Ethylene in seed dormancy and germination

The role of ethylene in the release of primary and secondary dormancy and the germination of non-dormant seeds under normal and stressed conditions is considered. In many species, exogenous ethylene, or ethephon – an ethylene-releasing compound - stimulates seed germination that may be inhibited because of embryo or coat dormancy, adverse environmental conditions or inhibitors (e.g. abscisic acid, jasmonate). Ethylene can either act alone, or synergistically or additively with other factors. The immediate precursor of ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid (ACC), may also improve seed germination, but usually less effectively. Dormant or non-dormant inhibited seeds have a lower ethylene production ability, and ACC and ACC oxidase activity than non-dormant, uninhibited seeds. Aminoethoxyvinyl-glycine (AVG) partially or markedly inhibits ethylene biosynthesis in dormant or non-dormant seeds, but does not affect seed germination. Ethylene binding is required in seeds of many species for dormancy release or germination under optimal or adverse conditions. There are examples where induction of seed germination by some stimulators requires ethylene action. However, the mechanism of ethylene action is almost unknown.
The evidence presented here shows that ethylene performs a relatively vital role in dormancy release and seed germination of most plant species studied.     

Germination of dormant seeds ofStylosanthes humilis as related to heavy metal ions

Cadmium, copper and zinc ions at high concentrations partially released scarified freshly-harvested seeds ofStylosanthes humilis from physiological dormancy. This response to toxic metals increased along with seed postharvest ageing. Cobalt and silver ions, and abscisic acid impaired metal-promoted germination. CAPES, CNPq, FAPEMIG and FINEP are gratefully acknowledged for the financial support granted for the conduction of this work.     

The Effects of Gibberellic Acid and Scarification on the Seed Dormancy and Germination in Luzula spicata

Luzula spicata L. seeds are completely dormant at maturity. A germination inhibitor is present at the micropylar end. Normally, the only effective means of eliciting germination is a precise scarification of the micropylar end which inactivates the inhibitor. Exogenous application of gibberellic acid, kinetin, KNO₃, and thiourea have no affect on the dormancy of unscarified seeds. Scarification of the hilar end of the seed does not elicit germination, but when gibberellic acid is applied to the hilar scarified seeds moderate germination results. Presumably, these seeds are dormant due to a deficit of endogenous gibberellin; a condition which can be overcome by the application of gibberellic acid to seeds scarified at a site in itself ineffective in producing germination. Apparently the gibberellic acid serves to initiate amylase activity in the endosperm, overcoming the inhibitor block. Luzula spicata seed dormancy is apparently unique in that a germination inhibitor is operative in conjunction with the commonly recognized gibberellin-amylase mechanism.     

Seed dormancy of Cardiospermum halicacabum (Sapindaceae) from three precipitation zones in Sri Lanka

• This study investigated seed germination of Cardiospermum halicacabum, a medicinally important invasive species.
• We compared mass, moisture content (MC), dormancy and dormancy‐breaking treatments and imbibition and germination of scarified and non‐scarified seeds of C. halicacabum from a low‐elevation dry zone (DZ), low‐elevation wet zone (WZ1) and mid‐elevation wet zone (WZ2) in Sri Lanka to test the hypothesis that the percentage of seeds with water‐impermeable seed coats (physical dormancy, PY) decreases with increased precipitation.
• Seed mass was higher in WZ2 than in DZ and WZ1, while seed MC did not vary among the zones. All scarified DZ, WZ1 and WZ2 and non‐scarified DZ and WZ1 seeds imbibed water, but only a few non‐scarified WZ2 seeds did so. When DZ and WZ1 seeds were desiccated, MC and percentage imbibition decreased, showing that these seeds have the ability to develop PY. GA₃ promoted germination of embryos excised from fresh DZ and WZ1 seeds and of scarified WZ2 seeds.
• At maturity, seeds from DZ and WZ1 had only physiological dormancy (PD), while those from WZ2 had combinational dormancy (PY+PD). Thus, our hypothesis was not supported. Since a high percentage of excised embryos developed into normal seedlings; this is a low‐cost method to produce C. halicacabum plants for medicinal and ornamental purposes.

