Grains of indica rice (Oryza sativa cv. Peiza 67) exhibit an increase in endo-β-mannanase activity, mostly after the completion of germination. According to tissue blots, the initial increase occurs in association with the embryo, and possibly the scutellum, although the largest sustained increase in activity is in the peripheral regions of the endosperm. The aleurone layer, being the only living region of the endosperm, is presumably the site of synthesis and secretion of the enzyme into the non-living, starch-laden region. β-Mannosidase activity is low throughout germination and subsequent seedling growth, particularly in the endosperm regions. Its activity profile does not mimic that of endo-β-mannanase. In the intact grain, gibberellin (GA) causes a relatively small increase in endo-β-mannanase activity, while abscisic acid (ABA) causes a large decrease; this inhibition is overcome to a considerable extent when GA is supplied along with ABA. β-Mannosidase activity is little affected by either GA or ABA. Embryoless half-grains imbibed in water exhibit only a small increase in endo-β-mannanase activity with time of imbibition, showing the necessity for a stimulus from the embryo for this to occur. Incubating half-grains in the presence of GA results in a large increase in enzyme activity; ABA reduces the amount of activity compared to the water controls. GA is capable of reversing the inhibitory effect of ABA with respect to endo-β-mannanase activity. As in the intact grains, β-mannosidase activity in the half-grains is unaffected by either GA or ABA. It is concluded that the major site for the production of endo-β-mannanase activity is the aleurone layer, and this event is influenced by the presence of the embryo; in the absence of the latter, the increase in enzyme activity is stimulated by GA. β-Mannosidase activity is low throughout germination and post-germination, it is not influenced by GA and ABA, and thus its activity is not regulated in a coordinated manner with that of endo-β-mannanase.

The relationship between tolerance of seeds to extreme desiccation and the presence of ‘dehydrinlike’ proteins was investigated in groups of related taxa from the unrelated plant families Aceraceae and Gramineae. Dehydrin-like proteins were identified by Western blot analysis using an antibody raised against a synthetic oligopeptide representing the 23-amino acid consensus sequence common to all group 2 late-embryogenesis-abundant (LEA) proteins.

Evidence is presented that seeds of Acer pseudoplatanus and A. saccharinum are desiccation intolerant (recalcitrant) whereas seeds of A. platanoides and A. rubrum are desiccation tolerant (orthodox). Despite these differences, dehydrinlike proteins at 60 and 20 kDa were detected in all four species.

Dehydrins at 20 kDa were also detected in seed samples of two aquatic grasses, Porteresia coarctata and Oryza sativa from the tribe Oryzeae, despite seeds of the former rapidly losing viability on drying, whereas O. sativa is one of the best-known examples of desiccation-tolerant seeds. In O. sativa, there was a correlation between contents of dehydrins detected and the proportion of individuals capable of withstanding extreme drying. However, the possibility of a causal link between these parameters is equivocal. Dehydrin-like proteins were also detected in desiccation-sensitive seeds of Zizania palustris, Z. latifolia and Z. texana and desiccation-intolerant seeds of Spartina anglica, all from the Gramineae.

The presence of group 2 LEAs is clearly not diagnostic of desiccation tolerance in seeds. However, a more direct correlation with the expression of other groups of LEAs cannot be discounted.

The breaking of dormancy in seeds can be elicited by many factors, including temperature and short exposure to low molecular weight amphipathic molecules such as primary alcohols, monocarboxylic acids and anaesthetics. Their action has been suggested to be mediated through effecting changes to membranes. Paradoxically, though, these molecules can inhibit the germination of some non-dormant seeds. Here, we review the structure–activity relationships between amphipathic molecules and dormancy breaking and, based on the known responses of membranes to them and to temperature changes, offer an alternative interpretation of the data and a new hypothesis to explain their action. We suggest that amphipathic molecules break dormancy by partitioning into the membrane, thereby increasing and optimizing phospholipid headgroup spacing. This, in turn, facilitates the binding and activation of a peripheral membrane protein component of a signal transduction pathway that is essential for the completion of germination. In cases where amphipathic molecules inhibit germination, it is predicted that they cause the optimal headgroup spacing to be exceeded, thus preventing subsequent association of the membrane with the peripheral protein component. The hypothesis is extended to explain membrane changes that can lead to dormancy breaking during dry after-ripening.

