Synthesis of Sorbitol in Apricot Leaves

Abstract

When 14CO2 was supplied as a 30-s pulse to photosynthesizing apricot leaves, the pattern of the various phosphate esters, organic acids and amino acids was consistent with the operation of the normal C3 photosynthetic pathway. Fructose 6-phosphate became labelled much more rapidly than glucose 6-phosphate and fructose was more rapidly labelled than glucose. Sorbitol was the major end-product of photosynthesis, becoming the predominant 14C-containing compound within 10 min of starting the 14CO2 pulse, and containing more than half the total radioactivity after 30 min. Sorbitol may be synthesized by way of fructose or fructose 6-phosphate rather than glucose or glucose 6-phosphate. Sorbitol phosphate was shown to be present in the tissue as 2% of the total hexose monophosphate, and to become labelled as rapidly as the hexose monophosphates. The pathway of sorbitol synthesis may therefore follow the sequence: fructose 6-phosphate → sorbitol phosphate → sorbitol.