     

Combinational dormancy in seeds of Sicyos angulatus (Cucurbitaceae,tribe Sicyeae)

Seeds with a water‐impermeable seed coat and a physiologically dormant embryo are classified as having combinational dormancy. Seeds of Sicyos angulatus (burcucumber) have been clearly shown to have a water‐impermeable seed coat (physical dormancy [PY]). The primary aim of the present study was to confirm (or not) that physiological dormancy (PD) is also present in seeds of S. angulatus. The highest germination of scarified fresh (38%) and 3‐month dry‐stored (36%) seeds occurred at 35/20°C. The rate (speed) of germination was faster in scarified dry‐stored seeds than in scarified fresh seeds. Removal of the seed coat, but leaving the membrane surrounding the embryo intact, increased germination of both fresh and dry‐stored seeds to > 85% at 35/20°C. Germination (80–100%) of excised embryos (both seed coat and membrane removed) occurred at 15/6, 25/15 and 35/20°C and reached 95–100% after 4 days of incubation at 25/15 and 35/20°C. Dry storage (after‐ripening) caused an increase in the germination percentage of scarified and of decoated seeds at 25/15°C and in both germination percentage and rate of excised embryos at 15/6°C. Eight weeks of cold stratification resulted in a significant increase in the germination of scarified seeds at 25/15 and 35/20°C and of decoated seeds at 15/6 and 25/15°C. Based on the results of our study and on information reported in the literature, we conclude that seeds of S. angulatus not only have PY, but also non‐deep PD, that is, combinational dormancy (PY + PD).     

Influence of the Seed Envelope and Growth Regulators Upon Seed Dormancy in Witchweed (Striga lutea Lour.)

Seed of the angiospermous parasite, witchweed (Striga luteaLour.), normally germinate poorly or not at all unless adequatelypretreated and exposed to a germination stimulant which is obtainedfrom plant root exudates. Under certain conditions, scarificationof the seed envelope promoted germination in the absence ofthe stimulant. With adequately (2-week) pretreated seed, a cutor puncture through the aleurone at the radicular end inducedtypical germination. A puncture through the aleurone elsewhereon the seed induced little or no germination. A cut throughthe aleurone at the cotyledon end or centre of the seed inducedatypical germination. A puncture through the aleurone elsewhereon the seed induced little or no germination. A cut throughthe aleurone at the cotyledon end or centre of the seed inducedatypical germination in which the radicle elongated but didnot penetrate the intact envelope over the radicle. Incubationin oxygen did not promote germination regardless of the siteof seed scarification. Ten per cent carbon dioxide reduced thegermination of punctured seed. Seed germinated equally wellwhen scarified over the radicle and incubated in air, nitrogen,darkness, or light. Brief treatments with sulphuric acid alsoinduced germination. The softening effect of stimulatory acidtreatments upon the aleurone was first evident 6 h later andonly occurred at the radicular end of the seed. Indol-3yl-aceticacid inhibited and ethylene stimulated germination of scarifiedand non-sacrified seed. Gibberellic acid(GA₃) had no apparenteffect upon nonscarified seed but promoted germination of scarifiedseed. Inadequately (I-day) or excessively (12- to 16-week) pretreatedseed germinated poorly or not at all when treated with the germinationstimulant, ethylene, GA₃, or sulphuric acid. Some seed germinatedslowly when scarified over the radicle, but the germinationrates and totals were less than those of scarified seed pretreatedfor 2 weeks. Additions of stimulant, ethylene or GA₃, aceleratedthe germination rates of scarified but inadequately pretreatedseed. Of the treatments tested, only GA₃, increased the slowgermination of the excessively pretreated scarified seed. Results indicated that the aleurone restrained radicle elongation.Scarification over the radicle removed the restraint and permittedradicle emergence. However, the ability of the radicle to elongate,as influenced by time of seed pretreatment and exogenous stimulant,GA₃ or ethylene, determined whether or not the scarified seedgerminated.     