Little is known about the seed ecology of annual holoparasites, such as Cuscuta (dodders), and how germination is timed to occur when seedlings can attach to a host. Seeds of Cuscuta epithymum, which is a rare and often threatened species in dry heaths of north-western Europe, were water-impermeable (physically dormant) at maturity in late summer. An 8-week period at 5°C, followed by incubation at 23, 15/6 or 30/20°C, resulted in 15–30% germination. Thus, in nature, only part of the seed population exposed to low winter temperatures and a subsequent warm spring potentially becomes water-permeable and can germinate in spring. When scarified seeds were cold-stratified at 5°C for 8 weeks and then incubated at 23°C, essentially all the viable seeds (c. 65%) germinated. It is concluded that seeds have a combination of physical (PY) and physiological (PD) dormancy (the first to be reported in the Convolvulaceae), which is interpreted as a double safety mechanism preventing germination at unfavourable times or places. After PY and PD were broken, seeds did not require light or fluctuating temperatures for germination. Some loss of PD (afterripening) can occur before PY is broken. These data indicate that a portion of the seed crop remains physically dormant in spring and potentially forms a persistent seed bank, a strategy that eliminates the risk of a total reproductive failure in a particular bad year.

The soil seed bank is the reserve of viable seeds found in the soil. This reserve contributes to plant population persistence in unpredictable environments; thus, determining its presence is basic to understanding recruitment patterns and population dynamics. Studies of soil seed banks in the Cactaceae are scarce, although these plants are ecologically dominant in American arid and semi-arid environments. Most studies have inferred the presence of seed banks by analysing morphological seed traits or germination of seeds stored in the laboratory for different periods of time. Few studies have determined their presence through evaluation of distribution, density and longevity of seeds in the field. To fill this information gap, we determined the existence of, and studied, the soil seed bank of Stenocereus stellatus, a columnar cactus endemic to central Mexico. This study reports the evaluation of these characteristics in the field and discusses whether this species forms a soil seed bank. We found a higher number of seeds under shrubs than in areas lacking vegetation. Recently dispersed seeds did not germinate because they have primary dormancy. This dormancy was broken after 6 months of burial in the soil. Seeds buried for 10 months entered secondary dormancy and they were not viable at 24 months, probably because of pathogen attack. Considering dormancy and seed longevity, we suggest that S. stellatus has the potential to form a short-term persistent seed bank. However, this should be confirmed by conducting studies on other S. stellatus populations throughout their geographical distribution.

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.

During a 7-day conditioning treatment of clumps of oil palm (Elaeis guineensis Jacq.) somatic embryos on a medium containing 0.75 M sucrose, the fructose and glucose concentrations remained constant, whereas a ten-fold and twenty-fold increase were noted for sucrose and starch concentrations, respectively. The only new sugar detected was arabinose which remained at a low concentration. After conditioning on media supplemented with various sugars and polyols at similar osmolarities, recovery of clumps of embryo growth was satisfactory except with ribose. After an additional desiccation period, survival was optimal with fructose, galactose, sucrose and glucose, intermediate with maltose and lower with other compounds. When embryos were cryopreserved without previous desiccation, survival was noted after conditioning treatment with sucrose only. In contrast, when freezing was performed after dehydration, survival could be obtained with several substances. It was optimal with sucrose, fructose, galactose and raffinose but was possible also with sorbitol and glucose. Intensity of recovery of proliferation was highest with embryos conditioned with sucrose.

The aim of the present study was to examine the micromorphology of the cork-oak (Quercus suber) acorn and to evaluate the efficiency of the pericarp as a barrier to water loss. When the desiccation curve obtained at 25 ± 1°C and 62% RH for physiologically mature acorns collected from the tree was compared with that corresponding to fully ripe acorns which had been shed, the latter showed a slightly lower rate of water loss. A marked reduction in the mean rate of water loss was recorded for intact acorns compared with those from which the pericarp had been removed. The external surface and transverse sections of the pericarp from both mature and fully ripe acorns were examined by scanning electron microscopy. Two zones presenting morphological and micromorphological differences were identified: an area including the point of attachment to the cupule and an apical zone covering the embryo. The microstructure of the pericarp showed an external thick cuticle and a single external palisade layer of closely packed cells with no intercellular spaces. Underlying this layer, there was a further parenchymatous layer of poorly differentiated, roughly isodiametric cells. The pericarp in the cupular zone consisted of only this undifferentiated layer between the two epidermal cell layers and contained vascular bundles with many xylem elements.

