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

Abstract

Sugars (sucrose + hexoses) produced photosynthetically by isolated mesophyll protoplasts of wheat (triticum aestivum) and tobacco (Nicotiana tabacum) were effluxed across the plasma membrane (3-10 .mu.mol 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 KCl, RbCl and LiCl (50 mm 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.