In the physiological sense, germination begins with seed water uptake and ends with the initiation of elongation by the embryonic axis, usually the radicle. The driving forces and constraints on expansion by the embryo are examined, particularly for seeds in which the embryo is surrounded by endosperm and testa tissues that restrict growth. Models have been developed to predict germination based on thermal time, hydrotime and combined hydrothermal time. These population-based models indicate that the timing of germination is closely tied to physiologically determined temperature and water potential thresholds for radicle emergence which vary among individual seeds in a population. The restraint imposed by tissues surrounding the radicle is a major determinant of the threshold water potential. Enzymatic weakening of these tissues is a key event regulating the timing of radicle emergence. Considerable evidence suggests that endo-β-mannanase is involved in this process in a number of species, although it is doubtful that it is the sole determinant of when radicle emergence occurs. Molecular and biochemical studies are revealing the complexity of events occurring in endosperm and embryo cells associated with the completion of germination. Unique permeability properties and the presence of enzymes associated with pathogen resistance suggest additional functional roles for the tissues enclosing the embryo. The insights gained from physiology and modelling are being extended by the application of molecular techniques to identify and determine the function of genes expressed in association with germination. Single-seed assay methods, in vivo reporters, specific modification of gene expression and mutagenesis will be critical technologies for advancing our understanding of germination

The relationship between endosperm cap weakening and endo-β-mannanase activity during priming and the time to germination after priming was studied in tomato (Lycopersicon esculentum) seeds. During priming of seeds in −0.4 MPa PEG, the mechanical restraint of the endosperm cap decreased while the endo-β-mannanase activity in the endosperm cap increased. There was no decrease in required puncture force and no increase in endo-β-mannanase activity in seeds during priming in −1.0 MPa PEG. Two classes of seeds could be distinguished during priming in −0.7 MPa PEG: one with decreased required puncture force and one without. A strong correlation was found between the lowering of the mechanical restraint and endo-β-mannanase activity. It was concluded that individual seeds have to cross a threshold water potential in order to develop enzyme activity and lower their mechanical restraint. A decrease in required puncture force and increase in endo-β-mannanase activity correlated with ice crystal-induced porosity in the endosperm cap cell walls in scanning micrographs. It was presumed that ice crystal-induced porosity reflects cell-wall hydrolysis. Germination time after priming correlated positively with required puncture force during priming, depending on the osmotic potential. Seeds in −1.0 MPa PEG improved their time to germination, without a decrease in the required puncture force. Therefore it was concluded that lowering of the endosperm restraint during priming positively affects the time to germination of primed seeds but is not a prerequisite for rapid germination.

Meadow microcosms were established from seed on low-fertility soil of known seed bank composition, and subjected to manipulations of simulated grazing, cutting date, temperature and fertility for seven years. The composition and density of the seed bank was then determined in five 2-cm soil layers (0–2, 2–4, 4–6, 6–8 and 8–10 cm). The seed bank contained three distinct groups of species: species present in the original soil, sown species, and ‘others’. The seed bank was little affected by the experimental treatments, presumably because the sown species made only a small contribution to the seed bank. Nearly all the species in the original soil are known to possess persistent seed banks and had survived, although at reduced density, for seven years. Density of the most abundant species in this group, Sagina procumbens, had changed very little over seven years, confirming the well-documented longevity of the seeds of this species. Seeds of sown species made up only about a quarter of the seed bank, despite accounting for virtually all the above-ground vegetation. Of the sown meadow species, only Plantago lanceolata and Alopecurus pratensis were relatively abundant in the seed bank. These results strongly support the conclusion of other authors that most meadow species, once lost owing to the effects of fertilizers or inappropriate management, will not reestablish from the seed bank. Among species which were neither sown nor present in the original soil, the majority possessed adaptations for wind dispersal and had presumably dispersed into the experimental plots from outside. The most abundant member of this group, Betula pendula, had dispersed from a nearby tree. Density of Betula seeds declined sharply with depth, consistent with the view that seeds on the soil surface are rapidly lost, mainly through germination, but seeds that become buried survive much better. Seeds of Betula appear to be persistent but not particularly long-lived.

