In 2009, the bicentenary of Charles Darwin's birth on 12 February 1809 is being celebrated. For seed scientists, celebrations of the contributions of the great biologist should also mark his involvement with seeds. Darwin was interested in seeds, particularly in their role in dispersal and distribution of plant species over long distances. His studies of seeds, laid down in several books and articles, contributed to the development of his ideas on evolution and the distribution of living organisms on the planet. In this review, the place of seeds in Darwin's work is surveyed and it is shown how he referred to them to support and illustrate some of his most important ideas.

Historic collections of biological material are important genetic resources for taxonomic, evolutionary and historical research. In this paper we describe a seed collection dating from 1862 to 1918 maintained at the Swedish Museum of Cultural History. The collection contains over 3000 well-documented seed samples of various agricultural crops, mostly cereals. A subset of 100 samples divided over ten species frequently represented in the collection and a range of ages were tested for germinability and DNA preservation. None of these accessions were found to contain viable seeds. DNA extracted from the seeds was degraded, but the amount of degradation varied between species. DNA quality was evaluated by yield, fragment size and size of amplification product. Quality was highest for DNA extracted from Pisum sativum and Vicia sativa. DNA extracted from Brassica napus, Beta vulgaris and Trifolium pratense was more fragmented, and DNA extracted from Triticum aestivum, Secale sereale, Hordeum vulgare, Avena sativa and Phleum pratense was most degraded. Polymerase chain reaction (PCR) amplification of ribosomal DNA fragments of up to 700 bp was permitted for most samples in all species. To test whether single-copy nuclear genes could be amplified from the extracted DNA, microsatellite markers were used on the Pisum sativum and Hordeum vulgare samples. Polymorphisms of microsatellite markers were detected between samples for both species. The results show that the 19th-century seed collection can be utilized to infer genetic relationships among obsolete cultivars as well as for other types of genetic research based on sequence or marker analysis.

Previous experiments had shown that microgravity adversely affected seed development in Brassica rapa L. We tested the hypothesis that gravity controls seed development via modulation of gases around the developing seeds, by studying how hypergravity affects the silique microenvironment and seed development. Using an in vitro silique maturation system, we sampled internal silique gases for 16 d late in the seed maturation sequence at 4 g or 1 g. The carbon dioxide level was significantly higher inside the 4-g siliques, and the immature seeds became heavier than those maturing at 1 g. Pollination and early embryo development were also studied by growing whole plants at 2 g or 4 g for 16 d inside chambers mounted on a large-diameter centrifuge. Each day the rotor was briefly stopped to permit manual pollination of flowers, thereby producing cohorts of same-aged siliques for comparison with stationary control material. The loss of starch and soluble carbohydrates during seed development was accelerated in hypergravity, with seeds developing at 4 g more advanced by 2 d than those at 1 g. Seeds produced at 4 g contained more lipid than those at 1 g. Taken together, these results indicate that hypergravity enhances gas availability to the developing embryos. Gravity's role in seed development is of importance to the space programme because of the plan to use plants for food production and habitat regeneration in extraterrestrial settings. These results are significant because they underscore the tight co-regulation of Brassica seed development and the atmosphere maintained inside the siliques.

This study examined whether the restricted habitat preference of the spring-flowering woodland geophyte Anemone ranunculoides L., compared with that of A. nemorosa growing in the same woodlands in northern Italy, could be explained by subtle differences in germination preference and emergence phenology. Immediately after harvest, seeds of A. ranunculoides were either sown on agar in the laboratory under simulated seasonal temperatures or placed in nylon mesh sachets and buried in the wild. Embryos, undifferentiated at the time of seed dispersal, grew during summer in the laboratory and in the wild, culminating in radicle emergence in the autumn, when temperatures fell to c. 15°C. Shoot emergence was delayed under natural conditions until soil temperature had dropped further to c. 10°C. Compared with populations of the closely related Anemone nemorosa L. occupying the same woodland habitat, which have been reported to have non-dormant radicles, A. ranunculoides displayed a narrower temperature tolerance for radicle emergence and high levels of germination were possible only after prolonged exposure to summer conditions, indicating physiological dormancy. However, unlike A. nemorosa, shoot emergence in A. ranunculoides was not dependent on winter temperatures, suggesting weaker epicotyl morphophysiological dormancy. Under a regime of diurnal temperature alternation, simulating the microclimate where there is little plant cover, germination failed almost completely; this could explain the absence of A. ranunculoides in open habitats.

