Daylength and Temperature Effects in the Development of Cold-hardiness, Acclimation, and Deacclimation in Diverse Blueberry Germplasm

The timing and rate of acclimation and deacclimation was measured in seven blueberry genotypes with different amounts of diverse species germplasm in their backgrounds. Primary differences observed among the genotypes were differences in maximum acclimation/hardiness levels and the date at which they were reached, and differences in the date at which maximum acclimation levels were no longer sustained and deacclimation commenced. ‘Bluecrop’, ‘Legacy’, ‘Tifblue’, and two rabbiteye-hybrid derivatives, US 1043 and US 1056, all reached maximum or near maximum cold acclimation by late December with LT_50 temperatures in a range from -22 to -27 °C. After reaching their maximum acclimation in late December, ‘Legacy’, ‘Tifblue’, and US 1043 began a sustained and relatively linear deacclimation, despite a continued decrease in ambient air temperatures over the next several weeks. Conversely, US 1056, ‘Bluecrop’, ‘Northsky’ and ‘Little Giant’ sustained their acclimation for longer intervals, and ‘Bluecrop’ and US 1056 did not begin to deacclimate until early March, and ‘Little Giant’ and ‘Northsky’ until late March. The differences suggest cold acclimation and the retention of cold hardiness have both daylength and temperature regulation. The ability to select for sustained cold acclimation in southern-derived materials should allow the expansion of this germplasm into many areas where it could not previously be used. As concerns of climate change increase, the ability to develop germplasm tolerant to greater extremes and greater fluctuations will prove increasingly valuable.