Seed dormancy is an important physiological stage in the life cycle of many seed-bearing plants. In an ever-changing environment, dormancy increases survival by distributing germination over time. In general, seed dormancy is a quantitative trait that is influenced by environmental factors. Depending on the species and accession, dormancy is controlled by nuclear factors, maternal factors or both, and dormancy can be genetically the dominant or recessive form of the trait. Wild oat (Avena fatua) is a well developed system for dormancy investigations. Dormancy in wild oat is regulated by three major genes as well as environmental factors, and dormancy is conditionally the recessive form of the trait. This paper summarizes current knowledge on genetic and environmental factors, and the interaction of these factors, on seed dormancy in several cereal grain and dicotyledonous species. Some applications of this knowledge to improve crops and manage weeds are outlined.

Advances in seed biology include progress in understanding the ecological significance of seed dormancy mechanisms. This knowledge is being used to make more accurate predictions of germination timing in the field. For several wild species whose seedlings establish in spring, seed populations show relevant variation that can be correlated with habitat conditions. Populations from severe winter sites, where the major risk to seedlings is frost, tend to have long chilling requirements or to germinate very slowly at low temperatures. Populations from warmer sites, where the major risk is drought, are non-dormant and germinate very rapidly under these same conditions. Seed populations from intermediate sites exhibit variation in dormancy levels, both among and within plants, which spreads germination across a considerable time period. For grasses that undergo dry after-ripening, seed dormancy loss can be successfully modelled using hydrothermal time. Dormancy loss for a seed population is associated with a progressive downward shift in the mean base water potential, i.e., the water potential below which half of the seeds will not germinate. Other parameters (hydrothermal time requirement, base temperature and standard deviation of base water potentials) tend to be constant through time. Simulation models for predicting dormancy loss in the field can be created by combining measurements of seed zone temperatures with equations that describe changes in mean base water potential as a function of temperature. Successful validation of these and other models demonstrates that equations based on laboratory data can be used to predict dormancy loss under widely fluctuating field conditions. Future progress may allow prediction of germination timing based on knowledge of intrinsic dormancy characteristics of a seed population and long-term weather patterns in the field.

Seed dormancy and germination of Senna marilandica and S. obtusifolia were compared in greenhouse and laboratory studies. About 90% of the S. obtusifolia seeds were green and had hard seed coat dormancy, whereas the other 10% were brown and nondormant. Seed-colour morphs did not occur in S. marilandica, and nearly 100% of the seeds had hard seed coat dormancy. Seeds of S. obtusifolia were significantly heavier than those of S. marilandica. Mechanical scarification was very effective in overcoming dormancy in seeds of both species. However, concentrated sulfuric acid, absolute ethanol and boiling water were less effective in breaking dormancy in seeds of S. marilandica than in those of S. obtusifolia. Further, incubating seeds at 30/15 to 40/25°C and dry-heat treatments at 80–100°C were ineffective in breaking dormancy in S. marilandica, but significantly increased germination percentages in S. obtusifolia. In neither species were simulated daily/seasonal temperature shifts effective in breaking dormancy. Scarified seeds of both species germinated over a wide range of temperatures in both light and darkness. Under near-natural temperature conditions, seeds of S. marilandica germinated in spring only, whereas those of S. obtusifolia emerged in late spring and throughout summer. Both species can form a long-lived seed bank. Dormancy break by high field temperatures in seeds of S. obtusifolia allows this species to germinate throughout the warm growing season and thus contributes to its success as a weed in arable crops.

Reversible phosphorylation is catalysed by protein kinases that transfer the γ-phosphate from ATP to amino acid residues of proteins. The process can be reversed by protein phosphatases. Phosphorylation can dramatically activate or inhibit enzymes and affect protein-protein interaction. Through phosphorylation protein kinases can amplify and propagate cellular signals. In plants and now seeds, protein kinases involved in hormone, defence and environmental stress responses are being identified. Increasingly, these protein kinases are being cloned and characterized, demonstrating the major role of reversible protein phosphorylation in seeds.

