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Shoot and Root Temperature Effects on Xylary Cytokinin Levels during Budbreak in Young Apple Trees


One-year-old apple trees (Malus domestica Borkh. cv. EMLA 106) were subjected to all combinations of 10° and 20°C root and shoot temperatures after their chilling requirement had been satisfied at 5°. Extracted xylem sap was tested for trans-zeatin riboside-like cytokinin activity using an immunoassay. Cytokinin activity in the xylem sap increased during the first 6 days of treatment regardless of temperature, but decreased only as budbreak occurred. This decline was in proportion to the amount of budbreak, which primarily was dependent on shoot temperature.