Although many studies have been conducted on seeds with a water-impermeable seed or fruit coat (physical dormancy), the primary site of water entry into these seeds after dormancy-breaking treatments is still controversial. Thus, the role of lens, hilum, micropyle and extrahilar regions in water uptake of seeds treated to break physical dormancy was examined in Vigna oblongifolia and Sesbania sesban (Fabaceae) following pretreatment with sulphuric acid and hot water. Morphology of seed surfaces in treated versus non-treated seeds of both species was examined with scanning electron microscopy. Most seeds of V. oblongifolia first cracked in the hilum when pretreated with sulphuric acid, but they cracked in both the hilum and extrahilar regions when pretreated with hot water. However, in S. sesban seeds, a crack formed only in the lens following either acid scarification or hot-water treatments, and the seeds imbibed water only through the lens. These results indicate that the primary site of water entry into seeds following physical dormancy break can vary with species and treatments. Slow, early imbibition via the hilum, and subsequent rapid imbibition via the lens, may not be detected unless seeds are monitored for several days. Time allowed for imbibition studies may, at least in part, explain various interpretations about the role of the lens in physical dormancy reported in the literature.

Cycling of physically dormant (PY) seeds between states insensitive and sensitive to dormancy-breaking factors in the environment has recently been demonstrated in Fabaceae and Convolvulaceae, and it may be a common phenomenon in seeds with water-impermeable seed coats. In contrast to seeds of many species with physiological dormancy (PD), those with PY cannot cycle between dormancy and non-dormancy (ND). In this paper, we evaluate the role of sensitivity cycling in controlling the timing of germination of seeds with PY in nature, and show that sensitivity cycling in seeds with PY serves the same ecological role as dormancy cycling in seeds with PD. Thus, sensitivity cycling in seeds with PY ensures that germination in nature occurs only at (a) time(s) of the year when environmental conditions for growth are, and are likely to remain, suitable long enough for the plant to complete its life cycle or to form a perennating structure. Further, we describe the experimental procedures necessary to determine whether sensitivity cycling is occurring, and discuss briefly the possible relevance of sensitivity cycling to dormancy classification.

Eleven sunflower (Helianthus annuus L.) inbred lines were evaluated in field and laboratory studies under drought and irrigation. In the field, lines B59, R419 and B67 had reduced seed and oil yield under drought, while no reduction was observed for R432, HAR4 and B71. Lines HA89, R415, R049, RHA274 and R423 presented intermediate responses. In laboratory tests, seeds of line B59 had reduced germination percentages at 200 and 400 mM mannitol, while germination of seeds of lines R432, B71, HAR4, RHA274 and HA89 was reduced only at 400 mM mannitol. Drought-sensitive B59 and drought-tolerant B71 grown under irrigation and drought conditions in the field were selected for hormone assays. Abscisic acid (ABA) and its catabolites in pericarp, embryonic axis and cotyledons of dry and germinated seeds of B59 and B71 were determined. ABA was the major component of the pericarp of dry seeds from B71 and B59 plants grown under drought. The embryonic axis of B71 dry seeds from drought-grown plants also showed high ABA content. The major findings from this study are: (1) the drought-sensitive and -tolerant lines exhibited different ABA and catabolite profiles; (2) water environment during maternal plant growth affected ABA content and the composition of catabolites in mature and germinated seeds; (3) ABA content did not affect germination performance in our conditions; and (4) the dry and germinated seed parts showed different ABA and catabolite profiles.

The soil moisture regime may affect dormancy of seeds and their sensitivity to signals that promote germination. We studied the effect of moisture regime on the sensitivity to light of dormant Bromus catharticus seeds, and on the emergence pattern of seedlings. Seeds were incubated under continuously hydrated, continuously dehydrated, or fluctuating moisture regimes in a controlled environment (25°C, darkness) for 2 months. After moisture treatments, seeds were exposed to red or far-red light pulses, or to darkness, to determine germinability. In addition, grassland mesocosms with intact seed bank and vegetation were irrigated or subjected to a drought regime in a glasshouse at summer temperatures. After 2 months, the temperature was reduced to correspond to grassland temperatures in autumn; the canopy was removed and half of the mesocosms were covered with filters that exclude red light. Density of B. catharticus seedlings was evaluated after 2 weeks. Dormancy decreased in continuously hydrated seeds but they still required red light for germination. In contrast, an important fraction of seeds that experienced continuously dehydrated or fluctuating moisture regimes germinated in darkness or after far-red light pulses. In the mesocosms that had experienced a soil drought, a higher density of seedlings emerged in the absence of red light than in the daily irrigated mesocosms. This indicates that a fraction of B. catharticus seeds acquired the capability to germinate under the canopy, especially in the drought moisture regime. Results indicate that the soil moisture environment experienced during dormancy affects the sensitivity to light of B. catharticus seeds, as well as the emergence pattern of seedlings.

