Like many fruit crops in the United States, blueberry (Vaccinium spp.) yields can be significantly reduced by late winter or early spring frost damage to open flowers across much of their range. These damaging frosts are expected to be on the rise as a result of global climate change and higher than average winter temperatures (Gu et al., 2008). The chilling requirements of most blueberry cultivars are satisfied by midwinter. If temperatures warm prematurely after this point, flower buds can resume development and deacclimate, leaving them susceptible to a late winter or early spring frost. In 2007 and 2012, widespread spring frosts resulted in dramatic damage to fruit crops like blueberries, raspberries, grapes, etc., in the eastern and midwestern sections of the United States (Warmund et al., 2008; Wisniewski et al., in press). Total crop losses were estimated at over $2 billion from the 2007 freezes (Warmund et al., 2008; Wisniewski et al., in press).
Most studies on cold-hardiness in northern highbush blueberry (Vaccinium corymbosum L.) have focused on midwinter hardiness of closed flower buds and flower buds in the early stages of deacclimation. In general, midwinter hardiness of flower buds of northern highbush range in tolerance from –20 to –30 °C. Significant variation among genotypes has been noted for both midwinter hardiness and timing of deacclimation (Arora et al., 2004; Bittenbender and Howell, 1976; Ehlenfeldt et al., 2007, 2012; Rowland et al., 2005, 2008).
Only a few studies have been carried out on frost tolerance of open flowers of blueberry, primarily on the southern rabbiteye species, V. virgatum Aiton (syn. V. ashei Reade) (Gupton, 1983; NeSmith et al., 1999; Spiers, 1978). Spiers (1978) showed that frost tolerance of floral buds is inversely related to the stage of bud development. As flower development increases, buds and flowers become less tolerant to freezing damage. Hancock et al. (1987) reported an association between stage of bud development and susceptibility to frost damage for highbush blueberry as well. Gupton (1983) evaluated five rabbiteye cultivars for frost tolerance of open flowers and found ‘Southland’ to be significantly more frost-tolerant than flowers of the other four cultivars examined (‘Delite’, ‘Woodard’, ‘Climax’, and ‘Tifblue’). NeSmith et al. (1999) examined flower damage of rabbiteye blueberry as well as fruit set after subfreezing temperature exposure. Of three flower parts evaluated, corollas were the most sensitive to freezing, followed by styles and then ovaries. Fruit set resulting from bee pollination declined after exposure to temperatures as high as –1 °C for 1 h in some cultivars. However, appropriately timed gibberellin (GA3) application, before or after freeze treatments, resulted in little reduction in fruit set with exposure to temperatures as low as –3 to –4 °C. Gibberellins applied after decapitation of styles have been shown to induce parthenocarpic fruit set in apple (Modlibowska, 1975). Thus, in blueberry, it is thought that if temperatures do not drop low enough to damage ovaries, GA3 application before or after a freeze can result in less fruit loss by stimulating parthenocarpic fruit set (NeSmith et al., 1999).
Here, we have investigated whether there is genotypic variation in frost tolerance of open highbush blueberry flowers that could be exploited in breeding highbush cultivars that are more tolerant to late winter/early spring frosts. Frost tolerance of open flowers of five highbush cultivars, Bluecrop, Elliott, Hannah’s Choice, Murphy, and Weymouth, was determined using two different freezing methods. Comparisons of the methods and genotypes were performed based on the freezing tolerances of several different flower parts, including corolla, style, filament, ovary, and ovules.
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