Showing papers in "Seed Science Research in 2015"

Dormancy in cereals (not too much, not so little): about the mechanisms behind this trait

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

A framework for the interpretation of temperature effects on dormancy and germination in seed populations showing dormancy

Abstract: Temperature is a key factor affecting both dormancy and germination. In non-dormant seeds, when temperature is within the thermal range permissive for germination, it just regulates germination velocity, while in seeds presenting dormancy it can also be affecting dormancy level, dormancy termination and the expression of dormancy itself. This dual effect of temperature on dormancy and germination often leads to misinterpretation of obtained germination results and confounds the analysis of temperature effects in seed populations presenting some degree of dormancy. In the present paper we discuss the effect of temperature in the regulation of dormancy level and its implications in dormancy expression, as an attempt to construct a conceptual framework that allows distinguishing between the effects of temperature on dormancy and germination. Finally, we present examples of how a better understanding of these effects could help us to interpret the mixed effects of temperature on both processes during incubation of seeds presenting dormancy.

Sodium chloride improves seed vigour of the euhalophyte Suaeda salsa.

Abstract: Suaeda salsa is an annual herbaceous euhalophyte in the family Chenopodiaceae that produces dimorphic seeds on the same plant under natural conditions. In order to determine the effect of salinity on seed quality traits during seed formation, seeds from plants grown under control conditions and on 200 mM NaCl were used to investigate the effect of NaCl on seed production and seed germination. Results showed that size and weight of both black and brown seeds generated from 200 mM NaCl-treated plants were markedly greater than those from controls. The germination percentage of brown seeds from both control and NaCl-treated plants was higher than that of black seeds. Furthermore, the germination percentage of the black seeds generated from 200 mM NaCl-treated plants was significantly higher than that of the control at different concentrations of NaCl, although germination percentage declined with the increase NaCl concentration. Surprisingly, NaCl did not affect germination of the brown seeds. The germination index and vigour index of both black and brown seeds from the control plants were significantly lower than those of seeds from the different NaCl treatments. Seed starch, soluble sugar, protein and lipid content of both black and brown seeds generated from the 200 mM NaCl-treated plants were significantly higher than those from the control. These results suggest that a certain concentration of NaCl plays a pivotal role in seed vitality of the euhalophyte S. salsa through increasing seed weight and contents of storage compounds such as protein, starch and fatty acids.

Physical dormancy in a changing climate

Abstract: Species with physically dormant (PY) seeds make up over 25% of plant species in a number of ecologically important ecosystems around the globe, such as savannah and Mediterranean shrublands. Many of these ecosystems are subject to temporally stochastic events, such as fire and drought; but are in areas projected to experience some of the most extreme climatic changes in the future. Given the importance of PY in controlling germination timing for successful recruitment, we ask how plastic the PY trait is, and if changes to the maternal environment from climate change could alter recruitment. This review focuses on: (1) the evidence for inter- and intraspecific variation in PY; (2) the genetic, maternal and environmental controls involved; and (3) the ecological consequences of (1) and (2) above. Evidence for (within-community) interspecific variation in conditions required to break PY is strong, but for intraspecific variation evidence is contradictory and limited by a paucity of studies. Identifying controllers of variation in PY is complex, there is some suggestion that conditions of the maternal environment may be important, but no consensus on the nature of effects. The implications of PY plasticity for the persistence of seed banks, species and communities under climate change are discussed. We highlight a number of key knowledge gaps, such as a lack of research estimating the components of variation in non-agricultural species, and identify a suite of seed attributes relevant to understanding the potential impacts of climate change on the population dynamics of PY species in the future.

Grain dormancy loss is associated with changes in ABA and GA sensitivity and hormone accumulation in bread wheat, Triticum aestivum (L.)

Abstract: Knowledge about the hormonal control of grain dormancy and dormancy loss is essential in wheat, because low grain dormancy at maturity is associated with the problem of pre-harvest sprouting (PHS) when cool and rainy conditions occur before harvest. Low GA (gibberellin A) hormone sensitivity and high ABA (abscisic acid) sensitivity were associated with higher wheat grain dormancy and PHS tolerance. Grains of two PHS-tolerant cultivars were very dormant at maturity, and insensitive to GA stimulation of germination. More PHS-susceptible cultivars were less sensitive to ABA inhibition of germination, and were either more GA sensitive or germinated efficiently without GA at maturity. As grain dormancy was lost through dry afterripening or cold imbibition, grains first gained GA sensitivity and then lost ABA sensitivity. These changes in GA and ABA sensitivity can serve as landmarks defining stages of dormancy loss that cannot be discerned without hormone treatment. These dormancy stages can be used to compare different cultivars, seed lots and studies. Previous work showed that wheat afterripening is associated with decreasing ABA levels in imbibing seeds. Wheat grain dormancy loss through cold imbibition also led to decreased endogenous ABA levels, suggesting that reduced ABA signalling is a general mechanism triggering dormancy loss.

