Response of Amaranthus retroflexus L. seeds to gibberellic acid, ethylene and abscisic acid depending on duration of stratification and burial

Freshly harvested, dormant seeds of Amaranthus retroflexus were unable to germinate at 25 and 35 °C. To release their dormancy at the above temperatures, the seeds were stratified at a constant temperature (4 °C) under laboratory conditions or at fluctuating temperatures in soil or by outdoor burial in soil. Fully dormant, or seeds stratified or buried (2006/2007 and 2007/2008) for various periods were treated with exogenous gibberellic acid (GA₃), ethephon and abscisic acid (ABA). Likewise, the effects of these regulators, applied during stratification, on seed germination were determined. The results indicate that A. retroflexus seed dormancy can be released either by stratification or by autumn–winter burial. The effect of GA₃ and ethylene, liberated from ethephon, applied after various periods of stratification or during stratification, depends on dormancy level. GA₃ did not affect or only slightly stimulated the germination of non-stratified, fully dormant seeds at 25 and 35 °C respectively. Ethylene increased germination at both temperatures. Seed response to GA₃ and ethylene at 25 °C was increased when dormancy was partially removed by stratification at constant or fluctuating temperatures or autumn–winter burial. The response to GA₃ and ethylene increased with increasing time of stratification. The presence of GA₃ and ethephon during stratification may stimulate germination at 35 °C. Thus, both GA₃ and ethylene can partially substitute the requirement for stratification or autumn–winter burial. Both hormones may also stimulate germination of secondary dormant seeds, exhumed in September. The response to ABA decreased in parallel with an increasing time of stratification and burial up to May 2007 or March 2008. Endogenous GA_n, ethylene and ABA may be involved in the control of dormancy state and germination of A. retroflexus. It is possible that releasing dormancy by stratification or partial burial is associated with changes in ABA/GA and ethylene balance and/or sensitivity to these hormones.     

The Time Required for Dormancy Release in Arabidopsis Is Determined by DELAY OF GERMINATION1 Protein Levels in Freshly Harvested Seeds

Seed dormancy controls the start of a plant's life cycle by preventing germination of a viable seed in an unfavorable season. Freshly harvested seeds usually show a high level of dormancy, which is gradually released during dry storage (after-ripening). Abscisic acid (ABA) has been identified as an essential factor for the induction of dormancy, whereas gibberellins (GAs) are required for germination. The molecular mechanisms controlling seed dormancy are not well understood. DELAY OF GERMINATION1 (DOG1) was recently identified as a major regulator of dormancy in Arabidopsis thaliana. Here, we show that the DOG1 protein accumulates during seed maturation and remains stable throughout seed storage and imbibition. The levels of DOG1 protein in freshly harvested seeds highly correlate with dormancy. The DOG1 protein becomes modified during after-ripening, and its levels in stored seeds do not correlate with germination potential. Although ABA levels in dog1 mutants are reduced and GA levels enhanced, we show that DOG1 does not regulate dormancy primarily via changes in hormone levels. We propose that DOG1 protein abundance in freshly harvested seeds acts as a timer for seed dormancy release, which functions largely independent from ABA.     

AtPER1 enhances primary seed dormancy and reduces seed germination by suppressing the ABA catabolism and GA biosynthesis in Arabidopsis seeds

Seed is vital to the conservation of germplasm and plant biodiversity. Seed dormancy is an adaptive trait in numerous seed‐plant species, enabling plants to survive under stressful conditions. Seed dormancy is mainly controlled by abscisic acid (ABA) and gibberellin (GA) and can be classified as primary and secondary seed dormancy. The primary seed dormancy is induced by maternal ABA. Here we found that AtPER1, a seed‐specific peroxiredoxin, is involved in enhancing primary seed dormancy. Two loss‐of‐function atper1 mutants, atper1‐1 and atper1‐2, displayed suppressed primary seed dormancy accompanied with reduced ABA and increased GA contents in seeds. Furthermore, atper1 mutant seeds were insensitive to abiotic stresses during seed germination. The expression of several ABA catabolism genes (CYP707A1, CYP707A2, and CYP707A3) and GA biosynthesis genes (GA20ox1, GA20ox3, and KAO3) in atper1 mutant seeds was increased compared to wild‐type seeds. The suppressed primary seed dormancy of atper1‐1 was completely reduced by deletion of CYP707A genes. Furthermore, loss‐of‐function of AtPER1 cannot enhance the seed germination ratio of aba2‐1 or ga1‐t, suggesting that AtPER1‐enhanced primary seed dormancy is dependent on ABA and GA. Additionally, the level of reactive oxygen species (ROS) in atper1 mutant seeds was significantly higher than that in wild‐type seeds. Taken together, our results demonstrate that AtPER1 eliminates ROS to suppress ABA catabolism and GA biosynthesis, and thus improves the primary seed dormancy and make the seeds less sensitive to adverse environmental conditions.     

