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The chilling requirements (CR) to break bud dormancy in a broad range of apple cultivars (Malus ×domestica Borkh.) and related Malus spp. were assessed by periodic sampling and forcing of field-grown shoots as a function of chill unit (CU) accumulation and/or by the total growing degree hours (GDH) accumulated from leaf fall until the time of budbreak under a simulated subtropical winter. The mean number of CU required to break dormancy of field overwintered shoots varied between 218 ± 113 for `Anna' and 1516 ± 113 for `Wright #1'. However, most genotypes had CR between 800 and 1200 CU. Much wider variation for the length of bud dormancy was observed in plants growing under simulated subtropic winter conditions. Genotypes that had shown the lowest CR values under Geneva, N.Y., winters generally had the highest year-to-year variation in CR estimates. Cultivar bud CR values obtained under cold winters are related to field-observed CR estimates in a subtropical environment, but absolute values may differ markedly. Furthermore, several genotypes that show reasonable adaptation to the subtropics have similar or higher CR than apple cultivars with standard CR under Geneva conditions. In addition, enough CU accumulated under the simulated subtropic winters to break dormancy of standard apple cultivars. However, complete dormancy removal was observed only in cultivars well-adapted to a subtropical environment. This result indicates that in addition to CU accumulation, there are important interactions among cultivars and environmental factors that are responsible for terminating bud dormancy. Several cultivars and wild species have shown resistance to delayed foliation. Among the species, M. brevipes, M. rockii, M. spectabilis, and M. turesii are more tolerant than M. baccata and its hybrids, which are recognized for their adaptation to the subtropic environment.
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.
Apple seedlings have a shallow dormancy, as has been observed in many other species. The length of bud dormancy in high-chilling-requirement seedlings does not reflect their genetic constitution well if dormancy is induced before they are 200 days old. Seedling populations sprayed with paclobutrazol and/or ethephon displayed bud dormancy periods resembling those of older populations of similar genetic constitution. Terminal bud formation and dormancy could not be induced by continuously exposing apple seedlings to low temperature (8 ± 1C) and short photoperiod, even after extended periods. Stomate operation may not be completely functional under these conditions. Terminal bud formation was induced by holding apple seedlings above 20C. Additional exposure to low temperature (8 ± 1C) induced leaf fall. These findings suggest the existence of an active regulatory factor that induces terminal bud formation and dormancy and is either turned on or synthesized above 15 to 17C. Chemical names used: β- [(4-chlorophenyl)methyl]- α -(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol(paclobutrazol);(2-chloroethyl)phosphoric acid (ethephon).
In a study of chilling requirement in Malus, broad-sense heritability estimates for the length of vegetative bud dormancy in 43 clones growing under simulated subtropical winter conditions were 0.76 ± 0.04 in 1986 and 0.81 ± 0.04 in 1987. Narrow-sense heritability estimates were 0.66 ± 0.13 in 1986 and 0.69 ± 0.13 in 1987. Seedlings with low chilling requirements (CR) were not observed in crosses where both parents had high bud-chilling requirements. `Koningszuur' did not transmit its long CR to its seedlings. Open-pollinated (OP) seedling populations from the Malus × domestics Borkh. cultivars Anna, Dorsett Golden, Ein Shemer, Khashabi, Winter Banana, and Zabaoani, and the species and interspecific hybrids M. baccata L. DE#98, M. brevipes Rehd., M. ×robusta (Carr.) Rehd. DE#485, M. × robusta No. 5 (`R5'), M. rockii, M. turesi Rehd. PI 34143, and `Rosedale' had at least 5% of their descendants in the lower CR classes. In all but one instance, 50% or more of `Anna' descendants had low CR. Many of these seedlings were within a few classes of the extreme low CR. It is postulated that the low-CR character present in `Anna' is controlled by at least one major dominant gene and that minor genes interact to modulate its effects. Very low-CR cultivars have a shallow bud dormancy. This highly heritable component for low bud CR is related to a failure to develop a deep dormancy state, rather than to acceleration of the termination of the dormancy process.
Relationships among estimates for the length of bud dormancy over a wide range of cultivars of Malus × domestics 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.