Secondary dormancy dynamics depends on primary dormancy status in Arabidopsis thaliana

Abstract: Seed dormancy can prevent germination under unfavourable conditions that reduce the chances of seedling survival. Freshly harvested seeds often have strong primary dormancy that depends on the temperature experienced by the maternal plant and which is gradually released through afterripening. However, seeds can be induced into secondary dormancy if they experience conditions or cues of future unfavourable conditions. Whether this secondary dormancy induction is influenced by seed-maturation conditions and primary dormancy has not been explored in depth. In this study, we examined secondary dormancy induction in seeds of Arabidopsis thaliana matured under different temperatures and with different levels of afterripening. We found that low water potential and a range of temperatures, from 8°C to 35°C, induced secondary dormancy. Secondary dormancy induction was affected by the state of primary dormancy of the seeds. Specifically, afterripening had a non-monotonic effect on the ability to be induced into secondary dormancy by stratification; first increasing in sensitivity as afterripening proceeded, then declining in sensitivity after 5 months of afterripening, finally increasing again by 18 months of afterripening. Seed-maturation temperature sometimes had effects that were independent of expressed primary dormancy, such that seeds that had matured at low temperature, but which had comparable germination proportions as seeds matured at warmer temperatures, were more easily induced into secondary dormancy. Because seed-maturation temperature is a cue of when seeds were matured and dispersed, these results suggest that the interaction of seed-maturation temperature, afterripening and post-dispersal conditions all combine to regulate the time of year of seed germination.

Seed dormancy is a dynamic state: variable responses to pre- and post-shedding environmental signals in seeds of contrasting Arabidopsis ecotypes

Abstract: Seeds have evolved to be highly efficient environmental sensors that respond not only to their prevailing environment, but also their environmental history, to regulate dormancy and the initiation of germination. In the present work we investigate the combined impact of a number of environmental signals (temperature, nitrate, light) during seed development on the mother plant, during post-shedding imbibition and during prolonged post-shedding exposure in both dry and imbibed states, simulating time in the soil seed bank. The differing response to these environments was observed in contrasting winter (Cvi, Ler) and summer (Bur) annual Arabidopsis ecotypes. Results presented show that environmental signals both pre- and post-shedding determine the depth of physiological dormancy and therefore the germination response to the ambient environment. The ecotype differences in seed response to ambient germination conditions are greatly enhanced by seed maturation in different environments. Further variation in response develops following shedding when seeds do not receive the full complement of environmental signals required for germination and enter the soil seed bank in either dry or imbibed states. Species seed dormancy characteristics cannot therefore be easily defined, as seed dormancy is a dynamic state subject to within-species adaptation to local environments.

Regulation of seed dormancy by abscisic acid and DELAY OF GERMINATION 1

Abstract: Physiological dormancy has been described as a physiological inhibiting mechanism that prevents radicle emergence. It can be caused by the embryo (embryo dormancy) as well as by the structures that cover the embryo. One of its functions is to time plant growth and reproduction to the most optimal season and therefore, in nature, dormancy is an important adaptive trait that is under selective pressure. Dormancy is a complex trait that is affected by many loci, as well as by an intricate web of plant hormone interactions. Moreover, it is strongly affected by a multitude of environmental factors. Its induction, maintenance, cycling and loss come down to the central paradigm, which is the balance between two key hormonal regulators, i.e. the plant hormone abscisic acid (ABA), which is required for dormancy induction, and gibberellins (GA), which are required for germination. In this review we will summarize recent developments in dormancy research (mainly) in the model plant Arabidopsis thaliana, focusing on two key players for dormancy induction, i.e. the plant hormone ABA and the DELAY OF GERMINATION 1 (DOG1) gene. We will address the role of ABA and DOG1 in relation to various aspects of seed dormancy, i.e. induction during seed maturation, loss during dry seed afterripening, the rehydrated state (including dormancy cycling) and the switch to germination.

Inbreeding depression and the cost of inbreeding on seed germination

Abstract: We review the literature on effects of inbreeding depression (ID) on seed germination for 743 case studies of 233 species in 64 families. For 216 case studies, we also review the relationship between mass and germination in inbred vs. outbred seeds. Inbred seeds germinated equally well as outbred seeds in 51.1% of 743 case studies, but better than outbred seeds in only 8.1%. In c. 50.5% of 216 cases, mass of inbred seeds was equal to (38.0%) or larger than (12.5%) that of outbred seeds. The magnitude of ID spans most of the − 1 to +1 range for relative performance for germination of inbred vs. outbred seeds; in contrast to what might be expected, seed germinability often is not negatively correlated with the coefficient of inbreeding (F) or positively corrected with population genetic diversity; neither heterosis nor outbreeding depression for germination is common in crosses between populations; and ID in most endemics is low and does not differ from that of widespread congeners. Our results on the effects of ID on seed mass and germination do not agree with the limited number of comparisons Darwin (1876) made on the effects of selfing vs. outcrossing on these two life-history traits. Recommendations are made on how to improve dormancy breaking and germination procedures in order to make the results of studies on ID more relevant to the natural world.

