Phytochrome control of growth cessation and initiation of cold acclimation in selected woody plants.

Short day enhancement of cold acclimation in twigs of Cornus (red-osier dogwood), Weigela, and Pyracantha (firethorn) was studied using dark interruptions with red or with red-far red radiation. Hardiness was estimated by freezing stem tissues to preselected temperatures and evaluating injury electrolytically. Dark-period interruptions with red radiation suppressed cold acclimation in Cornus and Weigela. When red light was followed by far red light, suppression was relieved. No radiation control of acclimation was found with Pyracantha. The short day enhancement of cold acclimation in Cornus and Weigela appears to be phytochrome-mediated.