Overwintering

Winter survival of woody perennials, including temperate tree fruit crops, is dependent on two phenological events: (1) the onset of dormancy during fall and (2) an ability to increase freeze tolerance upon exposure to low nonfreezing temperatures (e.g., a change from a freeze-susceptible to a freeze-resistant state—a process called cold acclimation). Once plants are in a dormant state, an exposure to a chilling period is required for floral and vegetative budbreak in the following spring. Chilling requirement prevents growth from occurring during periodic warm spells during winter and thus helps synchronize plant growth with the prevalence of favorable environmental conditions. Cold acclimation (CA), on the other hand, enables plants to survive the subfreezing temperatures present during winter. Due to the process of cold acclimation, woody plant tissues that would be killed by temperatures slightly below 0°C during summer and early fall may survive temperatures as low as -70°C during winter. The extent to which a particular species can acclimate is largely genetically determined. In the spring during an annual cycle, cold-acclimated plants begin to lose their acquired hardiness (deacclimation) while they also come out of the dormant state. Hence, transitions (onset and loss) in dormancy and cold hardiness partially overlap.

In nature, cold acclimation in woody plants is typically a two-stage process. The first stage (initial increase in freezing tolerance) is induced by short days, whereas the second stage is induced, primarily, by low temperatures. Therefore, full cold acclimation potential (maximum midwinter freezing tolerance) is normally achieved as an additive response of tree fruit crops to both environmental cues. Dormancy or rest in many woody perennials is also induced or enhanced by short photoperiods in the fall. A deviation from the aforementioned environmental regulation of cold acclimation and/or dormancy in woody plants, albeit possible, is considered an atypical response, and those genotypes which exhibit such differences often serve as valuable experimental systems to gain fundamental knowledge of the environmental physiology of these two processes.

The seasonal shifts in dormancy and cold hardiness status during the annual cycle of tree fruit crops imply that these are active processes of adaptation. Research indicates that the two processes are genetically regulated and lead to changes in metabolism and cell composition. Among the transformations that occur during overwintering of woody plants are distinct shifts in gene activity, changes in carbohydrate metabolism that lead to accumulation of specific types of sugars, changes in the composition of cell membranes, accumulation of abscisic acid (ABA, a plant growth hormone), and accumulation of unique classes of proteins (Chen, Burke, and Gusta, 1995). However, the simultaneous occurrences of dormancy and cold hardiness transitions make it difficult to associate physiological and molecular changes that specifically control one or the other phen-ological event. To overcome this problem, researchers have devised several physiological and/or genetic approaches and strategies (Wis-niewski and Arora, 2000) that are briefly described here.

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