The proposal is made that seed scientists need an internationally acceptable hierarchical system of classification for seed dormancy. Further, we suggest that a modified version of the scheme of the Russian seed physiologist Marianna G. Nikolaeva be adopted. The modified system includes three hierarchical layers – class, level and type; thus, a class may contain levels and types, and a level may contain only types. The system includes five classes of dormancy: physiological dormancy (PD), morphological dormancy (MD), morphophysiological dormancy (MPD), physical dormancy (PY) and combinational dormancy (PY + PD). The most extensive classification schemes are for PD, which contains three levels and five types (in the non-deep level), and MPD, which contains eight levels but no types. PY is not subdivided at all but probably should be, for reasons given. Justifications are presented for not including mechanical dormancy or chemical dormancy in the modified scheme. PD (non-deep level) is the most common kind of dormancy, and occurs in gymnosperms (Coniferales, Gnetales) and in all major clades of angiosperms. Since, first, this is the class and level of dormancy in seeds of wild populations of Arabidopsis thaliana and, secondly, Type 1 (to which seeds of A. thaliana belong) is also common, and geographically and phylogenetically widespread, it seems that biochemical, molecular and genetic studies on seed dormancy in this model species might have rather broad application in explaining the basic mechanism(s) of physiological dormancy in seeds.

Active oxygen species (AOS) are involved in various aspects of seed physiology. Their generation, which occurs during seed desiccation, germination and ageing, may lead to oxidative stress and cellular damage, resulting in seed deterioration. However, cells are endowed with detoxifying enzymes and antioxidant compounds that scavenge AOS and participate in seed survival. The detoxifying mechanisms play a key role in acquisition of desiccation tolerance of developing seeds, completion of seed germination and seed storability. However, AOS must also be regarded as molecules intervening in cellular signalling. They are involved in growth processes occurring at early embryogenesis during seed development, and participate in the mechanisms underlying radicle protrusion during seed germination. AOS might also have a regulatory function in the changes in gene expression during seed development, dormancy and germination. Their interplay with other molecules, particularly with hormones such as abscisic acid, suggests that they should be considered as key components of an integrated signalling network involved in many aspects of seed physiology.

This review focuses mainly on eudicot seeds, and on the interactions between abscisic acid (ABA), gibberellins (GA), ethylene, brassinosteroids (BR), auxin and cytokinins in regulating the interconnected molecular processes that control dormancy release and germination. Signal transduction pathways, mediated by environmental and hormonal signals, regulate gene expression in seeds. Seed dormancy release and germination of species with coat dormancy is determined by the balance of forces between the growth potential of the embryo and the constraint exerted by the covering layers, e.g. testa and endosperm. Recent progress in the field of seed biology has been greatly aided by molecular approaches utilizing mutant and transgenic seeds of Arabidopsis thaliana and the Solanaceae model systems, tomato and tobacco, which are altered in hormone biology. ABA is a positive regulator of dormancy induction and most likely also maintenance, while it is a negative regulator of germination. GA releases dormancy, promotes germination and counteracts ABA effects. Ethylene and BR promote seed germination and also counteract ABA effects. We present an integrated view of the molecular genetics, physiology and biochemistry used to unravel how hormones control seed dormancy release and germination.

Abscisic acid (ABA) is a plant hormone that regulates seed dormancy and germination. Seeds undergo changes in both ABA content and sensitivity during seed development and germination in response to internal and external cues. Recent advances in functional genomics have revealed the integral components involved in ABA metabolism (biosynthesis and catabolism) and perception, the core signalling pathway, as well as the factors that trigger ABA-mediated transcription. These allow for comparative studies to be conducted on seeds under different environmental conditions and from different genetic backgrounds. This review summarizes our understanding of the control of ABA content and the responsiveness of seeds to afterripening, light, high temperature and nitrate, with a focus on which tissues are involved in its metabolism and signalling. Also described are the regulators of ABA metabolism and signalling, which potentially act as the node for hormone crosstalk. Integration of such knowledge into the complex and diverse events occurring during seed germination will be the next challenge, which will allow for a clearer understanding of the role of ABA.

