In preparation for winter, woody perennial plants of the temperate zone have evolved mechanisms to first enter a state of dormancy in late summer or early fall and then, as fall progresses, to develop cold hardiness. The development of cold hardiness, termed cold acclimation, is triggered by environmental stimuli of shortening daylength and declining temperatures (Powell, 1987; Sakai and Larcher, 1987; Weiser, 1970). A plant's ability to survive low temperatures is dependent on several complex interacting factors. They include the timing of dormancy induction in the summer, the timing and rate of cold acclimation in the fall, the level of freezing tolerance reached while plants are in the cold-acclimated state, the maintenance of freezing tolerance during the winter, and the timing and rate of deacclimation and budbreak in the early spring, among others (Wisniewski et al., 2003). How these factors are controlled physiologically and genetically in woody perennials is not well understood.
The United States is the world's leading producer of blueberries. In a survey of blueberry research and extension scientists in the United States, lack of winter hardiness and susceptibility to spring frosts were identified as two of the most important genetic limitations of current cultivars (Moore, 1993). In nearly all blueberry growing areas in the United States, economic losses from early spring frosts can be significant. In the northern production regions, winter damage is considered the major factor limiting yields (Hanson and Hancock, 1990; Moore, 1994). In the Pacific Northwest, drastic drops in temperature in the fall to below freezing can cause flower bud damage in some crops, including blueberry (Doughty and Hemerick, 1975) and grape (Davenport and Keller, in press). As blueberry germplasm becomes more diverse, there is increased concern whether resulting selections and cultivars are sufficiently cold hardy for certain regions of the country, especially if southern-adapted germplasm comprises a significant part of the genetic background as it does for some of the newer releases (Ehlenfeldt et al., 2006).
To develop blueberry cultivars for specific regions of the country, a comprehensive understanding of blueberry cold hardiness is needed. Ideally, a blueberry cultivar should acclimate to cold quickly in the fall, have a high midwinter hardiness (actual level dependent on where the cultivar is to be grown), and deacclimate slowly during spring or during unseasonably warm spells in winter without adversely affecting the time of fruiting (Arora et al., 2004). Therefore, in recent years, we have begun comparing patterns of acclimation, deacclimation, and maximum midwinter cold hardiness of flower buds for several different genotypes with diverse genetic backgrounds (Table 1). A summary of this work and our conclusions from it are described subsequently.
Midwinter bud cold hardiness and germplasm composition of Vaccinium genotypes evaluated for timing and rate of deacclimation.zy
Arora, R., Rowland, L.J., Ogden, E.L., Dhanaraj, A.L., Marian, C.O., Ehlenfeldt, M.K. & Vinyard, B. 2004 Dehardening kinetics, bud development, and dehydrin metabolism in blueberry cultivars during deacclimation at constant, warm temperatures J. Amer. Soc. Hort. Sci. 129 667 674
Davenport, J. and M. Keller. How cold can you go? Frost and winter protection for grape. HortScience 43:1966–1969.
Ehlenfeldt, M.K. 1994 The genetic composition and tetrasomic inbreeding coefficients of highbush blueberry cultivars HortScience 29 1342 1345
Ehlenfeldt, M.K., Ogden, E.L., Rowland, L.J. & Vinyard, B. 2006 Evaluation of midwinter cold hardiness among 25 rabbiteye blueberry cultivars HortScience 41 579 581
Rowland, L.J., Ogden, E.L., Ehlenfeldt, M.K. & Vinyard, B. 2005 Cold hardiness, deacclimation kinetics, and bud development among 12 diverse blueberry genotypes under field conditions J. Amer. Soc. Hort. Sci. 130 508 514
Sakai, A. & Larcher, W. 1987 Frost survival of plants: Responses and adaptation to freezing stress Springer Berlin, Heidelberg, New York
Wisniewski, M., Bassett, C. & Gusta, L.V. 2003 An overview of cold hardiness in woody plants: Seeing the forest through the trees HortScience 38 952 959