ABSTRACT
Annually, woody perennial plants of the temperate zone must un dergo physiological changes in order to survive winter. As part of this annual growth cycle, by late summer to mid autumn plants enter a state of dormancy known as endodormancy and begin to develop cold hardi ness, referred to as the first stage of cold acclimation (Nissila and Fuchigami, 1978; Sakai and Larcher, 1987). Here, the prefix “endo” re fers to the inhibition of bud growth which is maintained within the bud itself, as distinguished from paradormancy which is controlled by a fac tor outside the bud (for example, apical dominance), and ecodormancy, which is imposed by environmental factors (Lang, 1987; Lang et al., 1987). Endodormancy and this first stage of cold acclimation are thought to be triggered or enhanced by short photoperiods (Downs and Borthwick, 1957; Nitsch, 1957; Perry, 1971; Vince-Prue, 1975; Nooden and Weber, 1978). Endodormancy is characterized by a chilling requirement, i.e., exposure to an accumulated number of hours of low, nonfreezing tem peratures in order for budbreak to occur the following spring (Lang et al., 1987). Thus, the chilling requirement serves to synchronize a plant’s growth upon exposure to favorable environmental conditions. During winter months, while buds are fully endodormant and then ecodormant, there is a further increase in cold hardiness to reach maximum hardi ness. This is the second stage of cold acclimation (Fuchigami et al., 1982). Some plants that would be killed by temperatures slightly below 0°C during the summer and early fall may survive temperatures as low as — 196°C during the winter when fully cold acclimated (Sutinen et al., 1992). These physiological changes culminate, upon the return of warmer temperatures, in resumption of growth and fully deacclimated plants.