Pollen (microgametophyte) competition: an assessment of its significance in the evolution of flowering plant diversity, with particular reference to seed germination

Abstract: A hypothesis by David L. Mulcahy published in Science in 1979 on pollen competition as an important force in the evolution of flowering plant diversity, via selection of favourable genes in the microgametophyte that are passed on to, and inherited in, the sporophyte, stimulated quite a bit of research on the topic. The primary aim of this opinion paper is to assess the evidence for the effect of microgametophyte competition on the sporophyte, with particular reference to seed germination. Pollen competition was associated with an increase in seed germination in 14 of 30 case studies (30 published papers). Seed mass was related to better germination in only two of the 14 cases in which there was a positive effect of pollen competition on germination. An evaluation of the four criteria that must be met to validate Mulcahy's hypothesis cast some doubt on the significance of pollen competition as an important selective force in the diversification of angiosperms. Insect pollination, via which many pollen grains are deposited on a stigma, may not be the answer to Darwin's ‘abominable mystery’ concerning the reason for the abrupt origin and rapid diversification of flowering plants during the Cretaceous.

Timing of seed germination correlated with temperature-based environmental conditions during seed development in conifers

Abstract: Ecological (climatic and geographic) variation in early life-history transitions is a vital determinant of the adaptive evolution of timing of seed germination. This study aimed to investigate the correlation between timing of seed germination and environmental conditions during seed development. We examined seed germination timing of 15 coniferous seed lots of lodgepole pine, ‘interior’ spruce and western hemlock collected from natural stands in British Columbia (BC), Canada, under manipulated [stratification, thermo-priming (15 or 20°C) and their combinations] and non-manipulated (control) conditions. Timing of seed germination showed strong and positive correlation with the temperature-based environmental condition during seed development. This pattern persisted across species and seed lots within species, substantiating the historic importance of environmental conditions during seed development and maturation to life-history traits. Moreover, the strategy of phenotypic plasticity affecting timing of seed germination was observed across the applied germination treatments. These results provide insight into the germination niche as affected by global warming, indicating that conifers' seed dormancy in BC (north of 54°N) tends to increase and the changes associated with early spring warm-up are expected to accelerate seedling emergence, as shortened winters would have a minimal effect on dormancy decay.

Are wildfires an adapted ecological cue breaking physical dormancy in the Mediterranean basin

Abstract: Many studies have claimed that fire acts as the chief ecological factor cueing dormancy break in seeds with a water-impermeable seed coat, i.e. physical dormancy (PY), in Mediterranean ecosystems. However, a proposal is made that seasonal temperature changes must be viewed as more meaningful dormancy-breaking cues because: (1) fire is erratic and may break PY in seasons during which seedlings cannot complete their life cycle; (2) fire may not occur for long periods, thereby only providing an opportunity for dormancy break and germination once in every several years; and (3) if fire opens the specialized anatomical structures called ‘water gaps’, in seconds, their evolutionary role of detecting environmental conditions becomes irrational. Although fire breaks dormancy in a proportion of seeds, given the risk of seed mortality and the post-fire environment providing cues for dormancy break, it is suggested that fire might possibly be an exaptation.

Seed dormancy in six cold desert Brassicaceae species with indehiscent fruits

Abstract: The dispersal unit of many species of Brassicaceae is an indehiscent fruit, but relatively few studies have tested the effect of the pericarp on seed germination in this family. Our aim was to determine the effect of the pericarp on seed dormancy in six species of Brassicaceae native to the cold desert of north-west China. Intact dispersal units and isolated seeds of Chorispora sibirica, Euclidium syriacum, Goldbachia laevigata, Spirorrhynchus sabulosus, Sterigmostemum fuhaiense and Tauscheria lasiocarpa were stored dry at ambient laboratory conditions for 0–12 months and tested for germination in light and in dark at 5/2, 15/2 and 30/15°C. The amount of water absorbed by fruits and by seeds within the fruits was determined. For four species, intact fruits, isolated seeds and isolated seeds plus the removed pericarps were used to test for the mechanical versus possible chemical role of the pericarp in seed dormancy. Fresh isolated seeds, which have a fully developed embryo, germinated to lower percentages and rates than afterripened seeds. Thus, seeds have non-deep physiological dormancy. The pericarp significantly reduced germination, but inhibition was not due to lack of water uptake by seeds or to chemical inhibitors. Afterripened seeds of the six species germinated to 0–50% inside the fruits. We conclude that the pericarp plays a dominant role in seed dormancy of the six study species, and it does so by mechanically restricting embryo growth. Thus, the pericarp has the potential to spread germination over an extended period of time.

