Temperature Effects on Dormancy Completion of Vegetative Buds in Apple

ADDITIONAL INDEX WORDS. rest, chilling requirements, apical dominance, Malus sylvestris ABSTRACT. The relative contribution of various temperatures to dormancy completion of lateral vegetative apple (Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf. ) buds was studied quantitatively on whole container-grown trees. Trees were exposed continuously to 10 different temperatures and also to daily alternating temperatures in a 24-hour cycle. In addition, fully chilled vertically and horizontally positioned shoots were compared under forcing conditions. No budbreak occurred in shoots chilled above 12.5 OC. There was a steep increase in budbreak as the chilling temperature fell from 12.5 to 7.5 OC. There was little difference in the level of budbreak on shoots chilled between 7.5 and 0 OC. The relative contribution of temperature to chilling accumulation in apple found in our study differs from what has been proposed for stone fruit and for apple in previous studies, especially at temperatures <6 OC. The length of exposure to forcing conditions required to initiate budbreak diminished as the chilling temperature was reduced. No additional bud- break was apparent on shoots chilled longer than 2100 chilling hours. The chilling requirement found here for lateral vegetative buds is much higher than that needed for terminal vegetative and flower buds. Trees that were exposed to daily alternating temperatures had lower levels of budbreak when the high temperature in the diurnal cycle was greater than 14 OC. Practically no budbreak was apparent on trees that were exposed to diurnal cycles with a high temperature of 20 OC for 8 hours. Budbreak on horizontally positioned trees was more than twice that on the vertically positioned trees, emphasizing the magnitude of the apical dominance effect and its strong masking of the chilling effect on lateral buds in vertically grown apple trees. Based on the data collected here we propose a new response curve for vegetative budbreak in 'Golden Delicious∑ apple, within a temperature range between 0 to 15 OC. Buds of most temperate-zone deciduous trees have a dormancy period in the winter. Low temperatures are the most significant fac- tor affecting dormancy completion, although, there are indications that heat, light intensity, and mist, during the endodormancy phase affect dormancy completion to a certain extent (Chandler, 1960; Erez et al., 1966; Erez et al., 1968; Freeman and Martin, 1981). Knowing the exact response of apple (Malus sylvestris var. do- mestica) buds to temperature is important for a basic understanding of dormancy and for the practical application of dormancy release or bloom delay techniques. Spring frost may cause severe damage to flowers and young foliage in temperate zones; therefore, predic- tion of the date of budbreak is an important issue for growers in those regions. Budbreak is affected by two temperature-dependent processes: a) the accumulation of chilling to the level required for dormancy completion; and b) the accumulation of the heat units required for the buds to develop to bloom and foliation. These two processes were shown to be interdependent, i.e., fewer degree-days of heating are required as the previously accumulated chilling in- creases (Couvillon and Erez, 1985a). In warm regions, temperatures in the winter are not low enough, or the duration of exposure is not