Estimating the timing of endodormancy release in apple by using budbreak forcing conditions

Bud dormancy is characterized by the temporal suspension of growth and developmental processes of temperate tree crops that allows them to withstand low temperatures during the winter period. Fruit trees have specific chill requirement to break dormancy and to proceed with normal flowering behavior; however, the lack of chill may negatively affect temperature related plant responses that lead to poor and asynchronous flowering. In this study, the time of endodormancy release was estimated in four apple cultivars (‘Spencer Seedless’, ‘Kanzi’, ‘Topaz’ and ‘Cameo’) by the time and rate of budbreak and flowering under budbreak forcing conditions. Therefore, apple branches with spurs and terminal buds were sampled weekly from the end of November over 14 weeks at the experimental orchard of the University of Hohenheim and then exposed to budbreak forcing conditions inside a growth chamber. Under forcing conditions, the time to budbreak gradually declined from 22 to 35 days to 1 day throughout the experimental period, whereas the budbreak rate continuously increased to nearly 100% in all cultivars when they had accumulated 1122-1205 Utah Chill Units in the middle of December. This time might indicate the end of transition from endo- to ecodormancy of all the studied cultivars. Our findings suggested that the transition between these dormancy phases has not an abrupt but rather a continuous nature. Flowering rate of the studied cultivars also increased until trees, depending on the cultivar, accumulated 1205-1340 Utah chill units and then reached a plateau. The time from budbreak to the first opened flower did not change throughout the experiment and was on average about two weeks for all cultivars. Besides the Utah chill units, chill portions were also calculated to facilitate the comparison with data from literature. Both models were used for the prediction of endodormancy release for the studied apple cultivars in 2014-2019. In contrast to the chill portions, Utah chill units showed much greater yearly variation of the predicted time of endodormancy release.