Induction of secondary dormancy in Amaranthus caudatus seeds

Nondormant A. caudatus seeds germinated in the darkat temperatures between 20 and 35° but not at 45 °C.Incubation at this temperature for at least 10 h inhibited seedgermination over the temperature range 20 to 35 °C,temperatures previously suitable for germination. Thus incubation at 45°C induced secondary dormancy. Mechanical or chemicalscarification or exposure to pure oxygen caused complete or almost completegermination of dormant seeds although more slowly in comparison to nondormantseeds. Secondary dormant scarified seeds required a lower concentration of ABAthan nondormant seeds to inhibit germination. The high temperature, whichinduced dormancy, 45 °C, caused the seed coat to be partiallyresponsible for secondary dormancy. Involvement of ABA (synthesis orsensitivity) in the induction and/or maintenance of this dormancy should beconsidered.     

The role of seed germination in the invasion process of Honey locust (Gleditsia triacanthos L., Fabaceae): comparison with a native confamilial

Identifying plant traits that promote invasiveness has been a major goal in invasion ecology. Germination plays a central role in the life cycle of plants and therefore could be a key trait in determining species invasiveness. In this study, seed germination of two confamilial, co‐occurring species that share ecological characteristics, the exotic invasive Gleditsia triacanthos L and the native Acacia aroma Gillies ex. Hook. & Arn., was compared. Seeds were obtained from individuals of three localities in the Chaco Serrano region of Córdoba, Argentina. Percent of seed germination and mean germination time were recorded in chemically and mechanically scarified seeds, and the former variable was also recorded in seeds subjected to: passage through the digestive tract of dispersers, fire simulations, fire simulation plus mechanical scarification, seed longevity, and dormancy break over time. In general, both species showed similar germination percentage. However, non‐scarified seeds of the exotic species lost physical dormancy when subjected to experiments of dormancy break over time, whereas, the native species had shorter mean germination time. The greater percentage of seed germination over time of the exotic species than of the native one might be triggering the spread of the former, whereas the shorter mean germination time might be hindering its expansion to more arid regions. The study of different mechanisms for achieving seed germination, particularly in hard seed species, could provide important information on the expansion of invasive species as well as useful knowledge for their management.     

Seed anatomy and water uptake in relation to seed dormancy in Opuntia tomentosa (Cactaceae, Opuntioideae)

BACKGROUND AND AIMS: There is considerable confusion in the literature concerning impermeability of seeds with 'hard' seed coats, because the ability to take up (imbibe) water has not been tested in most of them. Seeds of Opuntia tomentosa were reported recently to have a water-impermeable seed coat sensu lato (i.e. physical dormancy), in combination with physiological dormancy. However, physical dormancy is not known to occur in Cactaceae. Therefore, the aim of this study was to determine if seeds of O. tomentosa are water-permeable or water-impermeable, i.e. if they have physical dormancy. METHODS: The micromorphology of the seed coat and associated structures were characterized by SEM and light microscopy. Permeability of the seed-covering layers was assessed by an increase in mass of seeds on a wet substrate and by dye-tracking and uptake of tritiated water by intact versus scarified seeds. KEY RESULTS: A germination valve and a water channel are formed in the hilum-micropyle region during dehydration and ageing in seeds of O. tomentosa. The funicular envelope undoubtedly plays a role in germination of Opuntia seeds via restriction of water uptake and mechanical resistance to expansion of the embryo. However, seeds do not exhibit any of three features characteristic of those with physical dormancy. Thus, they do not have a water-impermeable layer(s) of palisade cells (macrosclereids) or a water gap sensu stricto and they imbibe water without the seed coat being disrupted. CONCLUSIONS: Although dormancy in seeds of this species can be broken by scarification, they have physiological dormancy only. Further, based on information in the literature, it is concluded that it is unlikely that any species of Opuntia has physical dormancy. This is the first integrative study of the anatomy, dynamics of water uptake and dormancy in seeds of Cactaceae subfamily Opuntioideae.     

Proteome analysis of Norway maple (Acer platanoides L.) seeds dormancy breaking and germination: influence of abscisic and gibberellic acids

Background  

Seed dormancy is controlled by the physiological or structural properties of a seed and the external conditions. It is induced as part of the genetic program of seed development and maturation. Seeds with deep physiological embryo dormancy can be stimulated to germinate by a variety of treatments including cold stratification. Hormonal imbalance between germination inhibitors (e.g. abscisic acid) and growth promoters (e.g. gibberellins) is the main cause of seed dormancy breaking. Differences in the status of hormones would affect expression of genes required for germination. Proteomics offers the opportunity to examine simultaneous changes and to classify temporal patterns of protein accumulation occurring during seed dormancy breaking and germination. Analysis of the functions of the identified proteins and the related metabolic pathways, in conjunction with the plant hormones implicated in seed dormancy breaking, would expand our knowledge about this process.     

