EVALUATION OF WINTER TEMPERATURES ON APPLE BUDBREAK USING GRAFTED TWIGS

Temperature is the main climate factor related to induction, maintenance and dormancy release in apple (Malus domestica Borkh.). The inadequate chilling exposure in apples causes budbreak problems, resulting in decrease in yield potential. Thus, the knowledge of physiological principles and environmental factors determining the dormancy phenomenon, especially winter temperature effects, it is necessary for the efficient selection of cultivars in a productive region. In addition, it is indispensable to adapt the orchard management aiming to decrease the problems caused by lack chilling during winter. The objective of this study was to evaluate the influence of different thermal conditions during the dormancy period on budbreak of apple cultivars. One-year-old twigs of 'Castel Gala' and 'Royal Gala' cultivars, grafted on M7 rootstock, were submitted to temperatures of 5, 10 and 15 degrees C for different exposure periods (168; 336; 672; 1,008 and 1,344 hours). After treatments execution, the plants were kept in a greenhouse at 25 degrees C. Budbreak was quantified when accumulated 3,444; 6,888; 10,332; 13,776; 17,220 and 20,664 GDH degrees C after temperature treatments. The cultivars responded differently to temperature effect during the winter period. The temperature of 15 degrees C during winter shows a greater effectiveness on 'Castel Gala' apple budbreak while in the 'Royal Gala' apples the temperatures of 5 and 10 degrees C show better performance. 'Castel Gala' cultivar (low chilling requirement) may supply its physiological necessities, may be capable to budburst, even when subjected to higher temperatures in relation to 'Royal Gala' apples (high chilling requirement).