Dormancy allows apple trees to survive seasonal changes and protects the meristem (growth tissue) from unfavourable climates.
Temperature and photoperiods (day length) are the main cues that trigger the different stages in the dormancy cycle, perceived through the plant’s molecular networks and endogenous signals (eg. plant hormones). This results in the regulation of plant growth and the tree’s dormancy cycle.
Bud dormancy has 3 phases: Paradormancy, Endodormancy and Ecodormancy.
Paradormancy is induced by factors such as the tree’s apical dominance and auxins. This dormancy period is defined by the inhibition of plant growth, and setting up buds for the next season.
Endodormancy is induced by cold temperatures and short photoperiods. Once the trees enter endodormancy they are required to meet a specific number of chilling units before they can move to the next phase of dormancy. This means they must go through a period of prolonged exposure to cold temperatures (above freezing). Chilling requirements for dormant buds determine the timing of budbreak, these chilling requirements vary between varieties but are mostly between 400 and 1000 hours. If warm weather or favourable growing conditions occur before winter chilling requirements are met (the plant is still in the phase of endodormancy), growth cannot resume.
Ecodormancy is when the meristems fully develop, and the bud regains its ability to respond to environmental factors. The buds remain in the Ecodormancy phase until the heating requirements are met through extended periods of warm weather, which in turn triggers budbreak. Budbreak is influenced by external signals from the plant, the bud itself and the external environment.
Figure 1: Advancing Endodormancy Release in Temperate Fruit Trees Using Agrochemical Treatments Jesús Guillamón Guillamón, Federico Dicenta and Raquel Sánchez-Pérez*
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