Can chlorophyll fluorescence be used to determine the optimal time to harvest rice seeds for long-term genebank storage?

Abstract: Chlorophyll fluorescence (CF) analysis was explored as a potential tool to identify the optimal time to harvest rice seed germplasm for maximum storage longevity. Seeds of 20 diverse genebank accessions were harvested at 24, 31, 38 and 45 d after peak flowering (DAF) and half of each seed lot was sorted by hand, following normal practice at the T.T. Chang Genetic Resources Center. CF analysis was carried out on both non-sorted and sorted seeds, while storage experiments were carried out on sorted seeds. Seed longevity (the time for viability to fall to 50%, p 50) was significantly correlated with the skewness, kurtosis, mode and mean of the CF histograms when the data for every accession at all the harvest times were included in the correlation analysis. However, these correlation coefficients were ≤ 0.481. The correlation coefficient between p 50 and DAF was similarly low (0.461). For individual accessions, there was wide variation in the correlation coefficients. While for some accessions, there appeared to be a strong relationship between p 50 and mean CF that could be used to guide when to harvest seeds in the field, for other accessions, a unique mean CF to inform when to harvest seeds or for use in seed sorting could not be identified; this was also true for DAF over the harvesting schedule used in this study. Given the number and diversity of accessions managed by a genebank, it seems unlikely that CF analysis would be an appropriate tool to help manage the regeneration or processing of seeds intended for storage in the genebank.

Pericarp structure of Glebionis coronaria (L.) Cass. ex Spach (Asteraceae) cypselae controls water uptake during germination

Abstract: Glebionis coronaria (L.) Cass. ex Spach is a common Mediterranean weed producing distinctive central and peripheral dormant cypselae with a hard fruit coat, which was previously hypothesized to impose physical dormancy. Analysis of water uptake in cypselae and in naked seeds showed that it preferentially takes place at the basal end of the fruit; however, seeds within an intact pericarp do not fully imbibe when compared with naked seeds. Germination was not significantly different between the two heteromorphs, and afterripening or cold stratification did not increase germination, while warm stratification at 35/20°C, as revealed by logistic regression, resulted in a significant improvement. However, loss of viability was also rapid at these high temperatures. Central and peripheral cypselae generally showed very low germination. In both heteromorphs, faster and higher germination (60–70%) was reached only after extensive scarification of pericarp tissue, and full germination was observed only after complete removal of pericarp tissue. Although the pericarp significantly reduced water uptake, no palisade layer(s) of macrosclereids could be observed. Xylem-vessel elements were found running through the basal end of the pericarp and forming the main point of water entry. We reject the hypothesis that G. coronaria cypselae have physical dormancy. Instead, water uptake and germination are impeded by: (1) directed water uptake, mainly through a pericarp-spanning channel-like structure; and (2) mechanical constraint on embryo growth exerted by the hard pericarp. The channel-like structure forms the principal system for controlling seed germination.

Structural aspects of dormancy in quinoa (Chenopodium quinoa): importance and possible action mechanisms of the seed coat

Abstract: Two possible sources of resistance to pre-harvest sprouting were evaluated in quinoa. They showed dormancy at harvest and significant variations in dormancy level in response to environmental conditions experienced during seed development. The aims of this work were to evaluate the importance of seed coats in the regulation of dormancy in this species, to investigate possible mechanisms of action and to assess association of seed coat properties with changes in dormancy level caused by the environment. Accessions Chadmo and 2-Want were grown under field conditions on different sowing dates during 2 years. Seed coats were manipulated and seed germination was evaluated at different temperatures. Seed coat perforation before incubation led to faster dormancy loss in both accessions. This effect decreased with delayed sowing date, and seeds expressed a level of dormancy not imposed by coats. This suggests the presence of embryo dormancy in the genus Chenopodium. Seeds of the accession 2-Want had a significantly thinner seed coat at later sowing dates, associated with a decreasing coat-imposed dormancy, but this pattern was not detected in Chadmo. The seed coat acts as a barrier to the release of endogenous abscisic acid (ABA) in quinoa, suggested by the increase in germination and a higher amount of ABA leached from perforated seeds. ABA is able to leach from seeds with an intact seed coat, suggesting that differences in seed coat thickness may allow the leakage of different amounts of ABA. This mechanism may contribute to the observed differences in dormancy level, either between sowing dates or between accessions.

