02: Bud dormancy
The classical definition of dormancy is relevant only to the shoot apical meristems of perennial, woody, temperate-zone plants and is weak in that it specifies neither physiology nor anatomy. Nonetheless, such plants do have a definite annual growth cycle composed of distinctly different, but intergrading, physiological states. Plant physiology must be in harmony with the environment throughout the year if vigorous growth is to occur. Dormancy may also be defined in terms of the mitotic activity of cells of buds or of resistance to stress ("hardening off"); the latter. frequently confused with "classical dormancy," is a physiological condition common to the entire plant, not just the shoot apical meristem. Budbreak speed, which may be a good indicator of seedling vigor, is currently evaluated by two general approaches (one relating number of chilling hours to dormancy release, the other comparing mortality and budbreak speed), each of which has its limitations. However, the speed with which buds resume growth in spring is a function of the physiological state of the bud and hence may not mirror whole-plant vigor. Budbreak speed may reflect seedling vigor less when stock has been weakened by natural stresses or mishandling, but more when the seedling's normal physiological sequence during dormancy has been disrupted. So far, other measures of seedling vigor, including the oscilloscope technique, dry-weight fraction, and hormone analysis, have been found unsatisfactory for describing probable differences in seedling physiology during dormancy or for predicting seedling vigor.
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Author(s): Denis P. Lavender