Sucrose, raffinose and stachyose accumulate in soybean [Glycine max L. (Merrill)] embryos during seed maturation. To determine the relationship of plant maternal composition on seed composition, soluble carbohydrates in three 1-cm2 leaf punches at three plant growth stages (R2, R3, R6) and in seed coat cup exudates in planta were analysed at four 30-min intervals on soybean plants (R5) with low-raffinose, low-stachyose (LRS) seeds expressing the mutant stc1 phenotype; low-raffinose, low-stachyose and low-phytin (LRSP1, LRSP2) seeds expressing the mutant mips phenotype; or normal raffinose, stachyose and phytin (CHECK) seeds expressing the Stc1 and Mips phenotype. Leaf sucrose (23.6 μg cm− 2), myo-inositol (9.3 μg cm− 2), d-chiro-inositol (6.7 μg cm− 2), d-ononitol (0.76 μg cm− 2), d-pinitol (50.1 μg cm− 2) and total soluble carbohydrates (107.1 μg cm− 2) were not significantly different between phenotypes. d-chiro-Inositol, myo-inositol, d-pinitol and sucrose were unloaded from soybean seed coat cups in planta at decreasing rates over the four sequential periods of sampling. Unloading rates of sucrose and myo-inositol were highest for LRS, d-pinitol was highest for LRSP2, and d-chiro-inositol was not different between LRS, LRSP1, LRSP2 and CHECK. Free cyclitols were 60% of total soluble carbohydrates in leaves and 20% in seed coat cup exudates. Except for sucrose and d-pinitol, seed phenotype had little influence on the composition of compounds unloaded from seed coats to maturing embryos of low-raffinose, low-stachyose seeds. Maternally supplied cyclitols may contribute, in part, to changes in the composition of cyclitol galactosides stored in mature seeds.

Seed shelf-life or longevity is difficult to predict or to measure on a practical time scale. Predictive models suggest that water has the same effect on ageing rate for all seed lots within a species and that initial seed quality is the dominating factor explaining within-species variation. These assumptions are used in ‘accelerated ageing’ or ‘controlled deterioration’ (AA/CD) tests, which are commonly used to predict seed longevity in commercial and research applications. In this study, we describe within-species variation of longevity for seeds of Secale cereale and S. strictum (cultivated rye and its wild progenitor) under typical dry storage conditions of a genebank, and show that initial seed quality is an important, but not sole, factor explaining measured longevity. We also test the correlation of seed longevity, measured under humid and dry conditions, using 50 cultivars of rye, wheat (Triticum aestivum) and the intergeneric cross triticale, to assess how well AA/CD tests predict seed shelf-life during dry storage. Known differences in longevity between wheat and rye were confirmed at all water contents, and triticale seeds demonstrated intermediate behaviour. Longevity measured for humid and dry conditions were weakly correlated when comparisons included all grain types and were not correlated in within-grain-type comparisons. Response to moisture varied among cultivars. These findings do not support assumptions made in seed ageing models that use AA/CD tests. Our results suggest that more traits are involved in the expression of seed longevity than those typically measured in studies of initial seed vigour.

The quality of a seed (germination and vigour) is established during its development and maturation, but can be improved by post-harvest processing and pre-sowing treatments. During commercial seed production, maturity is usually estimated visually, relying on experience of the growers, but seed researchers are working to find molecular markers that can be applied easily to help in establishing optimal harvest time. One marker is cell cycle activity expressed as DNA replication in the seeds, analysed by flow cytometry. This fast and accurate method for the estimation of DNA content in plant nuclei allows detection of nuclei at different replication stages in different seed tissues and thus makes it possible to follow changes in the physiological state of a seed. DNA replication, as a late event during germination, can also be used to mark completion of germination and transition to early seedling growth. This information can be useful in the evaluation of seed quality and for following the advancement of priming. Flow cytometric analysis of ploidy can be also used as a basis for control of purity of some polyploid species seed lots.

Eragrostis is a cosmopolitan genus of the family Poaceae. Several wild species, including E. pilosa (L.) Beauv., are harvested for food, but the only cultivated crop-species is tef [E. tef (Zucc.) Trotter]. Despite its importance as a staple food and its plasticity to diverse environmental conditions, little is known about the structural and physiological strategies that adapt tef seeds to endure diverse and variable moisture regimes. Here, we report the presence of slime cells, a type of modified epidermal cell, covering the fruit of tef and its wild relative, E. pilosa. The slime produced by Eragrostis belongs to the ‘true’ slime type, since it is exclusively composed of pectins. Pectin forms uniform layers on the cell wall inner surface, which are confined by a thin cellulose layer to prevent release into the cell lumen. In the presence of water, pectins quickly hydrate, causing swelling of the slime cells. This is followed by their detachment, which may be controlled by a thin cuticle layer on the fruit surface. The ability of slime to absorb and maintain moisture around the grain is thought to be an adaptive feature for Eragrostis growing in dry habitats. This retention of water by slime may create conditions that are suitable for rapid germination.