Theoretical models predict that seed size and seed-bank persistence evolve interdependently, such that strong selection for one trait corresponds with weak selection for the other. This framework has been supported and rejected by empirical data, and thus, conclusive evidence is lacking. We expanded the seed size–persistence framework to include seed-coat thickness, a defence trait previously correlated with seed survival in soil. To do this, we used Abutilon theophrasti accessions with varied evolutionary histories and we quantified associations among seed traits including morphology, size, coat thickness, dormancy (percentage of viable seeds that fail to germinate under optimum conditions) and seed-bank persistence (percentage of viable seeds remaining after 1 year of burial). Statistical models were developed to test the hypothesis that combined measurements of seed-coat thickness and seed size better explain variability in seed-bank persistence than seed-size data alone. Results indicated that measurements of seed size (length, width, mass) were negatively correlated with coat:width ratio (coat thickness relative to seed width) and coat:mass ratio (coat thickness relative to seed mass). Accessions characterized by smaller seeds with proportionally thicker seed coats were more dormant and more persistent in soil than accessions characterized by larger seeds with proportionally thinner seed coats. Seed-coat thickness data improved the explanatory power of logistic regression models for seed-size effects on both seed-bank persistence and dormancy. These results indicate that supplementing seed-size data with seed-defence data may clarify previously reported contradictory results regarding trade-offs between seed size and seed-bank persistence.

The effect of natural and artificial seed ageing has been compared in terms of physiological and biochemical responses of several maize genotypes. The physiological parameters were: viability, germinability, emergence in sand and dry matter accumulation. The biochemical parameters were: DNA synthesis and DNA polymerase activity. A close and direct relationship was found between seed deterioration and DNA metabolism in all maize materials which responded to ageing according to their genetic constitution; i.e., vigorous genotypes suffered less severe damage or recovery was faster than in the low-vigour genotypes. Coordination of events at appropriate times would seem a critical factor for proper seed germination.

Phenotypic plasticity in seed dormancy may allow plant species to cope with rapid environmental changes, such as climate warming. In controlled experimental settings, plasticity in dormancy has been found to relate to temperature during seed maturation, but this relationship has not been tested in field conditions. Here we analyse for the first time the variation in dormancy during five successive years in wild populations of the study species Centaurium somedanum, to determine how this variation is related to average temperatures during specific seasons of plant activity. We performed laboratory germination experiments to measure: (1) the degree of dormancy at dispersal; and (2) the sensitivity to dormancy-breaking factors. We calculated average temperatures during four seasons of plant activity (overwintering, vegetative growth, flowering and seed maturation) for each year, and tested the relationship between these temperatures and patterns of dormancy variation, using Generalized Linear Models. Dormancy varied among sites and years, seeds being more dormant in colder years. For the degree of dormancy at dispersal, we found a positive relationship with flowering temperature and a significant effect of collection site. For the sensitivity to dormancy-breaking factors, we found no significant differences among sites, a positive relationship with flowering temperature and a negative relationship with overwintering temperature. Phenotypic plasticity in dormancy in C. somedanum is thus especially marked in the sensitivity to dormancy-breaking factors. Temperatures during overwintering and flowering have differential effects on this plasticity, allowing the plant to detect the gradient of seasonality, a main ecological feature of its distribution. These results highlight the importance of taking into account more than average temperatures when assessing the response of plant phenotypic plasticity to climate change.

Seed development of tea was studied to identify the maturity index and the optimal time of seed collection. After harvest, the moisture content (28–30%fresh weight basis) of mature seeds, which germinated 100%, declined progressively (19% moisture content) after shedding, with a decrease in seed germination and viability. However, this viability loss could be prevented to some extent by storing seeds within intact fruits. The maximum rate of seed dry matter accumulation coincided with the accumulation of starch in the embryos and seeds at stage 8, the embryo maturation phase. Although the embryo abscisic acid (ABA) content was highest at stage 8, free ABA declined in the tea embryos throughout the remainder of the seed maturation cycle.

