Dormancy dynamics and cold hardiness of apple trees grown in a mild winter region.

• Apple trees grown in warmer regions experience shallow dormancy. • Cold hardiness is different between cultivars and between cultivation sites. • The Eva cultivar does not present endodormancy and deep cold hardiness. • The maximum cold hardiness occurs after the peak of endodormancy. • Cold deacclimation is rapid and with high heterogeneity. In temperate fruit plants, climate is the main factor correlating changes in plant physiology, mainly those related to entry and exit from dormancy and cold hardiness. Acclimation to cold and its relationship with dormancy are still unknown in areas with a subtropical climate. Therefore, this study aimed to investigate the dormancy dynamics and their interaction with the cold hardiness of different tissues of apple cultivars with different chilling requirements in two cultivation sites in a mild winter region in southern Brazil. The analyses were conducted from 2018 to 2022. The cultivars Gala, Fuji, and Eva were used in two environments with different altitudes. The dormancy dynamics were analyzed using the single-node cutting test, and the cold hardiness of the different tissues of each genotype was evaluated through electrolyte leakage in laboratory freezing tests. Cultivars Gala and Fuji had a greater depth of dormancy than Eva. Plants grown in orchards with higher chill accumulation had a greater depth of dormancy. Cold hardiness varied among cultivars, with the Eva cultivar having the lowest cold hardiness in mid-winter. The tissues (bud, bark, and wood) had different cold hardiness in early winter, with wood having the greatest cold hardiness, followed by bud and bark. As Eva did not go into deep dormancy, a superficial and short acclimation to cold was observed, in addition to rapid sprouting, even in winter. This condition predisposed it to cold damage to tissues in the pre-sprouting period. In this study, we found that maximum cold hardiness occurred after the end of the deep-endodormancy phase. Deacclimation of all cultivars occurred very quickly. [ABSTRACT FROM AUTHOR]