RELATIONSHIPS AMONG INDEXES FOR THE END OF BUD DORMANCY IN APPLE CULTIVARS AND RELATED MALUS SPECIES UNDER COLD WINTER CONDITIONS

Relationships among estimates for the length of bud dormancy over a wide range of cultivars of Malus x domestica Borkh. and related Malus spp. are reported. Flower bud dormancy state was estimated after fitting quadratic models for the number of days for 50% terminal budbreak (D50), bud development stage (BDS), and percent terminal budbreak (BB) after 21 days of forcing as a function of chilling unit (CU) accumulation. Cultivar, linear, and quadratic components of regression, as well as all interactions, were significant for D50, BDS, and BB when regressed against CU accumulation. Consequently, cultivars differ not only as to when changes in dormancy intensity take place, but also in the patterns and rates at which these changes proceed. The number of CU needed to reach arbitrary stages of each characteristic at the end of dormancy was calculated and compared for all genotypes. Rates of CU accumulation until BB, BDS, and D50 reached 50%, 1.5 and 15 days, respectively, were well-correlated. These CU values were considered the end of bud dormancy or genotype chilling requirement (CR). Estimations of D50 were less variable and less subjective, and a wider range of data points could be used in the analysis. In addition, values for D50 can indicate the growth potential of buds when other indices do not show changes. A few sampling times during the dormant season may give a preliminary idea about the CR of a given genotype.