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Substantial evidence indicates that a plasma membrane located, ATP requiring outwardly directed proton pump establishes a proton electrochemical gradient that is used to drive the transport of solutes into and out of plant cells ( 17,23,24). However, a proton electrochemical gradient across biological membranes may also be established by redox reactions involving the sequential operation of hydrogen and electron carriers (7,14 (12,21). A third model has recently been proposed in which plasma membrane redox activity transports only electrons to an exogenous acceptor. The resulting depolarization then activates the proton ATPase and accounts for the increase in H4 efflux observed on addition of Fe(CN)631 (18).Very little is known concerning the existence of plasma membrane redox activities in photosynthetic cells. The reduction of exogenous Fe(CN)63-by Elodea leaf cells and an associated decline in membrane potentials has been reported (1 1). Active ATP driven proton efflux and related transport processes are currently being investigated using isolated Asparagus mesophyll cells (3,5,20

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