FREEZING EXPOSURE RELEASES BUD DORMANCY IN BETULA-PUBESCENS AND B-PENDULA

Bud dormancy in woody plants is released by long-term exposure to non- freezing chilling temperatures, whereas freezing temperatures have bee n considered to have little or no effect. However, the present results demonstrate that short-term exposure to freezing can release bud dorm ancy in Betula pubescens (Ehrh.) and B. pendula (Roth). Short-term fre ezing during the dormancy induction phase improved the release of bud dormancy only if an adequate level of dormancy had been reached. In fu lly dormant or chilled plants both the percentage and the speed of bud -burst increased, the more so the lower the temperature. Our results r ule out the possibility that endogenous abscisic acid could be directl y involved in the physiological control of bud dormancy release. The f ast, easily applicable method presented here for bud dormancy release could further investigations into the biochemical and biophysical back ground to the process. The mechanisms of bud dormancy release and its relationship to cold acclimation are discussed in the light of these r esults, as also are the implications of the findings for modelling of bud dormancy.