Hormonal and temperature regulation of seed dormancy and germination in <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Fluctuating temperature plays a critical role in determining the timing of seed germination in many plant species. However, the physiological and biochemical mechanisms underlying such a response have been paid little attention. The present study investigated the effect of plant growth regulators and cold stratification in regulating Leymus chinensis seed germination and dormancy response to temperature. Results showed that seed germination was less than 2 % at all constant temperatures while fluctuating temperature significantly increased germination percentage. The highest germination was 71 % at 20/30 °C. Removal of the embryo enclosing material of L. chinensis seed germinated to 74 %, and replaced the requirement for fluctuating temperature to germinate, by increasing embryo growth potential. Applications of GA₄₊₇ significantly increased seed germination at constant temperature. Also, inhibition of GA biosynthesis significantly decreased seed germination at fluctuating temperatures depending upon paclobutrazol concentration. This implied GA was necessary for non-dormant seed germination and played an important role in regulating seed germination response to temperature. Inhibition of ABA biosynthesis during imbibition completely released seed dormancy at 20/30 °C, but showed no effect on seed germination at constant temperature, suggesting ABA biosynthesis was important for seed dormancy maintenance but may not involve in seed germination response to temperature. Cold stratification with water or GA₃ induced seed into secondary dormancy, but this effect was reversed by exogenous FL, suggesting ABA biosynthesis during cold stratification was involved in secondary dormancy. Also, cold stratification with FL entirely replaced the requirement of fluctuating temperature for germination with seeds having 73 % germination at constant temperature. This appears to be attributed to inhibition of ABA biosynthesis and an increase of GA biosynthesis during cold stratification, leading to an increased embryo growth potential. We suggest that fluctuating temperature promotes seed germination by increasing embryo growth potential, mainly attributed to GA biosynthesis during imbibitions. ABA is important for seed dormancy maintenance and induction but showed less effect on non-dormant seed germination response to temperature.     

A method for profiling classes of plant hormones and their metabolites using liquid chromatography-electrospray ionization tandem mass spectrometry: an analysis of hormone regulation of thermodormancy of lettuce (Lactuca sativa L.) seeds

A highly selective and sensitive method for the simultaneous analysis of several plant hormones and their metabolites is described. The method combines high-performance liquid chromatography (HPLC) with positive and negative electrospray ionization-tandem mass spectrometry (ESI-MS/MS) to quantify a broad range of chemically and structurally diverse compounds. The addition of deuterium-labeled analogs for these compounds prior to sample extraction permits accurate quantification by multiple reaction monitoring (MRM). Endogenous levels of abscisic acid (ABA), abscisic acid glucose ester (ABA-GE), 7'-hydroxy-abscisic acid (7'-OH-ABA), phaseic acid (PA), dihydrophaseic acid (DPA), indole-3-acetic acid (IAA), indole-3-aspartate (IAAsp), zeatin (Z), zeatin riboside (ZR), isopentenyladenine (2iP), isopentenyladenosine (IPA), and gibberellins (GA)1, GA3, GA4, and GA7 were determined simultaneously in a single run. Detection limits ranged from 0.682 fmol for Z to 1.53 pmol for ABA. The method was applied to the analysis of plant hormones and hormonal metabolites associated with seed dormancy and germination in lettuce (Lactuca sativa L. cv. Grand Rapids), using extracts from only 50 to 100 mg DW of seed. Thermodormancy was induced by incubating seeds at 33 degrees C instead of 23 degrees C. Germinating seeds transiently accumulated high levels of ABA-GE. In contrast, thermodormant seeds transiently accumulated high levels of DPA after 7 days at 33 degrees C. GA1 and GA3 were detected during germination, and levels of GA1 increased during early post-germinative growth. After several days of incubation, thermodormant seeds exhibited a striking transient accumulation of IAA, which did not occur in seeds germinating at 23 degrees C. We conclude that hormone metabolism in thermodormant seeds is surprisingly active and is significantly different from that of germinating seeds.     

