Sucrose and proton cotransport in Ricinus cotyledons

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Abstract
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.