Secondary dormancy is predominantly associated with seed behaviour in soil seed banks. Periodic changes in secondary dormancy may explain seasonal emergence of weedy species. Temperature and possibly soil water potential appear to be the predominant factors that determine the annual cycling of dormancy. Dormancy cycling is paralleled by modulations of germination responsiveness to germination stimulants, such as light and nitrate and to the width of the germination temperature window. Membranes have been proposed to be the primary target for the perception of temperature. It is hypothesized that alterations in properties of cellular membranes are involved in the regulation of dormancy. Possible mechanisms are discussed.

Wheat and barley seeds were subjected to accelerated aging conditions at 12% moisture content and 35°C temperature over 28 days of storage, and to heat-shock treatment carried out by 4 h of incubation at 40°C following 16 h of imbibition at 20°C. Heat-treated, aged seeds showed altered germination behaviour and increasing leakage electroconductivity as well as a reduced incorporation of [35S]-methionine into embryo proteins, in comparison with the corresponding untreated seeds. Two-dimensional electrophoresis of labelled proteins from embryos gave evidence of further quantitative and qualitative changes: (a) ‘normal’ germination protein synthesis in wheat and barley was slightly modified by age, but following heat-shock treatment a general reduction of most of the control polypeptides occurred; (b) heat-shock response resulted in the production of several HSPs with different MW and pl, but a uniform general decline in their synthesis was not observed. Highly labelled HSPs (e.g., those with MW 66.7 or 89.3–66.7 kDa in wheat or barley, respectively) did not change over the entire period of aging, while some smaller HSPs with MW >29 kDa either decreased in intensity or disappeared. Conversely, specific low molecular weight HSPs (MW 17–14.2 kDa) were synthesized more in extremely aged embryos of both species. The hypothesis that these polypeptides may be used as biochemical markers in monitoring both vigour loss and heat-shock induced tolerance in aged seeds is discussed.

Artificial aging of wheat grains (Triticum aestivum cv. Mercia) for 3 d resulted in the gradual loss of seed vigour without loss of viability, but the latter decreased rapidly over the next 24 h. Loss of vigour was expressed as a delay in germination i.e. an increasing ‘lag’ period with aging before the onset of radicle emergence, but the subsequent rate of seedling growth was not affected. There was an overall decrease in the incorporation of 35S-methionine into protein in aged embryos, and associated with the aging-induced delay in the completion of germination (i.e. start of radicle emergence) there was a delay in the production of certain polypeptides. In embryos of both non-aged and aged seeds these polypeptides appeared just prior to radicle emergence but several of these were never synthesized in embryos from non-viable seeds. The synthesis of mRNA coding for in vitro translated polypeptides, or groups of polypeptides, was similarly delayed in lowvigour embryos. Furthermore, some polypeptides were identified which were induced by the accelerated aging treatment. It is concluded that a reduced level of protein and mRNA synthesis in the embryos of low-vigour seeds may result in the delay in the accumulation of specific of mRNAs and polypeptides which are needed for the normal completion of germination, i.e. radicle emergence.

Prechilling seeds of Sitka spruce (Picea sitchensis [Bong.] Carr.) at 4°C with 30% moisture content for 12–14 weeks (84–98 d) removed conditional dormancy (i.e. they were then able to germinate at 10°C). The non-dormant status was preserved after redrying to 6% moisture content. However, conditional dormancy was gradually reimposed during subsequent air-dry storage at 4°C and 6% seed moisture content in all five seed lots tested. Further investigations with one seed lot showed that reimposition was reversed by a second prechill treatment, but was reimposed again during subsequent air-dry storage. The trend of dormancy reimposition within seed lots over time was quantified by negative exponential relations between ability to germinate at 10°C and duration of air-dry storage. The progress of dormancy reimposition was influenced by seed storage moisture content and was most rapid at 4–10%. At higher moisture contents (15 and 20%) the rate of the reimposition of conditional dormancy was much reduced, while at moisture contents of 25 and 30% further loss in dormancy occurred. Thus it is clear that dormancy reimposition occurred during storage at low water potential rather than solely during desiccation from high to low water potential.

The loss of percentage germination and vigour of non-sprouted and sprouted seeds of winter wheat was determined during storage for 80 days at 11 different combinations of moisture content and temperature. Before storage the initial percentage germination and vigour were reduced according to the level of sprouting. During storage the percentage germination and vigour decreased. The rate of germination loss increased with increasing seed moisture content and/or increasing temperature. The species-specific constants KE, CW, CH and CQ of the viability equation were not different for non-sprouted and sprouted seeds. Likewise, the relation between the percentage of normal and abnormal seedlings was independent of sprouting. The rate of vigour loss was also independent of the storage conditions and of sprouting.

