Coincident with the beginning and duration of dormancy in many plant organs and tissues is an ability to withstand increasingly harsher environmental conditions. For instance, with gradual temperature increases, a bud beginning high-temperature-induced ecodormancy has an increased ability to withstand higher temperatures without damage to the meristem. This process of increased environmental tolerance with exposure is called acclimation. Typically, acclimation is a relatively slow process—it may take days to weeks of slowly changing environmental conditions for a tissue or organ to acclimate to an environmental extreme. Rapid changes can still cause damage.
The loss of acclimation is called deacclimation. As conditions revert to those that are optimum for growth, tissues may deacclimate, or lose their ability to withstand environmental extremes. Similar to acclimation, deacclimation is a relatively slow process, which prevents the plant from losing its ability to withstand harsh conditions if there is a brief interlude of favorable conditions.
During endodormancy, cold temperature acclimation is very important. Tissues and organs acclimating to cold temperatures are said to increase in hardiness—the ability to withstand very cold temperatures. Generally, as endodormancy begins and plants are exposed to shortening days and decreasing temperatures, the tissues acclimate to the cold temperatures and gain in hardiness. Within a given species, the rate of acclimation may relate to the rate of temperature decline. For some woody fruit trees, studies demonstrate that plants are most hardy as they near completion of the endodormancy requirement. As a result of this phenomenon, plants may be somewhat vulnerable to late-autumn, early winter freezes and less susceptible to midwinter freezes. Once endodormancy is complete, plants may deacclimate and lose hardiness with exposure to warmer temperatures and lengthening photoperiod. As the tissues are in postendodormancy eco-dormancy, they retain some ability to regain hardiness within some reasonable temperature span. As growth begins after ecodormancy, hardiness fades and the tissues become very sensitive to freezing temperatures.
Knowledge of the causes, forms, and physiology of fruit tree dormancy and acclimation has improved in recent years. New research in the area of molecular biology has the potential to further increase understanding of these complex processes. However, a complete and simple story based upon molecular evidence has not yet unfolded.
Related Topics: BREEDING AND MOLECULAR GENETICS; GEOGRAPHIC CONSIDERATIONS; PLANT HORMONES; TEMPERATURE RELATIONS
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