Physiological Study of the Components of Winter-Hardiness in Rhododendron: Cold Acclimation, Deacclimation Kinetics, and Reacclimation Ability

Freezing is a major environmental stress during an annual cycle of temperate zone woody perennials. The timing and extent of seasonal cold acclimation and deacclimation are of critical importance for winter survival, particularly in view of the climate change, i.e., unpredictable extreme weather occurrences. For example, plants may acclimate inadequately if exposed to milder autumn, and may be damaged by sudden frosts. Alternatively, they may deacclimate prematurely due to unseasonable winter-warming, and be injured by the cold backlash. Efficient cold acclimation ability, high deacclimation resistance, and efficient reacclimation capacity are, therefore, important components of the winter survival in overwintering perennials. Endo-or ecodormant status (shallow versus deep) of the reproductive/vegetative apices can significantly impact these components of winter-hardiness, and presents another layer of complexity to this multi-component process. These components should be evaluated separately for a successful breeding program focused on improving winter hardiness. Also, understanding the physiology and underlying cellular mechanisms of cold acclimation, deacclimation and reacclimation is important to ensure winter survival and the sustainability of plant sources under changing climate. Some of our work with Rhododendron (broad-leaved evergreens and deciduous azaleas) is discussed to highlight these aspects of winter-hardiness.