BLUEBERRY COLD HARDINESS
Blueberry and the Need for Cold Hardiness-Related Research
The United States is the largest producer of blueberries. However, low temperature extremes reduce crop yields and impact the profitability and competitiveness of U.S. producers. Specifically, lack of sufficient winter hardiness and susceptibility to spring frosts have been identified as two of the most important genetic limitations of current highbush blueberry cultivars. Enhanced cold tolerance during the winter and early spring of elite varieties would be of great value to the blueberry industry.
Growth Cycle of Woody Perennials: Relationship to Dormancy and Cold Acclimation
Woody perennial plants of the temperate zone are exposed to freezing temperatures each winter. Their ability to survive is dependent on an evolved mechanism by which plants annually enter a state of dormancy and develop cold hardiness or freezing tolerance. Freezing tolerance is defined as the ability of a plant to survive extracellular ice whereas cold acclimation is defined as the capacity of a plant to increase its freezing tolerance, or survive lower freezing temperatures, upon exposure to short photoperiods and low temperature. The annual development of dormancy and cold acclimation in the fall occur simultaneously, as do the release from dormancy and deacclimation (loss of cold hardiness) in the spring.
Terminal buds of most temperate woody perennials are formed in the summer. Following terminal bud formation, there is generally a period of time in which axillary buds can be forced to grow by removal of the terminal bud, which exerts apical dominance. This type of bud dormancy which is controlled by something outside the bud itself, in this case terminal buds, is called paradormancy. By mid to late autumn, as daylength and average daily temperatures decrease, inhibition of bud growth comes to be maintained within the bud itself; thus, buds are referred to as endodormant. During this transition from paradormancy to endodormancy, woody plants also begin to develop freezing tolerance (first stage of cold acclimation). Endodormancy is characterized by a chilling requirement, i.e. exposure to an accumulated number of hours of low temperatures in order for budbreak to occur. This chilling requirement prevents growth from occuring during transitory periods of warm temperatures throughout a large portion of the winter, thus, assisting to synchronize a plant's growth with exposure to favorable environmental conditons. While buds are fully endodormant, there is a further increase in cold hardiness to reach maximum hardiness (second stage of cold acclimation). The fulfillment of the chilling requirement marks the end of endodormancy. Many plants have their chilling requirements satisfied by mid to late winter. However, they cannot resume growth until exposed to warm temperatures, conducive to growth. This type of dormancy, which is imposed by environmental factors, is called ecodormancy. These changes culminate, upon the return of warmer temperatures, in resumption of growth and fully deacclimated plants. Thus, the various stages of dormancy form a continuum in the plant's annual growth cycle.
Genetics and Molecular Genetics of Cold Hardiness
Genetic evidence from numerous plants, including woody perennials, indicates that cold hardiness is a quantitative trait. From a generation means analysis, we have established that inheritance of cold hardiness in blueberry can be explained by a simple additive/dominance model of gene action. Considerable molecular evidence indicates that development of cold hardiness or cold acclimation is a complex phenomenon involving changes in gene expression that result in the alteration in metabolism and composition of lipids, proteins, and carbohydrates. Genes induced during cold stress encode several different classes of gene products, enzymes required for the biosynthesis of osmoprotectants; lipid desaturases for maintaining membrane fluidity; protective proteins such as antifreeze proteins, dehydrins, chaperones, and mRNA-binding proteins; proteins involved in protein turnover including ubiquitin, ubiquitin associated proteins, and other proteases; detoxification proteins; and proteins involved in signal transduction such as transcription factors, proteins kinases and phospholipase C.