Standardizing Methods for Evaluating the Chilling Requirements to Break Dormancy in Seeds and Buds (Including Geophytes): Introduction to the Workshop

Abstract

Many plants and/or organs require chilling to break their dormancy (Coville, 1920; Doorenbos, 1953; Nienstaedt, 1967). Generally, the optimum temperature is between 0° and 10 °C (Coville, 1920; Doorenbos, 1953; Nienstaedt, 1967). Temperatures required may be continuous, sequential or alternating. The establishment of appropriate protocols for breaking dormancy will permit comparisons between laboratories and produce more effective treatments for commercial use. Romberger (1963) has identified three phases of dormancy: 1) correlative inhibition, 2) rest, and 3) quiescence. Correlative inhibition occurs when buds do not grow because of inhibitory effects caused by apical dominance or lateral dominance (Samish 1954). This period has also been referred to as “Summer Dormancy” (Doorenbos, 1953). The rest phase, also called “Winter Dormancy” (Doorenbos, 1953), occurs when a bud does not grow because of the effects of an internal factor within the bud (Doorenbos, 1953). Quiescence, “Imposed Dormancy” (Doorenbos, 1953), refers to the condition when bud growth is inhibited because of unfavorable environmental conditions (Samish, 1954). Lang (1987) referred to these three phases of dormancy as “Ecodormancy” (correlative inhibition), “Endodormancy” (rest), and “Paradormancy” (quiescence). The termination of the endodormancy (rest) period in temperate plant organs of various species requires a period of chilling (Coville, 1920; Doorenbos, 1953; Nienstaedt, 1967). The duration of the chilling period and the temperature required to overcome endodormnacy (rest) are genetically controlled (Bennett, 1950; Nakasu et al., 1982; Nooden and Weber, 1978; Samish, 1954; Sherman et al., 1977). The prerequisite of the chilling period for the termination of endodormancy (rest) was first observed by Knight (1801) and subsequently studied by many researchers (Chandler et al., 1937; Coville, 1920; Molish, 1908). The effective chilling period ranges from 250 to 1000 hours (Samish, 1954; Vegis, 1964). Erez and Lavee (1971) indicated in peach trees, 5 °C is the optimum temperature to satisfy the chilling requirement. Fluctuating temperature has been proposed to be more effective in satisfying the chilling requirements of deciduous fruit trees (Erez at al., 1979; Lyr et al., 1970; Samish and Lavee, 1962). However, Lavender and Cleary (1974) found that fluctuating temperatures under natural conditions were less effective in overcoming chilling requirements than a constant temperature in many coniferous species. In addition, chilling requirements may vary with age for a given plant species (Nienstaedt, 1966) and variation within species also occurs (Sharik and Barnes, 1976). Scalabrelli and Couvillon (1986) found that terminal buds required less chilling temperatures than the lateral buds of ‘Redhaven peach. Long photoperiods may partially compensate for the chilling requirement in many plant species (Garber, 1983; Nienstaedt, 1966; Olmested, 1951). They postulated that low temperature and photoperiod are not entirely independent since long daylengths could partially compensate for insufficient chilling. A continuous 24-h photoperiod can break dormancy in many plant species (Downs and Borthwick, 1956; Kawase, 1961; Wareing, 1956), while a continuous dark photoperiod will cause budbreak of trees only after their chilling requirements have been satisfied (Wareing, 1956). Artificial breaking of endodormancy (rest) period has been practiced since the early 1900s (Doorenbos, 1953). Early methods used to force spring-flowering shrubs were: injury (Jost, 1893), acetylene (Johanssen, 1896), ether (Johanssen, 1900), electric current (Bos, 1907), and warm water bath (Molisch, 1908, 1909). Dormancy-breaking treatments were also used on temperate fruit trees grown in warm regions of the world which lacked sufficient chilling temperature to overcome rest naturally (Black, 1952). The amount and time of application of dormancy breaking agents, depends on the stage of physiological growth and genetic makeup of the plants (Erez, 1987; Fuchigami and Nee, 1987). Black (1952) indicated that most dormancy-breaking treatments should be applied at the sublethal (near-lethal) levels in order to be effective. Workshop Papers and Authors