This study examines whether post-dispersal seed predators could be an important selective force in determining the seed bank strategies of grassland plants. It tests the hypothesis that species with persistent seed banks should sustain proportionally less predation of buried seeds than species which have transient seed banks and that this should be true irrespective of seed size. Results are drawn from a field experiment examining the relative susceptibility of surface versus buried seeds for 19 herbaceous taxa exhibiting different degrees of seed bank persistence. The data were consistent with the hypothesis that seed predators (rodents) influence the seed bank characteristics of seeds. Rodents removed proportionally more large seeds than small seeds and removed a smaller proportion of seeds with persistent rather than transient seed banks, independently of seed size. On average, burial reduced seed removal by almost 50%. The decrease in rates of seed removal following burial was marked for seeds with persistent seed banks but negligible for seeds with transient seed banks. Herbaceous plants with relatively large seeds (seed mass > 1 mg) that form persistent seed banks were either completely avoided or only consumed in small quantities by rodents. In contrast, large-seeded species with transient seed banks suffer high rates of seed predation. Models of life-history evolution predict trade-offs between seed dormancy and seed mass since dormancy and seed size are correlated traits that both reduce risk in variable environments and thus will show patterns of negative covariation. This paper presents an alternative explanation for this trade-off based on experimental evidence of a negative relationship between seed bank persistence and predation risk.

Lolium multiflorum Lam., a winter annual grass species originating from the Mediterranean region, has become widespread in native grasslands of the flooding pampa (Argentina). The dynamics of dormancy release of L. multiflorum seeds in simulated natural conditions was studied. Dormancy was overcome by exposing imbibed seeds to mild cool temperatures (15°C or below) for brief periods (pre-chilling). After-ripening progressively released dormancy, at a higher rate in seeds that remained under the canopy (30/20°C) than those that were dry-stored (20°C). Germination was promoted by red light only when the after-ripening requirement was partially fulfilled or after pre-chilling treatment. These results provide useful knowledge for designing grazing strategies aimed at obtaining well-developed ryegrass canopy of high forage quality during winter and spring.

Computer software called WeedCast was developed to simulate weed seed dormancy, timing of seedling emergence, and seedling height growth in crop environments in real-time and using actual or forecasted weather data. Weather data include daily rainfall and minimum and maximum air temperatures. Air temperatures are converted to average daily soil temperature at 5-cm soil depth using a series of equations that are specific for soil type, tillage system and previous year's crop-residue type. Daily rainfall and soil temperature estimates are combined to determine soil water potential (in megapascals) at 5-cm depth. Daily estimated soil water potential or soil temperatures are matched to empirically-derived threshold values that induce secondary dormancy in seeds of certain species. Soil growing degree days (GDD), calculated from soil temperatures, are used to project maximum emergence rates of weed seedlings. Emergence ceases on days when soil water potential falls below threshold values specific to each species. GDD based on air temperatures are used to estimate post-emergence seedling height growth. All three types of simulation provide information that allows users to answer important weed management questions in real-time. These types of questions include but are not limited to the following: (1) Are soil-applied treatments necessary? (2) How late can pre-emergence herbicides be applied? (3) When should mechanical control be implemented? (4) When should field-scouting commence and end? (5) When should post-emergence herbicides be applied?

Anhydrobiosis, or life without water, is the remarkable ability of certain types of plants and animals to survive almost total dehydration. This phenomenon requires a coordinated series of events within the cells of anhydrobiotes that protect their cellular components, particularly proteins and lipid membranes, from damage caused by the removal of water. Much of what is now understood about preserving biological samples during drying was learned by studying naturally desiccation-tolerant organisms and extended using model systems such as phospholipid vesicles. Most anhydrobiotic organisms accumulate disaccharides in their cells and tissues during the dehydration process. These carbohydrates, usually sucrose or trehalose, satisfy two criteria that appear to be necessary for protecting membranes during desiccation and during storage in the dry state. These requirements include: (1) depression of the gel-to-liquid crystalline phase transition temperature (Tm) in the dehydrated lipid to a temperature at or near that of the hydrated lipid, a process that appears to require a direct interaction between the carbohydrates and the lipid molecules of the membrane; and (2) formation of a carbohydrate glass with a relatively high glass transition temperature, leading to inhibition of fusion between the vesicles.

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.

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.