Cytokinin and Soluble Carbohydrate Concentrations in Xylem Sap of Apple during Dormancy and Budbreak

Abstract

Abstract Xylem sap from three apple ( Malus domestica Borkh.) rootstocks was vacuum-extracted during and after artificial chilling and during forcing of chilled and unchilled trees. Sap was assayed for cytokinins by immunoassay and for soluble carbohydrates (sorbitol, glucose, fructose, and sucrose) by enzymatic assays. Xylem cytokinin increased after 10 days of forcing at 21/18C regardless of chilling treatment. Cytokinin levels decreased significantly as budbreak occurred in fully chilled trees, but not in unchilled trees with very little budbreak. Xylem sucrose, glucose, and fructose concentrations decreased upon exposure to 21/18C, then increased after 30 days in both chilled and unchilled trees. Sorbitol level remained low and unchanged throughout growth. Full chilling was not necessary in apple for adequate cytokinin and carbohydrates to be transported to the developing buds via the xylem stream. Sorbitol apparently is not used in carbohydrate transport from reserve, as it is in photosynthesis.