A very important mineral storage compound in seeds is phytate, a mixed cation salt of phytic acid (myo-inositol hexakis phosphoric acid). This compound is important for several reasons: (1) It is vital for seed/grain development and successful seedling growth. (2) It is often considered to be an antinutritional substance in human diets, but it may have a positivenutritional role as an anti-oxidant and an anti-cancer agent. (3) It represents a very significant amount of phosphorus being extracted from soilsand subsequently removed with the crop. (4) It plays a role in eutrophication of waterways. A key part of this review is an estimate of the annualtonnage of phosphorus and phytic acid sequestered in up to 4.1 billion metric tonnes of crop seeds and fruits globally each year. We estimate thatnearly 35 million metric tonnes of phytic acid, containing 9.9 million metric tonnes of P, is combined with about 12.5 and 3.9 million metric tonnes of K and Mg respectively, to form each year over 51 million metric tonnes of phytate. The amount of P inthis phytate is equal to nearly 65÷ of the elemental P sold world wide for use in mineral fertilizers. Dry cereal grains account for 69÷ of the total crop seed/fruit production but account for 77÷ of the total phytic acid stored each year. Low phytate mutants, that are now available for some key staple food crops such as maize and barley, offer potential benefits in such areas as the sustainability of lands used to grow crops, the mineral nutrition of humans and animals, and reduction in pollution of waterways.

Seeds of different species are believed to have characteristic shelf lives, although data confirming this are scarce, and a mechanistic understanding of why this should be remains elusive. We have quantified storage performance of c. 42,000 seed accessions, representing 276 species, within the USDA National Plant Germplasm System (NPGS) collection, as well as a smaller experiment of 207 cultivars from 42 species. Accessions from the NPGS collection were harvested between 1934 and 1975, and had relatively high initial germination percentages that decreased at a variable rate during storage at both 5 and –18°C. Germination time courses, which represent the average performance of the species, were fitted to Avrami kinetics, to calculate the time at which germination characteristically declined to 50% (P50). These P50 values correlated with other longevity surveys reported in the literature for seeds stored under controlled conditions, but there was no correlation among these studies and seed persistence observed in the classic buried seed experiment by Duvel. Some plant families had characteristically short-lived (e.g. Apiaceae and Brassicaceae) or long-lived (e.g. Malvaceae and Chenopodiaceae) seeds. Also, seeds from species that originated from particular localities had characteristically short (e.g. Europe) or long (e.g. South Asia and Australia) shelf lives. However, there appeared to be no correlation between longevity and dry matter reserves, soluble carbohydrates and parameters relating to soil persistence or resource allocation. Although data from this survey support the hypothesis that some species tend to survive longer than others in a genebank environment, there is little information on the attributes of the seed that affect its storage performance.

The growing conditions of a parent plant may affect the degree of dormancy of its seeds. This has been demonstrated in numerous species, both wild and cultivated. The evidence comes from field observations and controlled experiments. A survey of the literature shows that some well defined patterns emerge, with certain environmental factors tending to have similar effects over a wide range of species. Lower dormancy (i.e., increased germinability) is generally associated with the following environmental conditions during seed development: high temperatures, short days, red light, drought and high nitrogen levels. The effects are probably the result of changes in the quantity, mobility or activity of growth substances such as abscisic acid. The ecological implications of the phenomenon are briefly discussed.

When seeds deteriorate, they lose vigour and become more sensitive to stresses upon germination. Eventually seeds lose the ability to germinate. The factors which determine the rate of this ‘aging’ are the temperature and moisture content at which seeds are stored and an ill-defined parameter, seed quality. While it has been known for many years that manipulation of these factors influences the longevity of seeds, the precise interactions among them are so poorly understood as to preclude the prediction of longevity for a particular seed lot. Concepts from studies of materials and food stability can be applied to seed aging research, and this may help us take a more integrative approach to understanding the kinetics of seed deterioration. These concepts describe the physical environment of the seed matrix in response to changing water contents and temperature. Water activity models describe the state of water in the seed, while the glass models describe the state of the aqueous solution. Both models presume that changes of state affect the nature and kinetics of chemical reactions. Thus, the physical and chemical environment within the seed are inextricably linked.

A suite of mechanisms or processes that together have been implicated in the acquisition and maintenance of desiccation tolerance in orthodox seeds is discussed in the context of the behaviour of desiccation-sensitive seeds, and where appropriate, parallels are drawn with the situation in vegetative plant tissues that tolerate dehydration. Factors included are: physical characteristics of cells and intracellular constituents; insoluble reserve accumulation; intracellular de-differentiation; metabolic ‘switching off’; presence, and efficient operation, of antioxidant systems; accumulation of putatively protective substances including LEAs, sucrose and other oligosaccharides, as well as amphipathic molecules; the presence and role of oleosins; and the presence and operation of repair systems during rehydration. The variable response to dehydration shown by desiccation-sensitive seeds is considered in terms of the absence or incomplete expression of this suite of mechanisms or processes.

