PLASMA-MEMBRANE REDOX SYSTEM IN GUARD-CELL PROTOPLASTS OF PEA (PISUM-SATIVUM L)

Guard cell protoplasts (GCP) from leaves of pea (Pisum sativum) were capable of reducing/oxidizing the membrane impermeable electron carriers, ferricyanide/NADH. The redox activity of GCP required the presence of both ferricyanide and NADH, although some ferricyanide reduction occurred even in the absence of NADH. The GCP preferred NADH to NADPH during ferricyanide reduction and the reduction was slow with DCPIP or cytochrome c. A stoichiometry of about 2 existed between moles of ferricyanide reduced and NADH oxidized by GCP. The redox activities of GCP were several times greater than those of mesophyll protoplasts from pea leaves. The ferricyanide reduction or NADH oxidation by GCP was unaffected by abscisic acid or sodium orthovanadate and fusicoccin indicating the non-involvement of plasma membrane ATPase in these redox reactions. The redox activities were markedly inhibited by chloroquine or 8-hydroxyquinoline. The findings are discussed in relation to the possible regulatory role of a guard cell plasma membrane redox system in stomatal function.