Energy-dependent Solute Uptake Into the Symplast of Leaves: ATP/KCl, ATP/Sucrose, ATP/D-serine and H⁺/ATP Stoichiometries of Transmembrane Transport

Abstract

KCl, sucrose, D-serine and some other solutes were fed through the petiole to leaflets of Solanum tuberosum and uptake into the symplast was monitored. Solute transport was accompanied by changes in membrane potential, apoplastic pH and respiration. After termination of solute feeding, membrane potential, apoplastic pH and respiration returned to initial steady state values. From transpiration, solute uptake was calculated and compared to ATP production during stimulated respiration, assuming an ATP/CO₂ ratio of 5. On this basis, calculated ATP/KCl ratios of energized transport approached 0.5. Similar ATP/solute ratios were observed with sucrose, mannitol, methylglucose and D-serine. With glucose, many ratios were somewhat above 0.5, possibly because of some metabolization of imported glucose. We conclude that solute uptake is energized by the proton motive force across the plasma membrane. Low ATP/substrate ratios suggest that the H⁺/ATP ratio of proton export by the plasma membrane ATPase is not 1 as presently assumed but 2 in potato leaves, and that the contribution of the alternative cyanide-resistant oxidase to leaf respiration is small, if not negligible, in the dark.

Key words

ATPase - energized transport - membrane potential - pH (apoplast) - plasma membrane - proton motive force

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Section Editor: U. Lüttge