Three categories of damage are envisaged: (i) reduction in cell volume which can lead to mechanical damage; (ii) aqueous-based degradative processes, probably consequent upon deranged metabolism at intermediate water contents. This is termed ‘metabolism-induced damage’ and its extent will depend upon the metabolic rate and the rate of dehydration; and (iii) the removal of water intimately associated with macromolecular surfaces leading to denaturation: this is referred to as desiccation damage sensu stricto. The effects of drying rate and the maturity status of seeds are considered in relation to the responses to dehydration, leading to the conclusion that the concept of critical water contents on a species basis is inappropriate. Viewing seed postharvest physiology in terms of a continuum of behaviour is considered to be more realistic than attempting precise categorization.

Rapid dehydration of excised embryonic axes (or other explants) from desiccation-sensitive seeds permits retention of viability (in the short term) to water contents approaching the level of non-freezable water. This opens up the possibility of long-term conservation, by cryopreservation techniques, of the genetic resources of species producing non-orthodox seeds.

Seed enhancements may be defined as post-harvest treatments that improve germination or seedling growth, or facilitate the delivery of seeds and other materials required at the time of sowing. This definition includes three general areas of enhancements: pre-sowing hydration treatments (priming), coating technologies and seed conditioning. Pre-sowing hydration treatments include non-controlled water uptake systems (methods in which water is freely available and not restricted by the environment) and controlled systems (methods that regulate seed moisture content preventing the completion of germination). Three techniques are used for controlled water uptake: priming with solutions or with solid particulate systems or by controlled hydration with water. These priming techniques will be discussed in this paper with reference to methodology, protocol optimization, drying and storage. Coating technologies include pelleting and film coating, and coatings may serve as delivery systems. Seed conditioning equipment upgrades seed quality by physical criteria. Integration of these methods can be performed, and a system is described to upgrade seed quality in Brassica that combines hydration, coating and conditioning. Upgrading is achieved by detecting sinapine leakage from nonviable seeds in a coating material surrounding the seeds. Seed-coat permeability directly influences leakage rate, and seeds of many species have a semipermeable layer. The semipermeable layer restricts solute diffusion through the seed coat, while water movement is not impeded. Opportunities for future seed enhancement research and development are highlighted.

The emphasis of modern dormancy research is almost entirely on the form of dormancy that is acquired during seed development, primary dormancy. Abscisic acid (ABA) appears to be intimately involved in its regulation. The action of abscisic acid has also been implied in many other developmental processes. The coincidence of developmental events, such as dehydration and completion of maturation, with the acquisition of primary dormancy suggests that dormancy is influenced by these processes. Germinability, both during development and after maturation, is sometimes directly correlated with ABA content. The lack of such a correlation may be explained by assuming a decisive role for the responsiveness to ABA or other overriding factors. ABA has been detected in all seed components. The different seed tissues may all contribute, to various extents, to the degree of whole seed dormancy. It is concluded that ABA action in dormancy regulation is not restricted to the embryo but is also located in endospermic tissue. In addition, a role of ABA in the morphological development of germination modifying seed tissues is proposed. The mechanism for ABA action appears to be associated with cell wall properties.

Stachyose, raffinose and related flatulence-producing oligosaccharides (α-galactosyl derivatives of sucrose) are associated with desiccation tolerance and storability of seed germplasm. Orthodox seeds of species with a sucrose-to-oligosaccharide ratio of <1.0 have storability half-viability periods >10 years while those >1.0 have a storability half-viability period <10 years. Seeds vary in their composition of oligosaccharides and some accumulate α-galactosyl derivatives of cyclitols. Known and proposed pathways for biosynthesis of soluble oligosaccharides, cyclitols and galactosyl derivatives of cyclitols are presented. Axes, cotyledons, embryos or seeds of 19 species in 7 families (all orthodox seeds) were analysed for sucrose, galactosyl derivatives of sucrose, cyclitols and galactosyl derivatives of cyclitols by high resolution gas chromatography. Sucrose and myo-inositol are universally present and galactinol is present in seeds accumulating stachyose series oligosaccharides. Seeds of some species of Leguminosae accumulate mostly stachyose series oligosaccharides, whereas seeds of other species accumulate varying levels of galactosyl derivatives of cyclitols in addition. Castor bean (Euphorbiaceae) seeds accumulate galactinol and buckwheat (Polygonaceae) embryos accumulate galacto-chiro-inositol instead of the stachyose series oligosaccharides. The mass ratio of sucrose:non-sucrose is related to storability and is applicable to seeds accumulating cyclitol derivatives. Galactinol and galacto-chiro-inositol are proposed to function in the same role as raffinose and stachyose in facilitating desiccation tolerance and storability.

