ABSTRACT
The capacity of woody perennial plants to survive winter freezes is dependent on two phenological events which occur during late summer or early fall: (1) the onset of endodormancy and (2) an increase in cold
hardiness upon exposure to low non-freezing temperatures called cold acclimation. Exposure to low temperature is also required for breaking endodormancy and resumption of growth the following spring. This low temperature or chilling requirement helps prevent growth during transitory periods of warm temperatures throughout a large portion of the winter, assisting to synchronize growth with exposure to favorable temperatures. Cold acclimation, on the other hand, enables plants to withstand subfreezing temperatures during the winter. Some plants which would be killed by temperatures slightly below 0°C during the summer and early fall may survive temperatures as low as — 196°C dur ing the winter when fully cold acclimated (Sutinen et al., 1992). Conse quently, chilling requirement and cold hardiness levels are major factors determining to what degree temperate-zone fruit crops will survive the winter and early spring without damage to shoot and flower buds.
