What is seed dormancy and how is it related to germination? - Definition of seed dormancy
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Seed dormancy could be considered simply as a block to the completion of germination of an intact viable seed under favourable conditions. In our Tansley review 'Seed dormancy and the control of germination' (Finch-Savage and Leubner-Metzger, 2006) we present an integrated view across the evolution, molecular genetics, physiology, biochemistry, modelling and ecophysiology of the control of seed germination by dormancy in an attempt to draw together these linked, but often separate disciplines.
Definition of seed dormancy: A more sophisticated and experimentally useful definition of dormancy has been proposed by Baskin and Baskin (2004). A dormant seed does not have the capacity to germinate in a specified period of time under any combination of normal physical environmental factors that are otherwise favourable for its germination, i.e. after the seed becomes non-dormant. A completely non-dormant seed has the capacity to germinate over the widest range of normal physical environmental factors possible for the genotype.
Discussion and opinion: Dormancy is a block to germination has evolved differently across species through adaptation to the prevailing environment so that germination occurs when conditions for establishing a new plant generation are likely to be suitable (Finch-Savage and Leubner-Metzger, 2006 and references therein). A diverse range of dormancy mechanisms (blocks) has evolved in keeping with the diversity of climates and habitats in which they operate. Definitions of dormancy are difficult because dormancy can only be measured by the absence of germination. We can observe completion of germination of a single seed as an all-or-nothing event, whereas dormancy of a single seed can have any value between all (maximum dormancy) and nothing (non-dormancy). Dormancy should not just be associated with the absence of germination, rather it is a characteristic of the seed that determines the conditions required for germination. When dormancy is considered in this way, any environmental cue that alters the conditions required for germination is by definition altering dormancy. Also by extension, when the seed no longer requires specific environmental cues it is non-dormant. Dormancy is a seed characteristic which defines the conditions required for germination and therefore any cue that widens the environmental requirements for germination should be regarded as a dormancy release factor. A wide range of factors can therefore alter (physiological) seed dormancy, e.g. temperature, light, nitrate or naturally occurring chemical signals (ABA and four other terpenes) in leachate from litter that covers the seeds in their habitat. However, there is an important distinction in the seeds response to these factors. 1) There are factors that are related to slow seasonal change. These factors (e.g. temperature) are integrated over time to alter the depth of dormancy, and the sensitivity to other factors (e.g. light). 2) There are other factors that indicate in a more immediate way that conditions are suitable for germination (e.g. light), which could be considered to terminate dormancy and therefore induce germination. Each of these factors therefore remove successive blocks to germination, but this process usually needs to be carried out in a set order for it to work, i.e. in the process described light must come last to be effective. In summary, seed dormancy is an innate seed property, which defines the environmental conditions that must be met before the seed can germinate. The intrinsic molecular mechanisms that determine dormancy have an embryo and/or a coat component. However, dormancy as a “whole seed”-characteristic controls germination and a classification system for seed dormancy has been proposed. See the 'seed dormancy webpage II ' for a phylogenetic table on the dormancy classification.
Primary versus secondary seed dormancy: Freshly harvested mature water-permeable dormant seeds are said to have primary dormancy, which has been induced with the involvement of abscisic acid (ABA) during seed maturation on the mother plant (Finch-Savage and Leubner-Metzger, 2006 and references therein). Subsequent dormancy release “in the field”, following dispersal, may involve the same factors that are commonly used “in the lab”: either after-ripening in the relatively dry state, or dormancy-release treatments in the imbibed state. These imbibed seed treatments include chilling (cold stratification), warm stratification, light, gibberellins (GA) and other hormones (Kucera et al. 2005 and references therein), smoke substances like butenolide and compounds like nitric oxide. In contrast to primary dormancy, secondary dormancy can be induced in seeds with non-deep physiological dormancy after seed dispersal, and is often associated with annual dormancy cycles in the seed bank
