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Proton Fluxes Associated with Sugar Uptake in Vicia faba Leaf Tissues ¹

Station Biologique de Beau-Site, 25 Faubourg St-Cyprien, 86000 Poitiers, France

Vicia 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 millimolar sucrose causes a transient (20 to 40 minutes) alkalinization (0.05 to 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 (Delrot, Despeghel, Bonnemain 1980 Planta 149: 144-148). Addition of 20 millimolar glucose, fructose, or 3-O-methylglucose induces weaker alkalinization than sucrose. Sequential additions of sugars show that: (a) sucrose- and hexose-induced proton fluxes are nearly saturated at 20 millimolar sugar (b) there is no competition between sucrose and hexoses for inducing proton influxes whereas (c) 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 minutes 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 millimolar sucrose (about 1.9 sucrose per proton taken up) but increases up to 6 sucrose per proton, between 5 and 15 millimolar sucrose. This confirms our previous study indicating that above 5 millimolar 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, thus suggesting that sucrose and K⁺ uptake can compete in dissipating the protonmotive force.

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