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 o and 20 o C root and shoot temperatures after their chilling requirement had been satisfied at 5 o . 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