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A classification system for seed dormancy: The 'whole-seed view' Marianna G. Nikolaeva devised a dormancy classification system reflecting that dormancy is determined by both morphological and physiological properties of the seed. Based on this scheme, C. Baskin and J. Baskin (1998; 2004) have proposed a comprehensive classification system which includes five classes of seed dormancy. The system is hierarchical with these five classes further divided into levels and types Baskin and Baskin (2004). See the 'seed dormancy webpage II ' for a phylogenetic table on the dormancy classification and examples for the different dormancy classes. (1) Physiological dormancy (PD) PD (dormancy class A according to Baskin and Baskin, 2004) is the most abundant form and is found in seeds of gymnosperms and all major angiosperm clades. It is the most prevalent dormancy form in temperate seed banks and the most abundant dormancy class “in the field”. PD is also the major form of dormancy in most seed model species “in the lab”, including Arabidopsis thaliana, Helianthus annuus, Lactuca sativa, Lycopersicon esculentum, Nicotiana spp., Avena fatua, and several cereals. The molecular mechanisms of PD are the focus of the Tansley review by Finch-Savage and Leubner-Metzger (2006). PD has three levels: deep, intermediate and non-deep. PD deep Embryos excised from PD-deep seeds either don’t grow or will produce abnormal seedlings. GA treatment does not break their dormancy. Ca. 3-4 months of cold (subtype a) or warm (subtype b) stratification are required before germination can take place. Examples: Acer platanoides (PD deep subtype a) (Aceraceae; Finch-Savage et al. 1998); Leptecophylla tameiameiae (PD deep subtype b) (Ericaceae; Baskin et al. 2005). PD intermediate Embryos excised from PD-intermediate seeds produce normal seedlings. GA promotes germination in some (but not all) species. Seeds require 2-3 months of cold stratification. Dry storage (after-ripening) can shorten the cold stratification period. Example: Acer pseudoplatanus (PD intermediate) (Aceraceae; Finch-Savage et al. 1998). PD non-deep The great majority of seeds have non-deep PD. Embryos excised from these seeds produce normal seedlings. GA treatment can break this dormancy and depending on species dormancy can also be broken by scarification, after-ripening in dry storage, and cold (0-10 °C) or warm (>15 °C) stratification. Based on patterns of change in physiological responses to temperature five types of non-deep PD can be distinguished (Baskin and Baskin 2004). Most seeds belong to type 1 or 2, in which the temperature range at which seed germination can occur increases gradually during the progression of non-deep dormancy release from low to higher (type 1, e.g. Arabidopsis thaliana) or from high to lower temperature (type 2, e.g. Helianthus annuus). In addition, the sensitivity of the seeds to light and GA increases as non-deep PD is progressively released. The molecular mechanisms of Arabidopsis seed dormancy cycling have been investigated by transcriptome analyses (Cadman et al. 2006). A model for the cycling of PD non-deep seeds is presented in the section below: "Induction, maintenance and release of physiological dormancy by plant hormones and environmental signals".
(2) Morphological dormancy (MD) MD
(dormancy class B according to Baskin and Baskin, 2004) is evident in seeds with embryos that are underdeveloped (in terms of size), but differentiated (e.g. into cotyledons and hypocotyl-radical). These embryos are not (physiologically) dormant, but simply need time to grow and germinate. This group does not include seeds with undifferentiated embryos. Example: Apium graveolens (Apiaceae).
(3) Morphophysiological dormancy (MPD) MPD (dormancy class C according to Baskin and Baskin, 2004) is also evident in seeds with underdeveloped (in terms of size) embryos, but in addition they have a physiological component to their dormancy. These seeds therefore require a dormancy-breaking treatment, e.g. a defined combination of warm and/or cold stratification which in some cases can be replaced by GA application. In MPD-seeds embryo growth/emergence requires a considerably longer period of time than in MD-seeds. Seeds with undifferentiated embryos like the Orchidaceae also have a morphological and a physiological component of dormancy, but they are not considered in this classification scheme (Baskin and Baskin 2004). Examples for MPD: Trollius (Ranunculaceae), Fraxinus excelsior (Oleaceae). There are eight known levels of MPD, based on the protocol for seed dormancy break and germination (see table below)
(4) Physical dormancy (PY) PY
(dormancy class D according to Baskin and Baskin, 2004) is caused by one or more water-impermeable layers of palisade cells in the seed or fruit coat (Baskin et al. 2000, Baskin, 2003, Baskin and Baskin 2004). In seeds PY-seeds, prevention of water uptake develops during maturation drying and the covering layer(s) control water movement (often associated with hardseededness). Seeds will remain dormant until some factor(s) render the covering layer(s) permeable to water. In nature, these factors include high temperatures, widely fluctuating temperatures, fire, drying, freezing/thawing and passage through the digestive tracts of animals. In seed technology, mechanical or chemical scarification can break PY dormancy. Once PY is broken, i.e. the seed or fruit coat becomes permeable to water, the seeds can germinate over a wide range of ambient conditions. Unlike PD-seeds, which may re-enter (secondary) dormancy after primary dormancy is broken, once the coat of PY-seeds becomes permeable it generally cannot revert to complete impermeability. Thus, the timing of dormancy break in nature is a more critical event in the life cycle of plants with PY, than it is in those with PD. The mechanism for PY-break must therefore be fine-tuned to the environment
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