Internode disks of tomato (Lycopersicon esculentum cv. Moneymaker) were shaken in glutamine and sucrose solutions. At low external pH (<±5.5), the uptake of these substances was accompanied with K(+) efflux, at high pH (>±5.5) with K(+) influx. Low concentrations of external K(+) (2 mmol l(-1)) stimulated the uptake of glutamine, which was strongly inhibited by the supply of high K(+) concentrations (20 mmol l(-1)). The effect of K(+) was particularly pronounced at high pH-values. Addition of CCCP in light reduced the uptake of glutamine to the same level as in the dark, and stopped the K(+) fluxes which coincided with the uptake. A model is presented wherein the movements of K(+) across the membrane are related to co-transport, depending on the membrane potential and the Nernst potential of K(+).

SUMMARYPerfusion experiments with excised internodes of tomato (Lycopersicon esculentum cv Moneymaker) showed that the uptake of glutamic acid and sucrose from the xylem vessels is accompanied with coupled proton cotransport and potassium antiport at low pH (<~5.5). At high pH (>5.5) both proton and potassium co-transport accompany the uptake. The results fit into the proton pump concept. INTRODUC~ONRecently we presented a model wherein protons and potassium ions (the latter only at high pH) are co-transported with carrier-substrate complexes across the membranes of tomato internode cells [1]. It may be summarised as follows:.At low pH (~5.5), protons are pumped out against an electrochemical gradient energised by the conversion of ATP into ADP by membrane-bound ATPase. The high proton-motive force (A pH + AE is high) drives protons, coupled to the carrier-substrate complex, through the membrane into the cells. After release of protons and substrate molecules, the carriers move back to the outer part of the membrane. The K ÷ ions leave the cells passively in order to reach their Nemst potential.At high pH (~5.5), the K ÷ ions tend to enter the cells passively, as the membrane potential is more negative than the Nernst potential of potassium ions. The proton-motive force which is smaller than at low pH (as A pH is smaller) drives the protons together with the carrier-substrate complex, through the membrane. The K ÷ ions compete with the protons for the binding sites of the carrier-substrate complex.Proton and potassium fluxes can be read off from changes in proton and