Sucrose and proton cotransport in Ricinus cotyledons : I. H(+) influx associated with sucrose uptake.

Ricinus cotyledons were used to investigate whether sucrose uptake by the phloem could be due to H(+)-sucrose cotransport. The addition of sucrose to the medium bathing Ricinus cotyledons created a pH shift to more alkaline values. The effect was not observed on the addition of glucose or fructose. Net H(+) influx and sucrose influx were both shown to be temperature sensitive, dependent on pH and external sucrose concentration. However, net H(+) influx was far more sensitive to pH than sucrose influx and was abolished at high pH; it was also observed for a shorter time period than sucrose uptake. The stoichiometry of H(+): sucrose was less than one and declined with increasing sucrose concentration and increasing pH. Two possible models are proposed to account for this low and changing stoichiometry. Given and H(+) pump, recirculation would seem an inevitable consequence of sucrose and proton cotransport.