Sorbitol Availability and SORBITOL DEHYDROGENASE Expression during Apple Bud Development from Budbreak to Bloom

Sorbitol is the primary photosynthate and translocated carbohydrate in apple (Malus domestica Borkh.) and is converted to fructose by SORBITOL DEHYDOGENASE (SDH) in sink tissues. The objectives of this work were to determine 1) the patterns of sorbitol availability and SDH activity and 2) if tissue-specific SDH expression occurs in flower buds from budbreak to bloom. Sorbitol was the main soluble carbohydrate in expressed xylem sap from the fully dormant stage until bloom, and it increased during this period. Bud SDH activity per g FW increased significantly from dormant bud to bloom. Four SDH genes were consistently expressed in flower buds during dormant and all postdormant developmental stages, while one SDH gene was expressed at full bloom only. The SDH genes exhibited tissue-specific expression at full bloom with three expressed in all floral tissues. This work indicated that SDH expression is developmentally regulated to some extent within emerging apple flower buds with an abundance of sorbitol available as substrate.