What is seed dormancy
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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