Fire-related cues and the germination of eight Conostylis (Haemodoraceae) taxa, when freshly collected, after burial and after laboratory storage

Abstract: Abstract The genus Conostylis (Haemodoraceae) is endemic to fire-prone south-western Australia. To gain an understanding of the effect of some fire-related germination cues, eight Conostylis taxa were tested in response to water, nitrate, smoke water and karrikinolide (KAR1) under light and dark conditions, when seeds were freshly collected and after a year of burial. The germination of all taxa tested was higher in response to smoke water and KAR1 than in water alone, whereas nitrate did not stimulate germination. Germination was higher in all taxa following 1 year of burial than in fresh seeds. Recently, glyceronitrile has been identified as another chemical in smoke water, apart from KAR1, that can stimulate the germination of certain species. The relative response of eight Conostylis taxa to KAR1, glyceronitrile and smoke water was examined in laboratory-stored seeds. Germination of these taxa was promoted by both smoke water and KAR1, except for C. neocymosa, which had high germination regardless of treatment. Four of the other seven taxa germinated to higher levels in at least one of the glyceronitrile concentrations tested (10, 50 or 100 μM) than in water alone. However, in only two of these taxa, C. aculeata subsp. septentrionora and C. juncea, was germination in glyceronitrile as high as that in smoke water. Thus, the response to glyceronitrile is not uniform across Conostylis taxa. Generally, germination was higher with KAR1 than glyceronitrile, suggesting that although some Conostylis taxa have the capacity to respond to glyceronitrile, KAR1 is the more important germination stimulant for this genus.

Nitrate controls testa rupture and water content during release of physiological dormancy in seeds of Sisymbrium officinale (L.) Scop.

Abstract: Seeds of Sisymbrium officinale display physiological dormancy and require nitrate to germinate. Rupture of the testa precedes radicle protrusion through the endosperm (germination sensu stricto). While both endosperm rupture and testa rupture (TR) required nitrate, endosperm rupture was fully inhibited by abscisic acid (ABA) but TR was not inhibited. The gibberellic acid (GA)-synthesis inhibitor paclobutrazol prevented TR, which was reverted by exogenous GA4 but not by nitrate. The orientation of TR was transverse, which prompted the question whether seeds elongate prior to radicle protrusion, concurrent with an increase in water content. Between 9 h and 1 d no increase in length or water content was observed. During incubation in ABA the length of imbibed seeds without TR did not increase between 1 and 5 d, whereas nitrate added to ABA induced TR and a 94% increase in length. At the same time the water content of seeds without TR increased by 18%, while the water content of seeds with TR increased by 38%. Length and water content were correlated in a single-seed analysis for seeds with TR, but not for seeds without TR. Increased length was also observed in Arabidopsis seeds with nitrate-induced TR. These results indicate that prior to endosperm rupture dormancy release by nitrate is accompanied by TR, seed elongation and an increase in water content. A new multiphasic model is proposed for the imbibition curve, where the second phase of the classical triphasic curve is split into three sub-phases, of which phases IIB and IIC are associated with TR.

A new seed coat water-impermeability mechanism in Chaetostoma armatum (Melastomataceae): evolutionary and biogeographical implications of physiophysical dormancy

Abstract: Determining the phylogenetic and biogeographic distribution of physical dormancy remains a major challenge in germination ecology. Here, our goal was to describe a novel water-impermeable seed coat mechanism causing physical dormancy (PY) in the seeds of Chaetostoma armatum (Melastomataceae). Although seed coat permeability tests indicated a significant increase in seed weight after soaking in distilled water, anatomical and dye-tracking analyses showed that both water and dyes penetrated the seed coat but not the embryo, which remained in a dry state. The water and dye penetrated the lumen of the exotestal cells, which have a thin outer periclinal face and thickened secondary walls with U-shaped phenolic compounds. Because of this structure, water and dye do not penetrate the inner periclinal face of the exotestal cells, indicating PY. Puncturing the seeds increased germination more than tenfold compared to that of the control, but GA3 did not increase germination further. A significant fraction of the seeds did not germinate after puncturing, indicating that embryos are also physiologically dormant (PD). This paper constitutes the first report of the water-impermeable seed coat in the Myrtales and the first report of physiophysical (PD+PY) dormancy in a shrub from a tropical montane area.

Interactions between substrate temperature and humidity in signalling cyclical dormancy in seeds of two perennial tropical species

