Relationship of Plasma Membrane Redox Activity to Proton Transport in Roots of Iron-Deficient Cucumber
Under iron deficiency, cucumber (Cucumis sativus L. cv. Pickler) plants respond with a great increase in the capacity to reduce ferric chelates at their root surface and with an acidification of the root medium. The transmembrane e− transfer to extracellular e− acceptors is associated with a H+ efflux which should be differentiated from the one induced by iron deficiency without the need of an extracellular electron acceptor. In the last years diverse plasma membrane redox systems have been studied. In some cases the ratio e−/H+ efflux during the reduction process has been established. For instance, le−/1H+ in isolated mesophyll cells of Asparagus sprengeri Regel (1) or 2e−/lH+ in roots of iron deficient Phaseolus vulgaris L. (3). However, Rubinstein and Stern (2) have shown in roots of Zea mays L. a redox system which involves only the transport of e− across the plasmalemma. In this case H+ efflux is an indirect effect of the depolarization of the membrane potential, causing the activation of the membrane H+-ATPase.