Phloem loading in Vicia faba leaves: Effect of N-ethylmaleimide and parachloromercuribenzenesulfonic acid on H+ extrusion, K+ and sucrose uptake

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Abstract
The effects of a penetrating (NEM) and a non-penetrating (PCMBS) sulfhydryl-specific reagent on proton extrusion, 86Rb and [U-14C]sucrose uptake by Vicia faba leaves have been studied. Proton extrusion was strongly or completely inhibited by 0.1 mM NEM. 86Rb and [U-14C]sucrose uptake were markedly reduced by NEM concentrations equal to or higher than 0.5 mM. Under our experimental conditions, PCMBS (1 mM) exerted a strong inhibition on [14C]sucrose uptake but did not inhibit proton extrusion and 86Rb uptake. The sensitivity of phloem loading to PCMBS is thought to be a consequence of sugar-carrier blockage and not of inhibition of the proton pump.