Year of Publication


Document Type





Plant Physiology

First Advisor

Douglas D. Archbold


Sorbitol, the primary photosynthate and translocated carbohydrate in apple (Malus x domestica Borkh.), is converted to fructose by SORBITOL DEHYDROGENASE (SDH; EC which is active in apple fruit throughout fruit development. Apple fruit set and early development is very sensitive to carbohydrate availability, but details on carbohydrate metabolism during this phase are limited. The first objective of this work was to determine if SORBITOL DEHYDROGENASE, the primary enzyme responsible for metabolism of the major phloem-transported carbohydrate sorbitol, is present and active during apple fruit set and early development. The second objective of this work was to determine if SDH genes are differentially expressed and how their patterns of expression may relate to SDH activity in apple seed and cortex during early fruit development. Nine different genes encoding SDH were determined from analysis of a cDNA library and genomic-clones. Northern, Western and ELISA analyses showed that SDH transcripts and SDH protein were present in the fruit during the first 5 weeks after bloom and comprised 7 to 8 % of the total extractable protein. Whole fruit SDH activity was highest at 2 to 3 weeks after bloom in each of three cultivars, Lodi, Redchief Delicious and Fuji. Seed SDH activity was found to be much higher than cortex SDH activity per mg and g FW, and seed SDH activity contributed significantly to whole fruit SDH activity during the first five weeks of development after bloom. Five of the nine SDH genes present in apple genome were expressed in apple fruit (SDH1, SDH2, SDH3, SDH6, SDH9). Expression of SDH6 and SDH9 was seed-specific and expression of SDH2 was cortex-specific. Using 2D SDS-PAGE and Western analyses, SDH isomers with pI values 4.2, 4.8, 5.5 and 6.3 were found in seeds, and SDH isomers with pI values 5.5, 6.3, 7.3 and 8.3 were found in cortex. The present work is the first to show that SDH is differentially expressed and highly active in seed and cortex during early development. Thus, SDH during apple fruit set and early development may play a primary role in defining fruit sink activity.