Cold Hardiness of Southern Highbush Blueberry (Vaccinium corymbosum L. interspecific hybrid) Floral Buds

Blueberry production in southeastern U.S. utilizes low chill cultivars of highbush blueberries, which are susceptible to freeze damage. Tolerance to freezing temperatures of floral buds in northern highbush blueberry (NHB; V. Corymbosum L.) has been previously described; however, southern highbush blueberry (SHB) has not been well characterized. The objective of the study was to determine SHB’s sensitivity to freeze in excised floral buds (EFB) and floral buds attached to the stem (AFB) through tissue freezing methods, differential thermal analysis (DTA) or controlled temperature thermal analysis (TA). Floral buds of ‘Emerald’ and ‘Farthing’ were sampled through dormancy to bud swell in 2015-2016 (Nov-Feb) from a farm in Lakeland, GA. The floral buds were held at -2 °C for 12 h then subjected to either DTA (-2 to -27 °C gradient program, decreasing 4 °C^.h^-1) or TA (-3 to -21 °C removing bud tissue at 3 °C increments). TA samples were stored for a week at 4 °C and evaluated for floral bud tissue damage. Moisture content was evaluated in untreated floral buds. Bloom development was evaluated on shoots that were treated in TA under forcing conditions. At the temperature where 50% lethality of floral bud tissue (LT₅₀) was observed, ‘Emerald’s lowest LT₅₀ was on 25 Jan at -18 °C within AFB and ‘Farthing’s lowest LT₅₀ was on 8 Feb at -18 °C within AFB. The corresponding EFB temperature was 11% greater at -11 °C for both cultivars. Neither cultivar’s EFB showed hardiness below -13 °C throughout dormancy. Comparing DTA exotherms and LT₅₀s for ‘Emerald’ and ‘Farthing’, there was significant separations between the temperatures at P<0.05 for both EFB and AFB, which suggests DTA did not capture the LT₅₀ inflection point. For ‘Emerald’ and ‘Farthing’, the average moisture over dormancy was 66%. ‘Emerald’ showed a 3% increase and ‘Farthing’ showed a 7% decrease in moisture from 2 Nov and 15 Feb samplings. Forced bloom development expressed a similar response when compared to the observations of LT₅₀ within AFB. However, in shoots exposed to temperatures of ≤ -18 °C, bloom was not observed when forced, suggesting subtle floral bud damage not observable under a light microscopy. In conclusion, SHB evaluated in this study had greater sensitivity to cold than previously described NHB and SHB ‘Legacy’. This work suggests the determination of LT₅₀ in SHB should be evaluated by using TA with AFB.