Release of Sucrose from Vicia faba L. Leaf Discs

Abstract

The release of sucrose from leaf discs of V. 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 > 85% of the total photosynthate released. The sucrose appeared to be derived from the mesophyll tissue directly and release did not require concurrent photosynthesis. Two separate channels for sucrose release were indicated. The 1st was sensitive to inhibition by 1 mM p-chloromercuribenzenesulfonic acid and the 2nd was promoted by lowering the Ca2+ concentration below 0.1 mM. 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 .mu.M carbonylcyanide p-trifluromethoxyphenylhydrazone. Lowering the Ca2+ concentration below 0.1 mM or raising the K+ concentration above 100 mM stimulated sucrose release. The stimulation by high K+ was not reversed by adding Ca2+. Apparently, Ca2+ removal or K+ addition changed the permeability of the mesophyll plasma membrane to sucrose.