Selenomethionine as a dormancy-breaking agent in seeds of Stylosanthes humilis

Physiological dormancy of scarified seeds of Townsville stylo (Stylosanthes humilis H.B.K.) was released by seleno-L-methionine (SeM), but not by L-methionine. This regulating effect was impaired by inhibitors of ethylene biosynthesis and action; in the first case SeM action was restored by 2-chloroethylphosphonic acid (CEPA) and 1-aminocyclopropane-1-carboxylic acid (ACC). The Se-aminoacid proved to be toxic in a time-dependent manner to seedling growth, inhibiting primarily the hypocotyl expansion. This toxicity is suggested to trigger ethylene biosynthesis, which would promote germination of dormant seeds.     

乙烯与水浮莲(Pistia stratiotes)种子需光性休眠的关系

水浮莲种子是一种奇特的需光种子。在黑暗中,GA_2或BA均不能代替光照诱导萌发,可是0.1μl/l乙烯却能引起部分种子萌发,在1000μ1/1乙烯的作用下,发芽率可达80％,接近全光照处理的萌发水平(91％发芽率)。ACC也能诱导水浮莲种子的萌发,0.1 mM浓度可获30％发芽率。在较短光照下,ACC对种子萌发有增效作用。在光照前应用ACC,其诱导效应大于两者同时施用。在照光萌发中,种子的内源ACC含量及乙烯释放量均显著增加。CoCl_2和AOA均能抑制光的诱导萌发。推论光打破休眠诱导萌发的作用是与乙烯的生成密切相关。     

Control of seed dormancy in Nicotiana plumbaginifolia: post-imbibition abscisic acid synthesis imposes dormancy maintenance

The physiological characteristics of seed dormancy in Nicotiana plumbaginifolia Viv. are described. The level of seed dormancy is defined by the delay in seed germination (i.e the time required prior to germination) under favourable environmental conditions. A wild-type line shows a clear primary dormancy, which is suppressed by afterripening, whereas an abscisic acid (ABA)-deficient mutant shows a non-dormant phenotype. We have investigated the role of ABA and gibberellic acid (GA₃) in the control of dormancy maintenance or breakage during imbibition in suitable conditions. It was found that fluridone, a carotenoid biosynthesis inhibitor, is almost as efficient as GA₃ in breaking dormancy. Dry dormant seeds contained more ABA than dry afterripened seeds and, during early imbibition, there was an accumulation of ABA in dormant seeds, but not in afterripened seeds. In addition, fluridone and exogenous GA₃ inhibited the accumulation of ABA in imbibed dormant seeds. This reveals an important role for ABA synthesis in dormancy maintenance in imbibed seeds. Received: 31 December 1998 / Accepted: 9 July 1999     

Action of ferric and aluminium ions on the dormancy breakage of <Emphasis Type="Italic">Stylosanthes humilis</Emphasis> seeds

Dormancy of scarified seeds of Stylosanthes humilis was broken by acidic Al³⁺ and Fe³⁺ solutions. Fe⁺³-stimulated seeds exhibited a high activity of 1-aminocyclopropane-1-carboxylate (ACC) oxidase and produced great amounts of ethylene, which showed correlated with the germination process. In addition, specific inhibitors of ethylene biosynthesis and action largely depressed the Fe³⁺-stimulated germination, leading to the conclusion that the ion broke dormancy by triggering ethylene production by the seeds. By contrast, inhibitors of ethylene biosynthesis and action did not impair germination of Al³⁺-stimulated dormant seeds. Moreover, ethylene production and activity of ACC oxidase of Al³⁺-treated seeds was substantially decreased by inhibitors of ethylene biosynthesis, but germination kept large. Together these data suggest that ethylene biosynthesis was not required in the chain of events triggered by Al³⁺ leading to dormancy breakage. Methyl viologen (MV), a reactive oxygen species-generating compound, broke dormancy of seeds to the same extent as Al³⁺ did. Germination of both Al³⁺- and MV-stimulated dormant seeds was inhibited by sodium selenate, an antioxidant compound; selenate, however had no effect on germination of Fe³⁺-stimulated seeds. Together these data indicate that the mechanisms underlying the germination of Al³⁺- and Fe³⁺-treated seeds are not the same.     

