Dormancy Patterns of Apple Trees Grown in a Mild Winter Region: A Breakpoint Model Analysis

Climate change poses a significant challenge to the production of temperate climate fruit trees, since the dormancy of these species is regulated by the chill and heat accumulation during winter and early spring. This study aimed to identify the dormancy evolution patterns of three apple cultivars ("Galaxy," "Fuji Suprema," and "Eva") in a mild winter region over five contrasting years (2018-2022) across two commercial orchards with different microclimates in southern Brazil. Dormancy status of lateral buds of branches was evaluated using the single-node cutting test (Mean Time to Budburst-MTB) and quantified using a linear breakpoint model to determine the rates of dormancy entry, maximum depth of dormancy, and rates of dormancy release. The colder orchard accumulated 51% more Chill Portions (Dynamic Model) than the warmer site, inducing a deeper endodormancy (alpha) but maintaining faster dormancy release rates (Cs). Inter-annual climatic variability strongly influenced the timing (theta) and the rate of entry (Ce). The low-chill cultivar "Eva" did not enter deep endodormancy. Early season chilling significantly accelerated dormancy induction (Ce) but also increased the chilling requirement for dormancy release. The results indicate that cultivars display distinct and highly plastic dormancy dynamics based on microclimate and yearly chilling, emphasizing the need to select cultivars with phenological patterns adapted to specific local conditions for crop resilience and sustainability under climate change.