Induction and Release of Bud Dormancy in Woody Perennials: A Science Comes of Age

The path to dormancy induction, maintenance, and release is a continuum and has been the topic of thousands of research articles to date. It would be an impossible task and indeed presumptuous of us to imagine that we could review all the research conducted on dormancy in the past century in this article. The multiple and complex nature of the dormancy phenomenon would require not one review but a series of in-depth reviews to cover the research on individual subdisciplines that come under the umbrella of dormancy. Its complexity and multiplicity of various subdisciplines stem from the fact that dormancy affects diverse plant structures (buds, seeds, bulbs, etc.) distinctly and that these dormant structures maintain distinct anatomical and physiological relations with neighboring parts. We, therefore, have chosen to discuss here only one, nevertheless highly significant, aspect of dormancy, i.e., bud dormancy in woody plants. As one reflects over nearly a century of work, it is apparent that, as with other disciplines, dormancy research has evolved as different aspects of bud dormancy (e.g., site of dormancy; photoperiod and environmental induction of dormancy; physiology of dormancy control, particularly phytohormones; chilling requirement—effective temperatures, bud differences, modification of chilling requirement by environment and/or cultural practices, models for calculating chilling requirement; dormancy-breaking chemicals and/or stress treatments) catching the fancy of horticulturists at different periods on the temporal curve of dormancy research. This research was extensively reviewed during the 1950s and 1960s (Doorenbos,1953; Leike, 1965; Romberger,1963; Samish, 1954; Vegis, 1964; Wareing, 1956, 1969), followed by more recent reviews and workshop proceedings in the 1970s, 1980s, and 1990s (Champagnat, 1989; Dennis, 1987; Erez and Lavee, 1974; Faust et al., 1997; Fuchigami and Nee, 1987; Kaurin et al., 1985; Lang, 1987, 1994; Nooden and Weber, 1978; Perry, 1971; Rowland and Arora, 1997; Saunders, 1978; Saure, 1985; Weiser, 1970; among others). To appreciate the continuity of significant research developments in this field, we recommend them as a must-read for students of plant dormancy. Although many significant mileposts have been reached in our understanding of the induction and release of bud dormancy in the past 50 to 60 years (reviewed in the above citations), research published up until the 1980s includes little information on experimental systems and approaches for studying the genetics of bud dormancy and the cellular and molecular events—gene expression and regulation, signaling mechanism(s), or mechanistic aspects—associated with regulation of bud dormancy. Many of us must wonder how H. Muller-Thurgau had already confirmed in 1885 that a shortened growth period of the shoots caused by water stress promotes early inception of bud dormancy and shortens its duration, i.e., reduces the chilling requirement. This observation was further supported by Chandler and Tufts in 1934 based on their observation that an extended growth period of shoots delays budbreak the following spring if there is insufficient chilling. Despite these early observations, today we still do not clearly understand the cellular biology of how environmental stress regulates bud dormancy. Perhaps the slow progress in our understanding of the basic biology of