To understand the beneficial effects of cold conditions during the alleviation of dormancy of walnut (Juglans regia L.), lipid mobilization, gluconeogenesis and changes related to ageing were compared in dormant kernels incubated in the cold and under deteriorating warm conditions. Stratifying kernels at 5°C for 30 d enhanced their germination capacity, whereas warm-incubated (27°C) kernels turned rancid after 20 d and had reduced germination. Kernel imbibition was sufficient to bring about lipid mobilization, irrespective of temperature of incubation. Although imbibed kernels displayed high isocitrate lyase activity, starch and soluble sugar accumulation occurred only under cold conditions. Deteriorative 64 kDa fatty acyl-ester hydrolase activity declined in cold-stratified kernels. Cold treatment also led to reduced lipid peroxidation and hydrogen peroxide in kernels. The activity of NADP+-isocitrate dehydrogenase, an NADPH-generating enzyme, declined in warm-incubated kernels. Thus, warm-incubated kernels undergo ageing associated with oxidative stress, but there are beneficial effects of cold stratification in preventing deteriorative ageing-related processes. Imbibition is sufficient to allow lipid mobilization to occur in dormant walnut kernels, although cold stratification accompanied by gluconeogenesis is essential for kernel germination.

A principal goal of seed germination modelling for wild species is to predict germination timing under fluctuating field conditions. We coupled our previously developed hydrothermal time, thermal and hydrothermal afterripening time, and hydration–dehydration models for dormancy loss and germination with field seed zone temperature and water potential measurements from early summer through autumn to develop predictions of germination timing for Bromus tectorum at a semi-arid site in north-central Utah, USA. Model predictions were tested with a validation dataset based on concomitant seed retrieval experiments in 2 years. Predictions were generally in agreement with observed field germination time courses, even though integration across multiple precipitation events was necessary. Success of the modelling effort hinged on two factors. First, we used a soil capacitance sensor that measured seed zone (5 mm soil depth) water content accurately over a wide range. Second, simulations were built using physiologically based threshold models that can incorporate differences in germination timing for multiple germination fractions and for multiple stages of dormancy loss. Our results suggest that simulation models using hydrothermal time concepts can predict field germination phenology accurately. Seeds in this study integrated their experiences in a widely fluctuating environment in a manner consistent with the assumptions of hydrothermal time. Such threshold-based models also have the advantage of generality, as these concepts can be applied to many different species, environments and weather scenarios.

The germination ecology of the winter annual Iberian endemics Iberis pectinata and Ziziphora aragonensis was investigated in order to better understand adaptations of rare species to their natural habitat and to improve ex-situ propagation techniques and management of their habitat. Specifically, we analysed the following aspects: (1) influence of temperature, light conditions and seed age on germination patterns; (2) phenology of germination; (3) germinative response of buried seeds to seasonal temperature changes; and (4) temperature requirements for induction and breaking of secondary dormancy. Germination was substantially lower in darkness than with a photoperiod in both taxa, with this difference being more pronounced in Z. aragonensis. Freshly matured seeds showed conditional physiological dormancy, germinating at low and medium temperatures but not at high temperatures (28/14 and 32/18°C). Germination capability increased with time of dry storage in both species, suggesting the existence of non-deep physiological dormancy. Under greenhouse conditions, germination of both taxa was mostly concentrated in autumn (October–November), while spring percentages were less than 1% of total accumulated germination recorded during the study. I. pectinata and Z. aragonensis seeds buried and exposed to natural seasonal temperature variations in an unheated greenhouse came out of conditional dormancy in summer and re-entered it in winter, thus exhibiting an annual conditional dormancy/non-dormancy cycle. Dormant seeds of both species which were stratified at 28/14 or 32/18°C during an 8-week period, were non-dormant when they were subsequently incubated over a range of temperatures from 5 to 25/10°C. Non-dormant seeds were induced into dormancy when stratified at 5 or 15/4°C for 8 weeks, showing a particularly low germination response at high temperatures. Recommendations for wild-population reinforcement programmes and for the management of the natural habitat of both endemics are discussed.

