Characterization of Southern Highbush Blueberry Floral Bud Cold Hardiness Through Dormancy in a Sub-Tropical Climate

Abstract

Southern highbush blueberry (SHB; Vaccinium corymbosumL. interspecific hybrids) is highly susceptible to freeze damage in sub-tropical climates, but the process is poorly understood. To address the issue, freeze tolerance experiments were conducted on two common cultivars of SHB, Emerald and Farthing, during the winters of 2015-16 and 2016-17. Floral buds (attached and excised from stems) were preconditioned overnight at 4.0 °C or -2.0 °C and exposed to temperatures of -3.0 to -21.0° C. The samples were then stored at 4.0 °C for a week and afterwards examined for bud damage. A lethal temperature threshold of 50% (LT ₅₀) was calculated on visual ratings for damage. Cold hardiness varied with preconditioning, bud type (attached or excised), and sampling date, and ranged from -6.8 °C to -20.2 °C in both cultivars. On average, LT ₅₀was lower in the attached than in the excised buds and when buds were preconditioned at -2.0 °C than at 4 °C. Attached buds displayed increasing hardiness as the chill hours accumulated, until the buds began to swell, at which time the hardiness decreased. Cold hardiness did not change in the excised floral bud in either cultivar or year. Preconditioning increased the hardiness of the attached buds at -2 °C, which may be artificially inflated considering sub-tropical climate. This work shows that attached SHB floral buds are sensitive to chill hour accumulation and deacclimation. Whereas, excised buds did not respond with similar sensitivity. For SHB, the best estimation for hardiness can be obtained through attached buds preconditioned at 4 °C.