Release of Sucrose from Vicia faba L. Leaf Discs

The release of sucrose from leaf discs of Vicia faba L. to a bathing medium was studied for evidence of a relationship between this release and mesophyll export of photosynthate in vivo. Sucrose was released specifically over hexoses and represented over 85% of total photosynthate released. The sucrose appeared to be derived from the mesophyll tissue directly and release did not require concurrent photosynthesis. The data indicated two separate channels for sucrose release. The first was sensitive to inhibition by 1 millimolar p-chloromercuribenzenesulfonic acid and the second was promoted by lowering the Ca²⁺ concentration below 0.1 millimolar. Flow through both channels was about equal when tissue that had been actively photosynthesizing for several hours was used. The rate of release was not dependent on the extracellular pH, but was inhibited by 10 micromolar carbonylcyanide p-trifluromethoxyphenylhydrazone. Lowering the Ca²⁺ concentration below 0.1 millimolar or raising the K⁺ concentration above 100 millimolar stimulated sucrose release. The stimulation by high K⁺ was not reversed by adding Ca²⁺. The data supported the postulate that Ca²⁺ removal or K⁺ addition changed the permeability of the mesophyll plasma membrane to sucrose.