Co-transport of Potassium and Sugars across the Plasmalemma of Mesophyll Protoplasts.

Sugars (sucrose + hexoses) produced photosynthetically by isolated mesophyll protoplasts of wheat and tobacco were effluxed across the plasma membrane (3 to 10 micromoles hexose equivalents per milligram chlorophyll per hour). The efflux was sensitive to uncouplers and oligomycin which indicated a requirement for energy. A proton gradient was probably not coupled directly to the transport because changing the proton gradient across the plasma membrane by varying the pH of the medium or by adding sodium acetate had no significant effect on the rate of sugar release.A release of K(+) was associated with sugar efflux from the protoplasts. The molar ratio of K(+) to sugar varied between 1.5 and 2.5, depending on the species. Exogenous CKl, RbCl, and LiCl (50 millimolar each), but not NaCl or CsCl, significantly inhibited sugar efflux. Conditions that reduced sugar efflux (exogenous KCl, LiCl, mersalyl, or oligomycin) also reduced K(+) release and caused a time-dependent reduction in photosynthetic sucrose formation and increased amino acid and starch formation. Results obtained support the postulate that a K(+) symport is involved in the transport of sugar across the energized plasmalemma of photosynthetically active mesophyll cells.