Patterns of Sorbitol Metabolism and Availability during Apple Fruit Set

Abstract

Acquiring sufficient carbohydrate is essential for successful apple fruit set. Sorbitol may be the dominant carbohydrate imported by growing fruit, and the rate of sorbitol accumulation may be a function of NAD-dependent sorbitol dehydrogenase (SDH; EC 1.1.1.14) activity. Prior work indicated that SDH activity from whole fruit (seeds plus cortex) increased for 2 or 3 weeks after initiation of fruit growth and then declined through 5 weeks. Using SDH activity assays, an SDH-specific antibody, and SDH-specific probes in Northern analyses, it is evident that SDH is expressed and is active in both apple seed and cortex tissue during the first few weeks of fruit growth. On a per unit protein basis, SDH activity in seeds increased by the pattern described above while that in fruit was generally lower and constant. During this same period of time, the sorbitol content of the expressed sap of apple shoots was analyzed. The sorbitol concentration was 50- to 100-fold higher than the sucrose concentration. The concentrations of both carbohydrates changed in parallel to the change in SDH activity of whole fruit and seeds. The lowest SDH activity and sap sorbitol levels preceded and/or coincided with the beginning of the natural fruit drop (or June drop) period.