Ion fluxes and pH changes induced by trans-plasmalemma electron transfer and fusicoccin inLemna gibba L. (strain G1)

Abstract

During the reduction of extracellular [Fe(CN)₆]³⁻ at the plasmalemma of intact, K⁺-starvedLemna gibba L. fronds, the external medium was acidified and K⁺ released, in the absence of inhibitors with rates of 10 e⁻/8.5 H⁺/1.5 K⁺ (μmol·(g FW)⁻¹·⁻¹). In K⁺ plants the larger K⁺ efflux caused a lag phase in extracellular acidification and a change in rates to 10 e⁻/6 H⁺/4 K⁺ and in the presence of CN⁻+salicylhydroxamic acid at pH 5 to 5.2 e⁻/0 H⁺/6.6 K⁺. The e⁻ transfer was accompanied by a membrane depolarization of up to 100 mV and a cytosolic acidification of about 0.6 pH units, but only in K⁺ plants, where the extracellular acidification was smaller. These results indicate that a stimulation of the plasmalemma H⁺-ATPase may be triggered either by a cytosolic acidification or by a strong membrane depolarization. It is concluded that the redox system catalyses only uncoupled e⁻ transfer without H⁺ transfer across the plasmalemma. The obligatory, but secondary charge compensation is partially achieved by the rapid K⁺ release upon membrane depolarization and partially by the activity of the plasma membrane H⁺-ATPase, but not by an e⁻/anion exchange. The extracellular acidification during [Fe(CN)₆]³⁻ reduction is generated by the conversion of a strong trivalent into a strong tetravalent anion. This acidification is caused by changes in the concentration ratio of strong cations to strong anions. Efflux of K⁺ and not the production of organic acids or NAD(P)H oxidation is the chemical cause of the measurable cytosolic acidification. Extracellular acidification was inversely correlated with intracellular acidification. Similarly, fusicoccin-induced pH changes were correlated with changes in the strong-ion concentration difference. Extracellular ± FC-dependent acidification and intracellular alkalinization of up to 0.6 pH units were strongly dependent on K⁺ fluxes. The ferricyanide-triggered trans-plasmalemma electron-transfer system is an example of how measurable pH changes are the consequence and not the cause of charge-transfer-induced changes in strong-ion fluxes.