The origin and significance of free radicals is described in the broad setting of animal and plant biology and in the specific context of seeds and seed viability. Evidence is given that free radicals play a central if not causal role in promoting molecular damage under the widest range of environmental stresses and induced ageing in mature plant tissuesIn ungerminated seeds, the evidence is less certain. The reason, we argue, is that many attempts to measure free radical processes in seeds are set not against gradients of damage or ageing (as they are in most other biological tissues) but against one of only two options, either seed germinability or mortality. Because free radical reactions differ quantitatively and qualitatively in living and dead tissues attempts to correlate radicalmediated damage with the viability of a population of seeds may be unreliable particularly when measurements are made at uncertain intervals after death. Despite this, the evidence that seeds, uniquely, are exempt from the ravages of oxygen assault is weak. Instead it ismore probable that oxygen plays a central role in seed mortality and may have significance in the evolution of seed persistence.

Soluble carbohydrates are one of multiple components required for the acquisition of desiccation tolerance during seed development and maturation. Sucrose and the raffinose series of oligosaccharides have been extensively studied in relation to seed desiccation tolerance. These galactosyl sucrose oligosaccharides are present in viable tissues of many edible seeds, especially the legumes, and contribute to flatulence after ingestion. A reduction in oligosaccharides of the raffinose series is desired by nutritionists but, if present at less than a threshold level, this this may result in reduced desiccation tolerance and storability of seeds. Some seeds that have very low amounts of raffinose and stachyose accumulate galactosyl cyclitols and small amounts of free cyclitols. Galactosyl cyclitols present in various seeds include galactosyl or methyl derivatives of myo-inositol, D-pinitol, D-chiro-inositol, D-ononitol, and scyllo-inositol. Castor bean seeds accumulate galactinol and buck-wheat seeds accumulate fagopyritol in addition to sucrose. Fagopyritol accumulation is associated with the acquisition of desiccation tolerance in buckwheat seeds. The galactosyl sucrose and galactosyl cyclitol soluble carbohydrates in maturing seeds have been proposed to have various roles in desiccation tolerance including non-toxic and non-reducing forms of seed storage products and intracellular osmotic agents contributing to the structural stability of organelles, membranes, enzymes and proteins, other macromolecules, and the glassy state. Ether derivatives of cyclitols may form liquid crystals. Methyl ether derivatives of cyclitols may have roles as cryoprotectants, desiccation protectants, and hydroxyl radical scavengers.

Desiccation tolerance is one of the most fundamental properties of seeds. It is acquired late in seed development and is considered necessary for the completion of the plant's life cycle, as an adaptive strategy to enable seed survival during storage or environmental stress, and to ensure better dissemination of the species. The role of water status in desiccated tissues and problems related to testing tolerance in seeds are reviewed. The molecular mechanisms of desiccation tolerance has received extensive consideration only during this past decade. There is a general consensus that desiccation tolerance involves the protection of cellular membranes from the deleterious effect of water removal and the resultant necessity to maintain the bilayer structure in the absence of an aqueous environment. Therefore, some aspects of desiccation-induced membrane injury are described. Several strategies for coping with cellular desiccation have been identified the presence of high amounts of non-reducing sugars, the efficiency of free radical-scavenging systems and the expression of desiccation- and/or ABA-regulated genes. These molecular mechanisms allowing cellular protection are reviewed together with their respective role in dessication tolerance. It is concluded that desiccation tolerance is not likely to be ascribed to a single mechanism but rather to a multifactorial property in which each component is equally critical.

The production of hydrophilic mucilages by the seed coat or pericarp, which are released upon seed hydration, is a commonly found adaptation in angiosperms, known as myxodiaspory. These are composed primarily of pectins and hemicelluloses that undergo substantive swelling upon hydration. Synthesized in the Golgi apparatus and secreted to an apoplastic space via secretory vesicles, mucilages can also contain cellulose microfibrils or cellulosic fibres that are synthesized at the plasma membrane in association with microtubules. Investigation of mucilage production in, and differentiation of, the mucilage secretory cells of the genetic model plant Arabidopsis thaliana has identified a number of regulatory genes and enzymes involved in pectin synthesis and secretion, in muro pectin modification and secondary cell wall synthesis. Studies of the role of mucilages in both a number of species and in Arabidopsis mutants affected in its production suggest that they have multiple ecological roles. These include facilitation of seed hydration, mediation of germination under waterlogged conditions, prevention of seed dispersal or predation by adherence to soil, and promotion of seed dispersal by attachment to animals. The precise role of mucilages appears to be dependent on species and their environmental context.

