Variations in kinetin-like activity in buds of <i>Betula</i> and <i>Populus</i> during release from dormancy

Abstract

Levels of kinetin-like activity in dormant buds and buds at various stages of release from dormancy were determined by an assay involving retardation by kinetin-like substances of chlorophyll degradation in tissue of senescing leaves of Raphanus test plants. Dormancy of buds of Betula papyrifera and Populus balsamifera cuttings was broken by exposure to 2-chloroethanol vapor or immersion in warm water. Water extracts of buds at various stages of dormancy were separated by ascending chromatography and active R f values identified. The amount of kinetin-like activity at various stages of bud dormancy was then determined by observing effects of active R f values of bud extracts on senescence (chlorophyll retention) of Raphanus plants. Kinetin-like activity was absent in dormant buds of both species and was present in various amounts after dormancy was broken. In Betula, such activity was found primarily at R f values of 0.3 to 0.5, and secondarily at 1.0. In Populus, kinetin-like activity was found at R f values 0.5 to 0.6 and 0.9 to 1.0. Kinetin-like activity was greater in non-dormant buds of Populus than in those of Betula. However, the trend of variation in activity with time after breaking of dormancy was similar in both species. After bud dormancy was broken, kinetin-like activity in both species increased progressively until shortly before buds opened, and it decreased thereafter. Evidence is cited that release of buds from dormancy involves a change in the balance of growth regulators, with various growth promoters increasing to overcome the effects of inhibitors. Endogenous cytokinins appear to be involved, together with other growth promoters, in regulating release of buds from dormancy.