The interactions between water content and temperature on freezing and desiccation damage were examined for Zizania (wild rice) embryos at several stages of maturity. The water content of excised embryos was manipulated by flash drying at 35°C or room temperature to between 2.5 g H2O/g dw and 0.05 g/g. Embryos were then exposed to temperatures ranging from 5 to −50°C. Viability following the drying and cooling treatments was assayed by leakage of electrolytes and germination in culture. Viability of embryos decreased when embryos were dried below a critical water content. The critical water content was greatest for the least mature embryos. Critical water contents were also temperature dependent and increased with decreasing temperature. Even though the critical water content varied with developmental status and temperature, the water activity corresponding to the critical water content appeared to be constant at 0.90. The most mature embryos survived temperatures as low as −50°C while the least mature embryos survived only to −18°C. These trends were predicted by ‘phase diagrams’ based on the physical properties of water in embryos at different stages of maturity (Vertucci et al., 1994a). Our results confirm the earlier prediction that long term preservation of Zizania grains is possible at −20°C and the degree of success will be related to the maturity status of the embryos.

Convolvulaceae is the only family in the asterid clade with species that produce physically dormant seeds, and most studies on germination in this family have focused on scarified seeds. However, no study has been done on the taxonomic/evolutionary pattern of seed dormancy in Convolvulaceae. We determined the moisture content of non-treated seeds and water uptake and germination percentages for non-treated and manually scarified seeds of 46 species in 11 of the 12 tribes in this family. Germination was tested over a range of temperatures in light/dark and in the dark. The effect of drying and storage at low temperatures was tested on seeds of Erycibe henryi and Maripa panamensis, the only species with high initial moisture content. Non-treated fresh seeds of Bonamia menziesii, M. panamensis and E. henryi imbibed water, whereas those of the other 43 species did not. Manually scarified seeds of all these 43 species took up large amounts of water. Therefore, seeds of 43 of the 46 species are physically dormant, and three are non-dormant. Seeds of all 46 species germinated after imbibition, except those of Cuscuta europaea, which are reported to have combinational dormancy. M. panamensis and E. henryi seeds are recalcitrant and those of the other 44 species orthodox. In Convolvulaceae, basal tribes or tribes derived from basal tribes contain species with non-dormant recalcitrant, physically dormant orthodox and combinationally dormant orthodox seeds. Physical dormancy of seeds in this family possibly evolved from a non-dormant recalcitrant seed-producing ancestor closely related to Erycibeae.

Seeds of the winter annual Bromus tectorum lose primary dormancy in summer and are poised to germinate rapidly in the autumn. If rainfall is inadequate, seeds remain ungerminated and may enter secondary dormancy under winter conditions. We quantified conditions under which seeds enter secondary dormancy in the laboratory and field and also examined whether contrasting B. tectorum genotypes responded differently to dormancy induction cues. The study also extends previous hydrothermal time models for primary dormancy loss and germination timing in B. tectorum by using similar models to account for induction and loss of secondary dormancy. Maximum secondary dormancy was achieved in the laboratory after 4 weeks at –1.0 MPa and 5°C. Seeds in the field became increasingly dormant through exposure to temperatures and water potentials in this range, confirming laboratory results. They were released from dormancy through secondary after-ripening the following summer. Different genotypes showed contrasting responses to dormancy induction cues in both laboratory and field. To examine secondary dormancy induction and release in the field in terms of hydrothermal time parameters, we first created a model that allowed mean base water potential (Ψb(50)) to vary while holding other hydrothermal time parameters constant, as in models for primary dormancy loss under dry conditions. The second model allowed all three model parameters to vary through time, to account for changes (e.g. hydrothermal time accumulation) that could occur simultaneously with dormancy induction in imbibed seeds. Shifts in Ψb(50) could explain most changes in dormancy status for seeds retrieved from the field, except during the short period prior to dormancy induction, when hydrothermal time was accumulating. This study illustrates that hydrothermal modelling, and specifically changes in Ψb(50), can be used to characterize secondary dormancy induction and loss in B. tectorum.

