What Is Wrong with Winter Chill Models in Warm Climates?

Abstract

Bud dormancy is an evolutionary strategy that protects temperate fruit trees from low temperatures during winter. With adequate winter chilling, it is a process consisting of a rapid entrance phase where growth inhibition increases to a maximum and is then released via an exit phase. This progression is, however, different in areas with insufficient winter chill. In such areas, the dormancy entrance phase is protracted, and the exit phase is often incomplete. Several chill models have been designed to describe the plants’ response to chill accumulation to match cultivar and site. Although it is known that these models are less accurate in warm winter areas, their accuracy during the two dormancy phases has always been assumed equal. The aim of this study was to investigate the accuracy of four chill models during the entrance and exit phases of bud dormancy of ‘Royal Gala’ buds from two contrasting climatic areas. One-year-old shoots were collected from commercial orchards and forced to determine their bud dormancy level. Before forcing, the shoots received varied amounts of chill additional to the field chill. The bud dormancy progression was represented as joint two- or three-line models to determine the entrance and exit phases. For each phase, the dormancy level of every sample was plotted against its chill accumulation based on four chill models (Chilling Hours, Utah, Positive Utah, and Dynamic Model). Results were compared to determine the ability of each model to predict the linearity of the dormancy progression. The results indicated that although all the models were able to describe the exit phase of bud dormancy successfully, none could describe a linear entrance phase. This suggests that during the onset of dormancy, buds respond to temperature differently to what is measured by the chill models making the models unreliable when winter chill is inadequate.