The role of Rhytidoponera metallica (Hymenoptera,Formicidae) in facilitating post‐fire seed germination of three ant‐dispersed legume species

The fire avoidance hypothesis proposes that a benefit of seed dispersal by ants (myrmecochory) is to protect seeds from being killed during fire and to facilitate post‐fire germination of seeds that require heat shock to break their physical dormancy. The aim of this study was to quantify the effect of fire and seed burial by a predominant seed‐dispersing ant, Rhytidoponera metallica (subfamily: Ectatomminae) on germination levels of three ant‐dispersed legume species (Pultenaea daphnoides, Acacia myrtifolia and Acacia pycnantha). Experimental burial of seeds within aluminium cans at a site prior to being burnt and at an adjacent unburnt site showed that fire increased germination levels, particularly for seeds buried at 1‐ and 2‐cm deep and that overall, germination levels differed among the three plant species. To quantify seed burial depths and post‐fire germination levels facilitated by R. metallica ants, seeds were fed to colonies prior to fire at the burnt and unburnt sites. Of the seeds buried within nests that were recovered, between 45% and 75% occurred within the upper 6 cm of the soil profile, although unexpectedly, greater percentages of seeds were recovered from the upper 0–2 cm of nests in the unburnt site compared with nests in the burnt site. Germination levels of buried seeds associated with R. metallica nests ranged from 21.2% to 29.5% in the burnt site compared with 3.1–14.8% in the unburnt site. While increased seed germination levels were associated with R. metallica nests following fire, most seeds were buried at depths below those where optimal temperatures for breaking seed dormancy occurred during the fire. We suggest that R. metallica ants may provide fire avoidance benefits to myrmecochorous seeds by burying them at a range of depths within a potential germination zone defined by intra‐ and inter‐fire variation in levels of soil heating.     

Seed dormancy and ABA signaling: The breakthrough goes on

The seed is an important organ in higher plants, it is an important organ for plant survival and species dispersion. The transition between seed dormancy and germination represents a critical stage in the plant life cycle and it is an important ecological and commercial trait. A dynamic balance of synthesis and catabolism of two antagonistic hormones, abscisic acid (ABA) and giberellins (GAs), controls the equilibrium between seed dormancy and germination. Embryonic ABA plays a central role in induction and maintenance of seed dormancy and also inhibits the transition from embryonic to germination growth. Therefore, the ABA metabolism must be highly regulated at both temporal and spatial levels during phase of dessication tolerance. On the other hand, the ABA levels do not depend exclusively on the seeds because sometimes it becomes a strong sink and imports it from the roots and rhizosphere through the xylem and/or phloem. These events are discussed in depth here. Likewise, the role of some recently characterized genes belonging to seeds of woody species and related to ABA signaling are also included. Finally, although four possible ABA receptors have been reported, not much is known about how they mediate ABA signaling transduction. However, new publications seem to show that almost all these receptors lack several properties to consider them as such.Key words: ABA/GA balance, ABA in woody plants, ABA-receptors, biosynthetic ABA mutants, rhizosphere ABA, seed dormancy     

The viability of seeds of rainforest species after experimental soil burials under tropical wet lowland forest in north-eastern Australia

Seed samples of 50 rainforest species representative of all successional stages from tropical lowland forests in north-eastern Australia were buried in nylon material bags under rainforest for periods up to 2 years. The seed samples were exhumed after 6 weeks, 6 months, 1 year and 2 years, and their viabilities assessed. Substantial portions of the samples of 31 species of pioneer and early and late secondary species retained viability for 2 years in the buried samples, and germinated immediately they were exposed to greenhouse conditions. In approximately one-third of these species, the dormancy was completely enforced by the burial conditions since fresh, mature seed germinated immediately in the greenhouse. The remainder of the long-lived seeds showed some evidence of variable initial periods of innate dormancy produced by their hard, impermeable coats. There was no indication that the burial conditions enforced or induced any dormancy in the seeds of primary forest trees. The mean viability of the primary forest seeds examined was 10% after 6 months burial. The soft-coated seeds were obligate immediate germinators; they germinated or died. The remainder showed variably delayed germination, and this was interpreted as a mechanical dormancy produced by their leathery, fibrous, or stony endocarps. The group of late secondary species sampled contained both obligate immediate germinators and seeds with well developed dormancy mechanisms. Many of the buried 'seedlings’ of primary forest species remained alive for a year or more attached to seeds, demonstrating the ability of the larger primary seeds to sustain seedlings under conditions adverse to photosynthesis and growth. Some seedlings of Castanospermum australe were alive and were transplanted successfully after 2 years burial. This enables primary forest species to stock seedlings on the forest floor in contrast to the secondary species which store seed in the soil.     