Vitamin E (tocols) is a key metabolite for efficient scavenging of lipid peroxy radicals that cause membrane breakdown during seed ageing. However, in rice, this hypothesis has been tested for very few lines only and without considering intraspecific variation in genomic structure. Here, we present a correlation study between tocols and seed longevity using a diverse rice panel. Seeds of 20 rice accessions held in the International Rice Genebank at the International Rice Research Institute, representing Aus, Basmati/Sadri, Indica, temperate Japonica and tropical Japonica variety groups, were used for tocols analysis (quantification of α-, β-, γ- and δ-tocopherol/tocotrienol by ultra-performance liquid chromatography) and storage experiments at 45°C and 10.9% seed moisture content. To examine the effects of DNA-haplotype on the phenotype, the 700 K high-density single-nucleotide polymorphism marker dataset was utilized. Both seed longevity (time for viability to fall to 50%; p50) and tocols content varied across variety groups related to the heterogeneity in the genetic architecture. Among eight types of tocol homologues, α-tocopherol and γ-tocotrienol were significantly correlated with p50 (negatively and positively, respectively). While temperate Japonica varieties were most abundant in α-tocopherol, Indica varieties recorded 1.3- to 1.7-fold higher γ-tocotrienol than those of other groups. We conclude that the specific ratio of tocol homologues rather than total tocols content plays an important role in the seed longevity mechanism.

In the presence of abscisic acid (ABA), contents of glucose and fructose decreased in carrot embryoids, whereas umbelliferose increased during dehydration. The acquisition of desiccation tolerance was tested by scoring germinated embryoids. It did not keep pace with the changes in soluble carbohydrate contents during development, which suggests that in these somatic embryoids di- and oligosaccharides are not the determining factors for anhydrobiosis. However, on slow dehydration, umbelliferose increased, while sucrose decreased and the monosaccharides declined completely. These changes were positively correlated with increased desiccation tolerance. A similar analysis of slowly dried, ABA-treated alfalfa embryoids showed that stachyose and sucrose increased and the monosaccharides declined with the acquisition of desiccation tolerance. These data support the contention that carbohydrates are involved in anhydrobiosis.

Priming of sugarbeet (Beta vulgaris L.) seeds induces increased solubilization of the basic B-subunit of 11-S globulin (a major seed storage protein in sugarbeet). Using a sensitive single-seed ELISA, the soluble and total B-subunit contents of individual untreated and primed sugarbeet seeds were measured. With the untreated seeds, there was a 160-fold range of the soluble B-subunit content among individual seeds. The individual primed seeds also exhibited large variations in their soluble B-subunit content, yet only over a five-fold range. Furthermore, the frequency distributions of soluble B-subunit content were markedly different for the primed and untreated seed populations; the primed seed population exhibited a substantially higher median than that for the untreated seed population. In marked contrast, and as expected from results with pooled seed samples, the distributions of total B-subunit content were superimposed when comparing untreated and primed seed populations.

Narcissus longispathus (Amaryllidaceae) is a perennial geophyte and a Mediterranean narrowly endemic species. At dispersal time, N. longispathus seeds are dormant and have underdeveloped embryos. This work aimed to determine requirements for dormancy break and germination and to compare dormancy traits with those of the two endemic Iberian congeners. Phenology of embryo growth and germination were studied by regularly exhuming seeds sown in near-natural conditions. Temperature and light requirements for embryo growth, breaking of dormancy and germination were determined by incubating seeds under controlled laboratory conditions. Mean embryo length in fresh seeds was 1.50 mm, and embryos had to grow to 3.80 mm before radicle emergence. Embryos grew to full size and seeds germinated when they were warm stratified for 2 months (optimum 1 month at 20/7°C+1 month at 15/4°C), then cold stratified at 5°C for 2 months, and finally incubated at cool temperatures (15/4°C) for 30 d. However, in seeds only subjected to either warm or cold stratification, the embryos hardly grew and did not germinate. In natural conditions, the embryos elongate in autumn–winter, and in late winter–early spring (March) almost all radicles and seedlings emerged. Velocity of embryo growth and germination percentages increased with seed storage duration. Seeds of N. longispathus have non-deep complex morphophysiological dormancy (MPD). This is the first report of such a level of MPD in Narcissus. Our data suggest that non-deep complex MPD may have been derived from intermediate complex MPD in the section Pseudonarcissi.