Dormancy patterns throughout the season were studied in more than 90 apple (Malus ×domestica Borkh.) cultivars and related Malus spp. The seasonal apple bud dormancy pattern resembles a normal curve: it starts to intensify soon after bud formation and reaches maximum intensity by the time of leaf fall/senescence. Genotypes were grouped into three general classes based on maximum dormancy intensity. Maximum intensity of bud dormancy measured in cold winters is inversely related to adaptation to the subtropics. Low-chilling requirement (CR) cultivars have a shallow depth of dormancy with very little alteration throughout the year. High-CR cultivars have intense bud dormancy, the first stage of which can be induced by growing these cultivars at temperatures above 20C. Genotypes differed in their rates of dormancy dissipation. The efficiency of chilling unit (CU) accumulation to break dormancy was negatively correlated with CR, which indicates the importance of factors other than CU accumulation in terminating bud dormancy in low-CR cultivars. The inherent length of bud dormancy plays a major role in determining the time of budbreak in the spring. Deviations may be related to the genotypic efficiency in which chilling modifies dormancy and possibly the basal temperatures to which buds respond. Chill unit requirement and heat unit requirement are dependent factors. Heat requirement comparisons may be meaningless if the dormancy intensities of the genotypes are not taken into consideration.
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