Fruit set and cytokinin-like activity in the xylem sap of sweet cherry (Prunus avium) as affected by rootstock

Using a trace-enrichment procedure involving solvent partitioning and high pressure liquid chromatography, followed by bioassay, two cytokinin-like substances believed to be zeatin and zeation riboside were detected in the xylem sap of sweet cherry ( Prunus avium L.). The levels of cytokinin varied between rootstocks with the higher yielding and fruit setting rootstocks producing the highest cytokinin levels. Most of the cytokinin activity occurred in the spring during bloom and early fruit set. A relationship between cytokinin level in the xylem sap and fruit set is proposed.