Dormancy—What We Know (and Don't Know)

Abstract

that seeds a) contain a whole plant (the embryo) and therefore exhibit internal correlations and b) manifest breaking dormancy by radicle elongation, whereas buds contain only shoot or floral tissue and growth depends on factors provided by other plant organs. Arias and Crabbé (1975) and Crabbé (1984) have demonstrated that chilling changes the developmental pattern of vegetative buds on single-node cuttings of tree fruit, with varying degrees of acrotony and basitony being exhibited depending on collection time. Correlative inhibition may occur even in the embryo; Thévenot and Côme (1973) and Thévenot et al. (1973) demonstrated that the presence of the cotyle-dons could either hasten or delay the growth of apple embryonic axes depending on the excision time during chilling. What induces dormancy? Dormancy can be divided into various phases. Although Champagnat (1983a) considers bud dormancy to be a continuous process beginning with apical dominance, Amen (1968) proposed four phases of seed dormancy-induction, maintenance, trigger, and germination. Most research has focused on breaking dormancy (trigger phase) because less is known about the conditions that induce dor-mancy, except for species in which growth is responsive to photope-riod. In species such as Acer pseudoplatanus L. and Betula pubescens J.F. Ehrh. (Kawase, 1961), exposing young, container-grown plants to short photoperiods stops growth, whereas growth is continuous under long photoperiods. How buds become dormant in many other species remains a mystery, although lateral buds seem to become endodor-mant when prevented from growing for long periods by apical dominance-paradormancy gradually becomes endodormancy (Champagnat, 1983a). To produce apples and peaches in tropical areas, trees must be defoliated before buds enter endodormancy. As leaf removal is delayed, the proportion of buds that develop becomes progressively lower (Edwards and Notodimedjo, 1987). The role of temperature in dormancy induction is not well defined. Several studies indicate that chilling may intensify dormancy in the fall, although it breaks dormancy later (Ben Ismail, 1989; Hatch and Walker, 1969; Kobayashi et al., 1983; Lavarenne et al., 1975; Walser et al., 1981). Hatch and Walker (1969) observed increased dormancy as measured by the concentration of gibberellic acid (GA 3) required to stimulate leaf bud expansion on excised peach and apricot (Prunus armeniaca L.) shoots in the fall, even as chill units accumulated. However, they did not include control (nonchilled) shoots for comparison. In a similar study, Walser et al. (1981) subjected peach trees or limbs to various temperatures, then removed cuttings and treated them with gibberellin (GA). Warm conditions delayed but did not prevent the onset of dormancy and increased dormancy in trees subsequently exposed to chilling temperatures. Ben Ismail (1989) demonstrated that growth of vegetative apple buds on isolated nodal sections was inhibited by chilling cuttings in early October. However, the response varied depending on collection time and length of exposure to low temperature. If chilling intensifies dormancy at one stage of development and releases it at another, the time at which one begins to record chill units becomes critical. How is dormancy maintained? According to Amen (1968), the maintenance phase continues until conditions (e.g., temperature or daylength) change. Amen implies that maintenance is a steady-state condition, with factors responsible for growth renewal (growth promoters?) being in balance with those that prevent growth (growth inhibitors?). Until conditions change, dor-mancy remains static. In certain structures [e.g., potato (Solanum tuberosum L.) tuber buds and lettuce (Lactuca sativa L.) seeds, Arabidopsis, and some cereal grains], dormancy disappears during dry storage; the only condition that changes is time. In some trees, growth So many types of dormancy have been described that any brief and meaningful review must be limited in scope. This paper will summarize what we know about dormancy and what remains to be learned. Most of this review will deal with endodormancy, with emphasis on tree fruit. Historical aspects and previous reviews Several reviews deal extensively with early observations and research on dormancy. The interested reader should consult Romberger (1963) for information on bud dormancy and Evenari (1980-81) for information on seed dormancy. Books dealing with seed dormancy include those by Bewley and Black (1982), Bradbeer (1988), and Khan (1982). Crabbé (1987) analyzes bud dormancy as a part of his treatise on shoot morphogenesis. Other recent reviews include those of What is dormancy? Recently, considerable attention has been devoted to dormancy nomenclature. Lang (1987) and Lang et al. (1985, 1987) proposed the terms ecodormancy, paradormancy, and endodormancy to describe three types of dormancy: environmental control, control within the plant but outside the structure, and control within the structure, respectively. Etymology aside, these concepts are well recognized. For example, Champagnat (1983b) points out that Chouard (1956) proposed a similar classification using the terms quiescence, correla-tive inhibition, and dormancy, respectively. Seed physiologists have difficulty accepting some of the new terms, under which nondormant, dry seeds would be classified as ecodormant. I prefer the term dormancy, and will use it synonymously with endodormancy unless otherwise noted. Every plant physiology text and many review articles list types of seed dormancy (coat-imposed vs. embryo dormancy; anatomical vs. physiological dormancy; immature vs. mature embryo, shallow vs. deep dormancy) and conditions that remove dormancy (dry storage, moist chilling, light). The many conditions necessary for removing dormancy make it unlikely that any one mechanism or condition, hormonal or otherwise, controls the process. Scientists are trained to seek simple explanations and universal principles, but these have proven difficult to find for the physiology of dormancy. Are bud dormancy and seed dormancy similar phenomena? Bud dormancy and seed dormancy have many common characteristics (Powell, 1987). For example, the temperatures that break the dormancy of seeds and buds of deciduous tree fruit exhibit similar optima, and secondary dormancy can be induced by exposing fruit to high temperatures before they accumulate a critical number of chill units. My students and I have studied seed dormancy based on the assumption that the knowledge gained could be applied to bud dor-mancy, perhaps providing a way to delay budbreak in the spring, thereby avoiding freeze injury. Seeley (1990) and del Real Laborde et al. (1990) have incorporated data obtained with apple (Malus domes-tica Borkh.) (del Real Laborde, 1987) and peach [Prunus persica (L.) Batsch.] (Seeley and Damavandy, 1985) seeds into the Utah Model (Richardson et al., 1974), which is used to predict the effects of fluctuating winter temperatures on breaking bud dormancy. J. Crabbé (personal communication) questions these assumptions on the grounds