Comparison of Mid-winter Cold Hardiness in Highbush Blueberry (Vaccinium corymbosum) Cultivars and Its Changes during Deacclimation

Abstract

Cold hardiness and soluble sugar contents were compared in shoots of twenty-one highbush blueberry cultivars (Vaccinium corymbosum cvs. Berkeley, Bluecrop, Bluegold, Bluehaven, Bluejay, Burlington, Chippewa, Collins, Dixi, Duke, Herbert, Jersey, Nelson, Northblue, Northland, Polaris, Rancocas, Sharpblue, Sierra, Spartan, Sunrise) in mid-winter. Relationships between cold hardiness and soluble sugar contents were also examined. The level of cold hardiness was expressed as temperature representing 50% injury occurred (LT50) and temperature at maximum rate of injury (Tmax) determined by electrolyte leakage analysis at various freezing temperatures. According to cold hardiness levels based on LT50, twenty-one highbush blueberry cultivars were ranked as Jersey>Northblue>Berkeley=Sierra>Northland>Dixi>Bluejay>Chippewa>Burlington>Bluegold>Spartan>Bluecrop>Sunrise>Duke=Rancocas>Herbert>Polaris>Collins>Bluehaven>Sharpblue>Nelson. Tmax was significantly correlated with LT50 (r2 = 0.96***). The cold hardiness was highly correlated with total soluble sugar contents (r2 = -0.74*** and r2 = -0.63*** for LT50 and Tmax, respectively). Of the detected soluble sugars, fructose and glucose contents were significantly associated with cold hardiness in the shoots of highbush blueberry cultivars, but sucrose and raffinose contents were not associated with cold hardiness. Seven highbush blueberry cultivars of Bluecrop, Jersey, Rancocas, Sharpblue, Sierra, Spartan, and Sunrise were selected depending on mid-winter cold hardiness and growth condition. And then, changes of their cold hardiness were examined during deaccliamation. According to the cold hardiness levels based on LT50, all cultivars except Sharpblue maintained their cold hardiness until February 20 and then steeply decreased until March 5. However, Sharpblue sharply decreased its cold hardiness until March 5. Since March 5, all cultivars except Sharpblue reacclimated as air temperature decreased. Degree of cold hardiness increased by -4.2, -3.5, -3.4, -2.5, -2.3, and -2.2℃ for Rancocas, Jersey, Bluecrop, Sierra, Sunrise, and Spartan, respectively on March 12. Bluecrop and Sunrise regained cold hardiness by -3.6 and-2.0℃, respectively, on March 26. Jersey also increased its cold hardiness slightly on April 2. Changes of cold hardiness based on Tmax were also similar. Although Spartan was more cold-hardy in mid-winter, it lost cold hardiness earlier and faster than Bluecrop, Sunrise, and Rancocas. Thus, deacclimation resistance was not always correlated with mid-winter cold hardiness. Furthermore, the water contents in the shoots tended to increase with decreasing cold hardiness. However, they were not always significantly correlated with the cold hardiness levels during deacclimation in all cultivars examined.