Proton Fluxes Associated with Sugar Uptake in Vicia faba Leaf Tissues

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Abstract
V. faba leaf fragments bring the pH of their incubation medium to about 4.7, whatever the initial pH value. At this pH, addition of 20 mM sucrose causes a transient (20-40 min) alkalinization (0.05-0.10 pH unit) of the medium. The alkalinization is not observed in the presence of p-chloromercuribenzenesulfonic acid which blocks the sucrose carrier involved in phloem loading without affecting the ATPase. Addition of 20 mM glucose, fructose, or 3-O-methylglucose induces weaker alkalinization than sucrose. Sequential additions of sugars show that sucrose- and hexose-induced proton fluxes are nearly saturated at 20 mM sugar; there is no competition between sucrose and hexoses for inducing proton influxes whereas glucose and 3-O-methylglucose are competing for a common system. Autoradiographs performed under the conditions used for the observation of proton fluxes show a slight accumulation of [14C]sucrose into the veins within 2 min of uptake, whereas [14C]glucose and 3-O-methyl [14C]glucose are localized in the mesophyll. These data support the proton-sucrose cotransport hypothesis of phloem loading and show that mesophyll cells are able to take up hexoses by symport with protons. The apparent sucrose/proton stoichiometry is constant below 5 mM sucrose (about 1.9 sucrose per proton taken up) but increases up to 6 sucrose per proton, between 5 and 15 mM sucrose. This confirms a previous study indicating that above 5 mM sucrose, a system which exhibits little pH dependence is involved in the uptake. Simultaneous measurements of H+ and K+ fluxes indicate that sucrose uptake is accompanied by a reduction of K+ uptake rate, suggesting that sucrose and K+ uptake can compete in dissipating the protonmotive force.