Accumulation of Sorbitol and Glucose by Leaf Slices of Rosaceae

Abstract

Pear (Pyrus domestica Medik.) leaf slices, which form sorbitol photosynthetically, took up sorbitol from solution by an active transport mechanism. Uptake was linear with time, was sensitive to metabolic inhibitors and cold, occurred against a concentration gradient, and was not reversed by washing the tissue. Glucose uptake was similar to sorbitol uptake, but showed consistent differences in response to tissue aging, sugar concentration, light, and to competing sugars and sugar analogues. There was little competition between sorbitol and glucose, suggesting that the two may be taken up by separate mechanisms. The relationship of uptake to concentration could be interpreted as biphasic with constants resembling those for sugar uptake in other tissues: for sorbitol, (a) Km 3 × 10-3M and Vmax 1000 nmol per g fresh wt per h, (b) Km 1 × 10-1 M, Vmax 19 500; and for glucose, (a) Km 1 × 10-3 M and Vmax 600, (b) Km 0.6 × 10-1 M and Vmax 10 500. The ability of various species to accumulate sorbitol was compared. Species which normally contain sorbitol accumulated it as readily as glucose, while other species accumulated it much less rapidly. In pear leaf tissue, sorbitol was poorly metabolized while glucose was largely converted to sorbitol. In rose (Rosa sp., cultivated) leaf tissue (which does not contain sorbitol), sorbitol and glucose were both extensively metabolized to sucrose and other compounds. In pear, sorbitol may be segregated into a non-metabolic pool, thus serving as a specific storage compound in place of sucrose as in other plants.