Changes in Xylem Constituents in Response to Rest-breaking Agents Applied to Apple before Budbreak

Xylem sap was vacuum-extracted weekly from 1-year-old apple shoots from trees treated with dinitro-o-cresol (DNOC) oil, hydrogen cyanamide, or untreated controls. Sampling began 1 week before treatment and continued until 2 weeks after budbreak had occurred in the control trees. Sorbitol, calcium, magnesium, potassium, and zeatin-type cytokinin concentrations were determined by enzymatic, atomic absorption, and immunoassay methods, respectively. The rest-breaking treatments resulted in earlier and more intense budbreak. Xylem sap cytokinin concentrations increased rapidly in response to the rest-breaking chemicals and peaked just before or at budbreak. The rapid increase in cytokinin was closely followed by increases in calcium and magnesium concentrations in the sap. Potassium concentration appeared to be unaffected by rest-breaking treatment. Sorbitol levels dropped rapidly as a result of the rest-breaking treatments and appeared to be used rapidly in budbreak and early bud growth.