The endogenous ABA contents of dormant and nondormant barley grains were determined following application of different compounds to break dormancy. The chemicals used for breaking of dormancy in intact dormant grains were weak and strong acids, alcohols, hydrogen peroxide, cyanide, nitrate, salicylic acid, gibberellic acid and fusicoccin. The dormancy-breaking compounds could be classified into two major groups: compounds that caused a decrease in endogenous ABA (class I) and compounds which did not affect endogenous ABA (class II). Class I compounds included gibberellic acid, ethanol, hydrogen peroxide, nitrate, salicylic acid; class II compounds were fusicoccin, acid (H2SO4), sodium azide, n-caproic acid. In addition, these dormancy-breaking compounds were able to stimulate the germination rate when applied to embryos isolated from dormant grains. The concentrations necessary for stimulation of germination of isolated embryos were much lower than the concentrations for breaking the dormancy of intact grains. After embryos were isolated from dormant grains and incubated in water, ABA was determined in both embryos and in the incubation media. The class I compounds stated above also reduced ABA content in the incubation medium of isolated embryos, while class II compounds had no effect on ABA content of the medium. External application of ABA could overcome the effect of dormancy-breaking compounds of class I but not of class II. The results suggest that in the presence of the agents belonging to class II, ABA responsiveness of isolated embryos from dormant grains is decreased, compared to nontreated embryos.

Capeweed (Arctotheca calendula) seeds were found to be dormant at harvest. Effects of duration and temperature of storage under ‘laboratory’ and ‘natural’ conditions, growth regulators, stratification and age of seeds, were studied on the germination of dormant seeds. Three factors imposed on seeds were found to promote germination of capeweed: (i) allowing seed to age, either in storage, buried in soil or during stratification (germination of 18-month-old seeds was up to 60% higher than that of fresh seeds); (ii) the presence of light (the average germination percentage of seeds exposed to light during storage was 3.2-fold greater than that of seed stored in the dark); and (iii) the application of growth regulators, particularly gibberellic acid, enhanced by scarification (GA3 and ethephon promoted germination by up to 58% when applied to scarified seeds). Other factors, e.g. temperature and depth of storage, influenced germination but were less critical to its success. These results provide a preliminary assessment of the importance of these factors in controlling dormancy in seeds of capeweed.

The papers in this special section of Seed Science Research are products of a symposium on Seed Biology and Technology: Applications and Advances, held in Fort Collins, Colorado, on 13–16 August, 1997. The symposium was convened as a cooperative effort of Regional Research Project W-168 within the United States Department of Agriculture (USDA) Cooperative States Research, Extension and Education Service (CSREES) system. Regional Research Projects are authorized by the Hatch Act, which established the Agricultural Experiment Station (AES) system in the United States (US Code). This is a system in which land-grant institutions in each state conduct research and education programmes relevant to agriculture, the environment and society. Regional Research projects are a mechanism ‘for cooperative research in which two or more State agricultural experiment stations are cooperating to solve problems that concern the agriculture of more than one state.’ Such projects ‘can provide the solution to a problem of fundamental importance or fill an important gap in our knowledge from the standpoint of the present and future agriculture of the region’

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

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.

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.

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.

Seeds of the common bean, Phaseolus vulgaris, contain two inhibitors of mammalian and insect α-amylases (αAls) that show specificity towards the amylases of different insect species. Expression in pea (Pisum sativum) and azuki bean (Vigna angularis) of a chimeric gene consisting of the cDNA of bean αAl-1 and a seed-specific promoter makes the seeds of these legumes resistant to three species of Old World bruchids whose amylases are inhibited by αAl-1. This was the first successful genetic engineering of insect resistance in seeds. To understand the basis of the specificity between amylases and inhibitors we cloned a second bean inhibitor (αAl-2) with different specificity, and we cloned the cDNA of the New World bruchid, Zabrotes subfasciatus. The amylase of this bruchid is inhibited by αAl-2, but not by αAl-1. Knowledge of the amino acid sequences and of the three-dimensional structure of the pancreatic α-amylase–αAl-1 complex allows us to predict the peptide domains and amino acids of the proteins that are important for protein–protein recognition and inhibition of enzyme activity.

Growers expect high-quality, genetically pure seed. As a result, seed companies maintain quality control programmes that monitor seed from harvest to purchase. An array of ‘traditional’ seed quality tests, including mechanical tests and tests of genetic purity, seed germination and vigour, and seed health tests, is used, and seed quality assessment techniques continue to evolve. Advances in molecular genetics are allowing the release of new varieties differing essentially in one gene. New molecular biology approaches offer the potential to identify these subtle genetic differences. Advances in seed enhancements, such as pelleting, priming and pregermination, require increased scrutiny of seed quality before and after the enhancement process. New developments in computer imaging for improved purity and germination/vigour analyses are being developed. These novel approaches to seed quality assessment become important as new genetic improvements are conveyed in the seed at increased cost to the grower.