Abstract: Abstract The involvement of environmental factors in dormancy cycling is well known in temperate annual species, but it is not known how interaction between soil temperatures and humidity can modulate dormancy in perennial tropical species. In this study the effects were evaluated of substrate temperature and humidity on the modulation of the acquisition and overcoming of secondary dormancy in the buried seeds of two endemic Eriocaulaceae species from the rocky fields (campos rupestres) vegetation in south-eastern Brazil. Fresh seeds of Comanthera bisulcata and Syngonanthus verticillatus were buried and subsequently maintained at temperatures of 15, 20, 25 and 30°C, under three substrate humidity levels (boggy, humid and humid/dry). The seeds were exhumed every 3 months and tested for germination (20°C, 12 h photoperiod) and viability (tetrazolium test). The seeds of both species acquired dormancy after burial in all of the treatments. During the experimental period they demonstrated cycles of acquisition and overcoming of dormancy that were most evident in the treatments involving alterations of the substrate humidity (humid/dry regime) that coincided with the environmental conditions found naturally in the region of origin of the species. The seeds gradually lost dormancy during the dry period and re-acquired it when exposed again to humidity; dormancy would once again be overcome during the subsequent dry period. Burial promoted the acquisition of dormancy in C. bisulcata and S. verticillatus seeds; the lowest temperature tested favoured overcoming dormancy; and varying the humidity regime signalled the acquisition and the overcoming of secondary dormancy.

Longevity of seeds and soil seed bank of the Cerrado tree Miconia chartacea (Melastomataceae)

Abstract: Miconia chartacea is a widely distributed tree in Brazil, occurring at altitudes ranging from 300 m to 1900 m in the Caatinga, Cerrado and Atlantic Forest biomes. In this work we attempted to classify M. chartacea seeds regarding their behaviour during storage and their germination syndrome and to determine, from a storage test in Cerrado soil and laboratory conditions in situ and ex situ, the longevity of seeds, as well as the capacity of the species to form a soil seed bank. The results suggested that M. chartacea seeds form a transient soil seed bank in the Cerrado and can be classified as orthodox in terms of storage behaviour, although the seeds are dispersed with a relatively high water content. The life span of seeds was favoured in soil-stored seeds in comparison with dry storage at 25°C, whereas storage at low temperatures prevented a decrease of the seed's germinability with storage time (330 d). M. chartacea seeds are dispersed during the dry season and germinate during the next rainy season, which can be classified as an intermediate–dry germination syndrome. Seeds of this species are dispersed in the Cerrado when temperatures and soil moisture are relatively low, which favours the formation of a soil seed bank, considering that the seeds tolerate desiccation and their longevity is favoured by low temperatures. A transient seed bank type is favoured by the loss of viability in storage at warm temperatures linked to the rainy season, and the predictable seasonal variations in climate in the region, with germination being restricted to the beginning of the rainy season.

The crypsis hypothesis: a stenopic view of the selective factors in the evolution of physical dormancy in seeds

Abstract: Physical dormancy (PY) in seeds/fruits, which is caused by the water-impermeable palisade layer, has long been considered a mechanism for synchronizing germination to a favourable time for seedling survival and establishment. Recently, a new hypothesis (crypsis hypothesis) was proposed as the main selective factor for the evolution of PY. However, there are some misconceptions in this hypothesis. Our objective is to critically evaluate the crypsis hypothesis and to point out that there are multiple adaptive roles of PY. The fundamental argument in the crypsis hypothesis, that PY evolved as an escape mechanism from predators, is not valid according to the evolutionary theory of Darwin. According to Darwin's hypothesis, variations (dormancy in our case) within a population occur randomly, i.e. there is no direct function of a variation at the time of its origin. Different selection pressures operating in the environment increase or decrease the fitness of individuals with the variation. Water-gap anatomy in seeds/fruits and phylogenetic relationships of species with PY suggest that PY has evolved several times in angiosperms. Thus, we argue that not only predatory pressure but also several other environmental pressures (e.g. proper timing of germination, ultra-drying of seeds, dispersal and pathogens) were involved in increasing the fitness of species producing seeds with PY. The significance of PY in the survival of the species under the above-mentioned environmental pressures and other misconceptions of the crypsis hypothesis are discussed in detail.

Secondary dormancy in Brassica napus is correlated with enhanced BnaDOG1 transcript levels

Abstract: Dormancy has evolved in plants to restrict germination to favourable growth seasons. Seeds from most crop plants have low dormancy levels due to selection for immediate germination during domestication. Seed dormancy is usually not completely lost and low levels are required to maintain sufficient seed quality. Brassica napus cultivars show low levels of primary seed dormancy. However, B. napus seeds are prone to the induction of secondary dormancy, which can lead to the occurrence of volunteers in the field in subsequent years after cultivation. The DELAY OF GERMINATION 1 (DOG1) gene has been identified as a major dormancy gene in the model plant Arabidopsis thaliana. DOG1 is a conserved gene and has been shown to be required for seed dormancy in various monocot and dicot plant species. We have identified three B. napus genes with high homology to AtDOG1, which we named BnaA.DOG1.a, BnaC.DOG1.a and BnaC.DOG1.b. The transcripts of these genes could only be detected in seeds and showed a similar expression pattern during seed maturation as AtDOG1. In addition, the BnaDOG1 genes showed enhanced transcript levels after the induction of secondary dormancy. These results suggest a role for DOG1 in the induction of secondary dormancy in B. napus.

