Sucrose and proton cotransport in Ricinus cotyledons : II. H(+) efflux and associated K(+) uptake.

In Ricinus cotyledons, evidence for proton extrusion came from observation of direct acidification of the medium in the presence of potassium salts. Increasing K(+) influx with increasing pH suggested a link between K(+) influx and H(+) efflux by an H(+) pump. The kinetics of K(+) influx and H(+) efflux were consistent with a 1:1 stoichiometry K(+):H(+), which may indicate either electrical coupling or carrier mediated exchange. The results were consistent with an H(+) pump setting up an electrochemical potential gradient which provides the driving force for an H(+)-sucrose cotransport and the movement of K(+). With reference to this, a model for phloem loading is suggested.