Fruit tree buds also express the three basic forms of dormancy. Ecodormancy is apparent in many woody plants. Early in the season, when temperatures are mild and sunshine is abundant, there is a flush of vigorous vegetative growth. Later in the season, if conditions become too hot or dry, growth will slow and may cease, and a terminal bud may be set at the apex of a shoot. However, if favorable conditions return, the plant produces a new flush of growth. Sometimes, several cycles of growth are observed in woody plants as they respond to changes in temperature, water availability, and light during the growing season.
Below the apex of a shoot, lateral buds may express paradormancy, as mentioned previously. Tree fruit vary in their degree of apical dominance and thus the manifestation of paradormancy. Apples, pears, and sweet cherries typically exhibit strong paradormancy of lateral buds and strong dominance of the apex. Thus, heading-back pruning cuts are used to stimulate the development of lateral branching. Many stone fruit, particularly peaches and tart cherries, have weaker apical dominance and tend to more readily develop lateral branches below the apex. Paradormancy appears to be a stronger influence on vegetative buds than on floral buds.
In temperate fruit trees, as daylength shortens and temperatures cool, buds express ecodormancy. With increasingly shorter daylength and lower temperatures, endodormancy, especially pronounced in floral buds, is then induced. Some reports indicate that the point in time that a bud changes from ecodormancy to endodormancy is a stage of vegetative maturity. It is from this point in time that there is an accumulation of cool temperature exposure, or "chill," by the plant. After a specific period of chill exposure, endodormancy ends. The period of chill required for buds to physiologically change and have endodormancy removed is referred to as "chilling requirement."
Various models have been proposed for predicting when fruit trees will complete endodormancy. A model for peach trees is based on the hourly accumulation of chill units (CUs) (Richardson, Seeley, and Walker, 1974). It is a partial sine wave or extended quadratic model in which 1 CU is accumulated when flower buds are exposed for 1 hour to a temperature of 7.2°C. No CUs are accumulated at temperatures below freezing or at 12.8°C, and there is a negative CU (-1 CU) response at 21 °C. In 1990, Linvil introduced a modified model utilizing daily high and low temperatures. Researchers propose that the onset for the accumulation or vegetative maturity of chill begins after flower buds have received a threshold number of CU (approximately 50) without interruption by negative chill at high temperatures.
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