Mating system modulates degree of seed dormancy in Hypericum elodes L. (Hypericaceae)

Abstract: Knowledge of processes responsible for seed dormancy can improve our understanding of the evolutionary dynamics of reproductive systems. We examined the influence of the breeding system on primary seed dormancy in Hypericum elodes, an Atlantic–European softwater pools specialist plant that exhibits a mixed mating strategy (the ability to both self- and cross-pollinate) to set seeds. Seeds were obtained through hand pollination treatments performed in a natural population during three consecutive years. Primary dormancy of seeds recovered from each pollination treatment was measured by analysing the seed germination response at dispersal and after various periods of cold stratification. While all collected seeds exhibited physiological dormancy, the degree of primary dormancy was associated with the pollination treatments. Weak and rapid loss of primary dormancy characterized seeds recovered from self-pollinated flowers, while stronger dormancy was found in seeds obtained from cross-pollination. The association between pollination treatments and primary dormancy indicated that the mating system should be considered as a source of variation for dormancy degree, proportional to self- and cross-pollinations (selfing rate) within populations of this species. These results suggest that by shedding seeds with various degrees of dormancy, plants may distribute their offspring across time by means of polymorphism in germination response. We conclude that seed germination alone is not an appropriate fitness measure for inbreeding depression estimates, unless dormancy is removed.

Seed afterripening and germination photoinhibition in the genus Crocus (Iridaceae)

Abstract: Mediterranean characteristics are attributed to the genus Crocus, which is inadequately studied in terms of seed germination. An afterripening requirement is very common in environments with warm and dry periods, and photoinhibition has been detected in many angiosperms inhabiting dry and open areas. The effects of afterripening and light on seed germination were investigated for the first time in 23 native Greek Crocus taxa, collected from various localities with either a Mediterranean or a temperate climate. Germination experiments were conducted in continuous darkness and in light at the optimal temperature for each taxon, with both freshly collected and afterripened seeds; warm stratification (20°C, darkness) was also examined in 22 taxa. A number of selected taxa were additionally investigated with respect to afterripening outdoors, afterripening and warm stratification at higher temperatures (35 and 25°C, respectively), stratification at 20/10°C, dry storage at low temperatures, response to gibberellic acid and phenology of embryo growth. It was postulated that an afterripening requirement is a characteristic of the genus Crocus, and we found that it can be fulfilled in nature during the Mediterranean dry summer. Also, for the vast majority of the taxa, warm stratification and stratification at 20/10°C can both meet the afterripening requirement. Embryos of the taxa studied are underdeveloped and have to grow prior to germination. Intrageneric differences of seed germination were observed only towards light, with photoinhibition being predominant in taxa from drier environments.

Analysis of QTLs for the micromorphology on the seed coat surface of soybean using recombinant inbred lines

Abstract: The seed coat of soybean (Glycine max (L.) Merrill) must protect the seed but allow water intake. Overprotection, causing impermeability, is assumed to be due to the presence of an impermeable layer in the seed coat, although validation of this assumption has relied on imbibition testing, which tends to be influenced by microfractures in the seed coat. Recent micromorphological analyses using laser-assisted topography microscopy revealed links to the surface roughness (SR) of the seed coat. To verify genetic links between hardseededness and SR, we analysed quantitative trait loci (QTLs) governing SR formation using 148 recombinant inbred lines (RILs) with a genetic linkage map covering 2663.6 cM of all 20 linkage groups of soybean, with 355 DNA markers and 5 phenotype markers. Five QTLs were detected, including previously identified hardseededness QTLs for ratio of seeds absorbing water, namely RAS1 and RAS2, which accounted for 20% of the phenotypic variance, and one near a locus inhibiting seed coat colour (I). These results indicate that the impermeability of soybean seed is genetically related to the reduction of SR.

Non-deep simple morphophysiological dormancy in seeds of Viburnum lantana (Caprifoliaceae), a new dormancy level in the genus Viburnum

Abstract: Seed germination requirements of Viburnum lantana were investigated by experiments both in the laboratory and outdoors. Embryo length, radicle emergence and shoot emergence were analysed to determine the level of morphophysiological dormancy (MPD) of seeds. Mean embryo length in fresh seeds was 1.30 mm, and required growth to at least 2.51 mm to germinate. The critical embryo length was 4.1 mm. In the laboratory, the embryo reached 3 mm length after 20 weeks of warm-temperature incubation (20/7 or 25/108C), which in fact represents a combination of warm þ cold stratification. In seeds subjected to cold stratification (1.5 or 58C) for 24 weeks, embryos hardly grew. Gibberellic acid stimulated embryo growth and germination. In the outdoor phenology test, the embryos grew from 1.30 mm, i.e. fresh seeds sown in September, to 2.98 mm at the end of the following March. In the ‘move-along’ test (laboratory), starting with temperatures of warm stratification [i.e. 25/108C (4 weeks) ! 20/78C (4 weeks) ! 15/48C (4 weeks) ! 58C (12 weeks) ! 15/48C (4 weeks)], and in the outdoor phenology study on seeds exposed to a similar temperature sequence, radicle emergence percentages reached 73% after 28 and 35 weeks, respectively. V. lantana does not exhibit a delay between root and shoot emergence, dismissing any kind of epicotyl dormancy. Seeds of V. lantana have non-deep simple MPD, a level not detected previously in the genus Viburnum, with the physiological dormancy component overcome by a combination of warm and cold stratification, preferably in that order.