Ameliorative effect of brassinosteroid and ethylene on germination of cucumber seeds in the presence of sodium chloride

Brassinosteroids are a class of plant polyhydroxysteroids with a diverse of functions in plant growth and development, while ethylene is a gaseous hormone involved in regulation of numerous physiological processes. To evaluate the roles of BR and ethylene in seed germination under conditions of salt stress, effects of 24-Epibrassinolide (EBR) and 1-aminocyclopropane-1-carboxylic acid (ACC) on seed germination of cucumber (Cucumis sativus) seeds in the presence of 250 mM NaCl were investigated. Seed germination was significantly inhibited by the presence of NaCl in the incubation medium, and the inhibitory effect was significantly alleviated by addition of EBR and ACC to the incubation medium containing NaCl. There was an increase in ethylene evolution during seed germination and this increase was suppressed by salt stress. The reduction in ethylene evolution from imbibed seeds by salt stress was attenuated by EBR. Salt stress inhibited ACC oxidase (ACO) activity and EBR reversed the salt stress-induced decrease in ACO activity. Salt stress reduced expression of gene encoding ACO (CsACO2), and EBR reversed the salt stress-induced down-regulation of CsACO2. The alleviative effect of EBR on seed germination in the presence of NaCl was diminished by antagonist of ethylene synthesis, aminoethoxyvinylglycine. These results indicate that both ethylene and BR are likely to be associated with suppression of seed germination under salt stress and that the mitigating effect of BR on salt stress-induced inhibition of seed germination may occur through its interaction with ethylene synthesis.     

Ethylene-dependent action of gibberellin in seed germination of Amaranthus caudatus

The role of gibberellins in the germination of seeds of Amaranthus caudatus L. was examined. Tetcyclacis (BAS 106), an inhibitor of gibberellin biosynthesis, inhibited germination of the seeds. The inhibition caused by BAS 106 was antagonised by gibberellin A₄₊₇ (GA₄₊₇). Ethephon (2-chloroethylphosphonic acid) and 1-aminocyclopropane-1-carboxylic acid (ACC) could replace GA₄₊₇. Ethephon and ACC counteracted also the side effects of BAS 106 that are not reversible by GA₄₊₇. The rate of seed germination was not increased by gibberellin in the presence of aminoethoxyvinylglycine (AVG). AVG increased the effect of BAS 106. GA₄₊₇ could not reverse the effect of BAS 106 when AVG was simultaneously applied. The results indicate that the biosynthesis of endogenous gibberellins may be required for germination of A. caudatus seeds and that main physiological effects of GA₄₊₇ on seed germination may depend on ethylene biosynthesis.     

Ethylene production is associated with germination but not seed dormancy in red rice

BACKGROUND AND AIMS: The relationship between ethylene production and both seed dormancy and germination was investigated using red rice (weedy rice) as a model species. METHODS: Both fully dormant and after-ripened (non-dormant) naked caryopses were incubated with or without inhibitors of ethylene synthesis [aminoethoxyvinylglycine (AVG)] and perception [silver thiosulfate (STS)], or in the presence of the natural ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). The kinetics of ethylene emissions were measured with a sensitive laser-photoacoustic system. KEY RESULTS: Dormant red rice caryopses did not produce ethylene. In non-dormant caryopses, ethylene evolution never preceded the first visible stage of germination (pericarp splitting), and ethylene inhibitors completely blocked ethylene production, but not pericarp splitting. Accordingly, endogenous ACC appeared to be lacking before pericarp splitting. However, early seedling growth (radicle or coleoptile attaining the length of 1 mm) followed ethylene evolution and was delayed by the inhibitors. Wounding the dormant caryopses induced them to germinate and produce ethylene, but their germination was slow and pericarp splitting could be speeded up by ethylene. CONCLUSIONS: The findings suggest that, in red rice, endogenous ethylene stimulates the growth of the nascent seedling, but does not affect seed dormancy or germination inception. Correspondingly, this phytohormone does not play a role in the dormancy breakage induced by wounding, but accelerates germination after such breakage has occurred.     

