously been presented that meet the requirements for both rapidity and reversibility. Stomatal regulation of gas permeability in leaves involves electrically driven fluxes of inorganic osmoticants, so we investigated the possibility of a somewhat similar mechanism in nodules. We used microelectrodes to monitor membrane potential in intact, attached nodules of Glycine max, Medicago sativa, Lotus corniculatus, and Trifolium repens while controlling external O₂ concentration and, in the case of C. max, measuring P_O with a nodule oximeter. A 1- to 2-min exposure to 100 kPa O₂ was found to induce rapid and reversible membrane depolarizations in nodules of each species. This depolarization (which, to our knowledge, is unique to nodules) is accompanied by reversible decreases in P_O in G. max nodules. An osmoelectrical mechanism for control of nodule gas permeability, consistent with these data, is presented.