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.