The control of seed germination by red and far-red light is one of the earliest documented phytochrome-mediated processes Phytochrome is now known to be a small family of photoreceptors whose apoproteins are encoded by different genes Phytochrome B (phyB) is present in dry seeds and affects germination of dark imbibed seeds but other phytochromes could also be involved Phytochrome A (phyA) appears after several hours of imbibition and mediates very-low-fluence responses PhyB and other phytochromes different from phyA mediate the classical low-fluence responses The phytochrome involved in high-irradiance responses of seed germination (inhibition of germination under continuous far-red) has not been unequivocally established, although phyA is the most likely candidate Phytochrome can affect embryo growth capacity and/or the constraint imposed by the tissues surrounding the embryo At least in some species, gibberellins participate in the signalling process In the field, phyA has been implicated in the perception of light during soil cultivations, and phyB would be involved in the perception of red/far-red ratios associated with the presence of gaps in the canopy This review describes recent advances in phytochrome research, particularly those derived from the analysis of germination in specific mutants, and their connection with traditional observations on phytochrome control of seed germination

β-1,3-Glucanase (βGlu) expression in seeds plays important roles in the regulation of seed germination, dormancy and in the defence against seed pathogens. A thick β-1,3-glucan layer is typical for the seed envelope of cucurbitaceous species, confers seed semipermeability and is degraded during germination. In many species with coat-imposed dormancy, the seed envelope confers a physical constraint to radicle emergence. In the solanaceous species, the micropylar endosperm and testa have this function, and endosperm weakening appears to be a prerequisite for germination. Class I βGlu is transcriptionally induced in the micropylar endosperm of tobacco, tomato and other solanaceous seeds just prior to radicle emergence. βGlu induction and germination are tightly linked in response to plant hormones and environmental factors, e.g. they are both promoted by gibberellins and inhibited by abscisic acid (ABA). Sense and antisense transformation of tobacco reveals two sites of βGlu action: after-ripening-mediated release of testa-imposed dormancy and endosperm rupture during germination. The use of an ABA-inducible chimeric sense-transgene resulted in overexpression of class I βGlu in seeds and provided direct evidence that βGlu contributes to endosperm rupture. A model integrating βGlu, seed dormancy, after-ripening and germination is presented, and possible mechanisms for βGlu action are discussed. It is proposed that βGlu not only helps defend seeds against pathogens, but is also a key factor in regulating coat-imposed dormancy and germination of seeds in response to environmental and hormonal cues.

The strong mechanistic relationship between climatic factors and seed dormancy and germination suggests that forecast climatic changes will significantly affect seed bank persistence. This review focuses on the potential impact of changing temperature, rainfall and fire regimes on the longevity of long-term persistent seed-banks. Currently, there are few studies investigating the mechanistic responses of demographic processes, such as seed-bank dynamics, to forecast climate change. However, from the work that has been published, several key points have been highlighted. First, increased air temperatures will produce significantly higher soil temperatures in open and sparsely vegetated habitats. Some evidence shows that this could accelerate the decline of seed viability and compromise bet-hedging strategies of species in dryland regions. Second, changes to rainfall season may determine the relative success of recruitment, with lower levels of success producing net losses to seed bank longevity. Finally, higher temperatures are likely to produce increased fire frequency, compromising the persistence of plant populations dependent on long-lived seed banks. Improving our understanding of both the mechanistic response and adaptive capacity of seed banks to climate change will provide a solid basis for improved predictions of future species distributions and risk of extinction, particularly in ecosystems subjected to temporally stochastic disturbances. It is necessary to develop functional groups based on key life-history trait responses to changing environmental conditions, to enable broader-scale predictions of distribution and persistence in the future.

As in other cultivated species, dormancy can be seen as a problem in cereal production, either due to its short duration or to its long persistence. Indeed, cereal crops lacking enough dormancy at harvest can be exposed to pre-harvest sprouting damage, while a long-lasting dormancy can interfere with processes that rely on rapid germination, such as malting or the emergence of a uniform crop. Because the ancestors of cereal species evolved under very diverse environments worldwide, different mechanisms have arisen as a way of sensing an appropriate germination environment (a crucial factor for winter or summer annuals such as cereals). In addition, different species (and even different varieties within the same species) display diverse grain morphology, allowing some structures to impose dormancy in some cereals but not in others. As in seeds from many other species, the antagonism between the plant hormones abscisic acid and gibberellins is instrumental in cereal grains for the inception, expression, release and re-induction of dormancy. However, the way in which this antagonism operates is different for the various species and involves different molecular steps as regulatory sites. Environmental signals (i.e. temperature, light quality and quantity, oxygen levels) can modulate this hormonal control of dormancy differently, depending on the species. The practical implications of knowledge accumulated in this field are discussed.