The objective of this study was to fit a hydrothermal germination model to germination data for a seedlot of radiata pine (Pinus radiata D. Don). Seeds were incubated for 50 d at constant temperatures and water potentials (T = 12.5–32.5°C, Ψ = 0 to − 1.2 MPa). Most seeds completed germination within 50 d, but for low Ψ and/or non-optimal temperatures (T < 17.5°C, T>25°C) many seeds did not complete germination. In general, germination data conformed to the hydrothermal model. Departures from the model were encountered for slow-germinating seeds at suboptimal temperatures (T ≤ 20°C). To account for these departures, two alternative hydrothermal models were fitted with an additional term for an upwards shift in seed base water potential with increasing time to germination. The alternative models more correctly predicted germination time than the original model. Similarly, reduced percentage germination at supra-optimal temperatures (T>20°C) was explained by including a term in the hydrothermal model which shifted the base water potential of seeds upwards towards zero, which in turn reduced the predicted rate that hydrothermal time would be accumulated by seeds. The rate of this upwards shift in base water potential was dependent on time to complete germination and ambient water potential as well as supra-optimal temperature.

Arrowleaf sida (Sida rhombifolia) is a tropical weed that grows in disturbed areas. Its control by hand pulling and mowing are only partially effective because arrowleaf sida is difficult to pull and quickly sprouts after cutting. We studied the seed-bank dynamics of S. rhombifolia in the region of Campinas, São Paulo State, Brazil for 2 years, following a known number of seeds placed artificially at five burial depths. Predators and pathogens were responsible for the loss of c. 40% of the seeds during the first year of the experiment. The importance of predation declined with depth. Mortality due to fungal attack did not differ among the burial depths, except for seeds placed at the surface (where it was negligible). Seedling emergence only occurred during the first year, after the beginning of the rainy period and when the study area was still free of a dense vegetation cover. Seedlings did not emerge from depths greater than 5 cm. We conclude that seedling emergence and death are significant causes of seed losses from the seed bank of S. rhombifolia but their relative importance is highly dependent on depth of burial.

Recently, forest seed banks were proven to not only reflect former (decades-old) but also ancient (centuries-old) land use. Yet, as land-use intensity determines the magnitude of seed-bank changes in recent forests, this study aims to identify whether an ancient land-use gradient would also be reflected in the seed bank. On a forested 1600-year-old archaeological site, five different land-use intensities were mapped and sampled. Apart from seed density, species richness and composition, functional seed-bank types, defined by nine seed-bank-related plant traits, were related to the land-use intensity gradient. The land-use gradient from gardens to undisturbed sites was still clearly reflected in the soil seed bank. Six emergent functional seed-bank types, characterized by specific plant traits, changed significantly in abundance, parallel to the land-use gradient. In particular, dispersal agent (and related traits) proved an important explanatory trait of present (functional) seed-bank patterns. Poor dispersers (large and heavy seeds) were not found in the intensively used areas, contrary to animal-dispersed species. Wind-dispersers may have been inhibited in the extension of their distribution by recruitment bottlenecks (low seed production) and/or competitive exclusion. Additionally, the agricultural land-use probably introduced ruderal species into the seed bank of the most intensively used areas, yielding a simultaneous increase in vegetation–seed-bank dissimilarity with land-use intensity, eliminating present vegetation as a driver behind the differences over the seed-bank gradient. We conclude by arguing how coppice-with-standards management possibly maintained the seed-bank gradient.

Aerosol smoke and smoke-water can break dormancy and promote seed germination of many plant species. In this study we investigated changes in gene expression after imbibition of light-sensitive Lactuca sativa L. cv. ‘Grand Rapids’ achenes with dilute smoke-water compared to water control samples kept in the dark or continuous light, using the fluorescent differential display technique. Although no difference was detected in the smoke-water versus water control samples germinated in light, smoke-water treatment resulted in the differential display of several expressed sequence tags (ESTs) when compared to water control samples kept in the dark. The most pronounced fragments isolated correspond to known genes related to germination, with functions in cell wall expansion, regulation of translation, the cell division cycle, carbohydrate metabolism and abscisic acid (ABA) regulation. Real-time polymerase chain reaction (PCR) validation revealed that the transcript abundance of the genes, HVA22, short-chain dehydrogenase/reductase and late embryogenesis abundant protein, are upregulated after smoke treatment when compared to control achenes kept in the light. The results indicate that smoke has a dual effect. On the one hand, the smoke can induce genes that may be linked to ABA action, whereas, on the other hand, it elicits a faster germination rate by inducing a similar pattern in gene expression as light treatment. Smoke effects could be manifested mainly through the induction of the cell division cycle, cell wall extension and storage mobilization.