Cytokinins in Populus×robusta: Changes during Chilling and Bud Burst

Cytokinin levels in sap and vegetative buds of Populus × robusta Schneid have been determined during chilling and bud burst. From non‐detectable levels in December and January, parallel increases in cytokinin levels occur in sap and buds during February and March, both in material from the field and that held at 2°C in the dark. The maximum in the sap occurs two weeks prior to natural bud burst, and 3 weeks, prior to the maximum attained in the buds. Excised twigs, forced to bud burst, show a similar pattern. The role of roots as a possible source of cytokinins is discussed. Partition chromatography on Sephadex LH‐20 indicates that at least 5 cytokinins are present in buds, two of which have similar elution volumes to zeatin and zeatin riboside‐The main activity in the sap is confined to a zeatin riboside‐like component.