Seed dormancy and germination of the three tropical medicinal species Gomphandra luzoniensis (Stemonuraceae), Nothapodytes nimmoniana (Icacinaceae) and Goniothalamus amuyon (Annonaceae)

Abstract: The three tropical species Gomphandra luzoniensis, Nothapodytes nimmoniana and Goniothalamus amuyon contain important cancer-fighting drugs; however, little is known about how to propagate these species from seeds. Thus, the aim of this study was to determine the germination requirements of seeds of each of these three species in order to provide an effective protocol to produce plants. Fresh seeds of G. luzoniensis germinated up to 73% at high temperatures in light in 4 weeks, and embryos were underdeveloped. Most seeds had morphological dormancy (MD), but a proportion of them had morphophysiological dormancy (MPD). Fresh seeds of N. nimmoniana germinated up to 50% in light in 4 weeks, embryo length increased by 17% before radicle emergence and ≥ 89% of the seeds had germinated after incubation for 6 weeks in light at high temperatures. Thus, about 50% of the seeds have MD and about 50% MPD. Fresh seeds of G. amuyon incubated at 30°C in light for 4 weeks germinated to 69%, whereas at the other incubation temperatures germination took longer than 4 weeks. Embryo length increased 213% before radicle emergence, and after 8 weeks of incubation at high temperatures ≥ 80% of the seeds had germinated. As in the other two species, the seed population consisted of a mixture of MD and MPD. Incubation of seeds of these three species at high temperatures (e.g. 25, 30 and 30/20°C) for up to 2 months is recommended for germination and thus seedling production.

Variations in seed micromorphology of Paphiopedilum and Cypripedium (Cypripedioideae, Orchidaceae)

Abstract: The micromorphology of a seed is linked to its dispersal and germination, but evolutionary and ecological aspects in Orchidaceae remain unclear. We investigated the seed characters of Paphiopedilum and Cypripedium that might be associated with life form and involved in possible ecological adaptations. A phylogenetic comparative analysis of nine seed micromorphological characters was performed in 24 species from two genera with close phylogenetic relationships but significant differences in their ecological characteristics. Species within Paphiopedilum had larger embryos and a smaller percentage of air space (AS) than those of Cypripedium species. Compared with 16 terrestrial species, two epiphytic Paphiopedilum species had larger embryos and smaller AS. Those larger embryos might ensure more successful seedling establishment while the higher amount of air space in both terrestrial Paphiopedilum and Cypripedium may increase seed buoyancy and enable them to disperse over longer distances. Whereas AS and seed length (SL) are phylogenetically conservative, most other characters examined here had weak signals, indicating clear convergent evolution. Across species, SL was positively correlated with AS, indicating a high degree of seed size-dispersal coordination. These findings may imply a trade-off of seed characters in relation to the possible ecological adaptations required for seedling establishment versus dispersal.

The crypsis hypothesis explained: a reply to Jayasuriya et al. (2015)

Abstract: In imbibing seeds, resumption of metabolism leads to the unavoidable release of volatile by-products that are perceived as cues by rodent seed predators. The crypsis hypothesis proposes that the primary function of a water-impermeable, hard seed coat is to reduce rodent seed predation by rendering seeds olfactorily cryptic. In an opinion paper, Jayasuriya et al. (2015) find the crypsis hypothesis unscientific and ‘not consistent with Darwin's theory of evolution by natural selection’. It is unfortunate that Jayasuriya et al. (2015) did not appreciate that the crypsis hypothesis offers an alternative explanation for the evolution of water-impermeable seeds: released seed volatiles are cues used by rodents to locate seeds, and variation in seed-coat permeability leading to differences in seed volatile release represents the variable under selection. Furthermore, the sealing of water-impermeable seed coats imposes a cost of increased generation time and, therefore, dormancy-release mechanisms are expected to subsequently evolve in response to local environmental conditions. We also disagree with most other claims by Jayasuriya et al. (2015), who failed to appreciate how species with dimorphic seeds – one morph with permeable and the other with impermeable seed coats – benefit from rodent caching behaviour and population dynamics. We welcome this opportunity to clarify and elaborate on key features and the evolution of water-impermeable seed coats according to the crypsis hypothesis.