Dormancy and germination: How does the crop seed decide?

Whether seeds germinate or maintain dormancy is decided upon through very intricate physiological processes. Correct timing of these processes is most important for the plants life cycle. If moist conditions are encountered, a low dormancy level causes pre‐harvest sprouting in various crop species, such as wheat, corn and rice, this decreases crop yield and negatively impacts downstream industrial processing. In contrast, a deep level of seed dormancy prevents normal germination even under favourable conditions, resulting in a low emergence rate during agricultural production. Therefore, an optimal seed dormancy level is valuable for modern mechanised agricultural systems. Over the past several years, numerous studies have demonstrated that diverse endogenous and environmental factors regulate the balance between dormancy and germination, such as light, temperature, water status and bacteria in soil, and phytohormones such as ABA (abscisic acid) and GA (gibberellic acid). In this updated review, we highlight recent advances regarding the molecular mechanisms underlying regulation of seed dormancy and germination processes, including the external environmental and internal hormonal cues, and primarily focusing on the staple crop species. Furthermore, future challenges and research directions for developing a full understanding of crop seed dormancy and germination are also discussed.     

Hydrogen sulfide counteracts chlorophyll loss in sweetpotato seedling leaves and alleviates oxidative damage against osmotic stress

Sawgrass (Cladium jamaicense) is the predominant plant and vegetation community in the Florida Everglades. Germination of sawgrass seeds in the laboratory or nursery has been difficult and problematic, yet little is known about the physiological mechanistic regulation of the sawgrass seed germination process. In the present study, we examined the factors and mechanisms that influence sawgrass seed germination. We found that removal of seed husk and bracts, pre-soaking with bleach (hypochlorite), breaking the seed coat, or combinations of these treatments promoted the rate and success of germination, whereas presence of seed-encasing structures or treatment with husk/bract extract inhibited germination. We further detected the presence of abscisic acid (ABA) in the husk and bract. Experiments with ABA and gibberellin biosynthesis inhibitors fluridone and tetcyclacis suggested that ABA already presented in the pre-imbibed seeds, and not derived through post-dormancy de novo synthesis, contributed to the inhibition of seed germination. Examination of bleach and mechanical treatments indicated the physical barrier presented by the seed-encasing structures provided additional mechanism for the long-term delay of seed germination. Based on the results of this study and others, we discussed the implications of sawgrass seed dormancy and germination in relation to its natural habitat and proposed a hypothesis that the protracted seed dormancy in sawgrass offered an adaptive advantage in the pre-anthropogenic Everglades environment, but may become a liability in the current man-managed Everglades water system.     

Seed dormancy in alpine species

In alpine species the classification of the various mechanisms underlying seed dormancy has been rather questionable and controversial. Thus, we investigated 28 alpine species to evaluate the prevailing types of dormancy. Embryo type and water impermeability of seed coats gave an indication of the potential seed dormancy class. To ascertain the actual dormancy class and level, we performed germination experiments comparing the behavior of seeds without storage, after cold-dry storage, after cold-wet storage, and scarification. We also tested the light requirement for germination in some species. Germination behavior was characterized using the final germination percentage and the mean germination time. Considering the effects of the pretreatments, a refined classification of the prevailing dormancy types was constructed based on the results of our pretreatments. Only two out of the 28 species that we evaluated had predominantly non-dormant seeds. Physiological dormancy was prevalent in 20 species, with deep physiological dormancy being the most abundant, followed by non-deep and intermediate physiological dormancy. Seeds of four species with underdeveloped embryos were assigned to the morphophysiologial dormancy class. An impermeable seed coat was identified in two species, with no additional physiological germination block. We defined these species as having physical dormancy. Light promoted the germination of seeds without storage in all but one species with physiological dormancy. In species with physical dormancy, light responses were of minor importance. We discuss our new classification in the context of former germination studies and draw implications for the timing of germination in the field.