A model for proton and potassium co-transport during the uptake of glutamine and sucrose by tomato internode disks.

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(+).