Carbohydrate availability modifies sorbitol dehydrogenase activity of apple fruit

Complete defoliation of the stem or spur and girdling the phloem (D/G) subtending a single apple (Malus domestica Borkh.) fruit at 70 or more days after bloom resulted in reduction of fruit growth. Extractable sorbitol dehydrogenase (SDH) (enzyme code, 1.1.1.14) activity and sorbitol and starch content of the cortex tissue of fruit receiving D/G treatment subsequently declined. Co-extraction of control and D/G cortex tissues yielded expected levels of SDH activity, indicating that the loss of extractable SDH activity in D/G fruit was not due to presence of an inhibitor or proteolytic activity. Incubating cortex sections from D/G fruit in a buffered 200 mM sorbitol or glucose solution increased extractable SDH activity, and incubating cortex sections from control fruit in the sorbitol solution maintained the activity. However, neither 200 mM fructose or 27 mM PEG, the latter with the same osmotic potential as the sorbitol solution, affected extractable SDH activity of D/G fruit. The results indicate that carbohydrate availability may affect extractable SDH activity of apple fruit, and that specific carbohydrates such as sorbitol and glucose may be signals for modulating this activity.