Engineering seed dormancy by the modification of zeaxanthin epoxidase gene expression

Abscisic acid (ABA) is a plant hormone synthesized during seed development that is involved in the induction of seed dormancy. Delayed germination due to seed dormancy allows long-term seed survival in soil but is generally undesirable in crop species. Freshly harvested seeds of wild-type Nicotiana plumbaginifolia plants exhibit a clear primary dormancy that results in delayed germination, the degree of primary dormancy being influenced by environmental culture conditions of the mother plant. In contrast, seeds, obtained either from ABA-deficient mutant aba2-s1 plants directly or aba2-s1 plants grafted onto wild-type plant stocks, exhibited rapid germination under all conditions irrespective of the mother plant culture conditions. The ABA biosynthesis gene ABA2 of N. plumbaginifolia, encoding zeaxanthin epoxidase, was placed under the control of the constitutive 35S promoter. Transgenic plants overexpressing ABA2 mRNA exhibited delayed germination and increased ABA levels in mature seeds. Expression of an antisense ABA2 mRNA, however, resulted in rapid seed germination and in a reduction of ABA abundance in transgenic seeds. It appears possible, therefore, that seed dormancy can be controlled in this Nicotiana model species by the manipulation of ABA levels.     

细叶楠种子休眠与萌发对低温层积的生理响应

细叶楠(Phoebe hui Cheng ex Yang)是我国特有的珍贵树种,种子具有生理休眠特性。本文以细叶楠种子为实验材料,研究不同低温层积处理时间(0、20、40、60 d)对细叶楠种子发芽率、营养物质(淀粉、可溶性糖、可溶性蛋白)、过氧化物酶(POD)以及内源激素脱落酸(ABA)、赤霉素(GA₃)、生长素(IAA)、玉米素核苷(ZR)的影响,分析细叶楠种子萌发与其生理指标的相关性。结果显示：(1)低温层积可以有效打破细叶楠种子的休眠,且处理60 d效果最佳。(2)在低温层积过程,细叶楠种子萌发时间缩短,发芽率提高;种子内淀粉含量降低,可溶性糖和可溶性蛋白含量升高;POD活性上升;ABA含量下降,GA₃、IAA含量增加,而ZR含量先上升后下降。(3)细叶楠种子休眠的解除与萌发与其体内营养物质含量、POD活性以及内源激素含量的变化密切相关。     

生长素调控种子的休眠与萌发

植物种子的休眠与萌发,是植物生长发育过程中的关键阶段,也是生命科学领域的研究热点。种子从休眠向萌发的转换是极为复杂的生物学过程,由外界环境因子、体内激素含量及信号传导和若干关键基因协同调控。大量研究表明,植物激素脱落酸(Abscisic acid, ABA)和赤霉素(Gibberellin acid, GA)是调控种子休眠水平,决定种子从休眠转向萌发的主要内源因子。ABA与GA在含量和信号传导两个层次上的精确平衡,确保了植物种子能以休眠状态在逆境中存活,并在适宜的时间启动萌发程序。生长素(Auxin)是经典植物激素之一,其对向性生长和组织分化等生物学过程的调控已有大量研究。但最近有研究证实,生长素对种子休眠有正向调控作用,这表明生长素是继ABA之后的第二个促进种子休眠的植物激素。本文在回顾生长素的发现历程、阐释生长素体内合成途径及信号传导通路的基础上,重点综述了生长素通过与ABA的协同作用调控种子休眠的分子机制,并对未来的研究热点进行了讨论和展望。     

The changing window of conditions that promotes germination of two fire ephemerals, Actinotus leucocephalus (Apiaceae) and Tersonia cyathiflora (Gyrostemonaceae)

BACKGROUND AND AIMS: Following a period of burial, more Actinotus leucocephalus (Apiaceae) and Tersonia cyathiflora (Gyrostemonaceae) seeds germinate in smoke water. The main aim of this study was to determine whether these fire-ephemeral seeds exhibit annual dormancy cycling during burial. This study also aimed to determine the effect of dormancy alleviation on the range of light and temperature conditions at which seeds germinate, and the possible factors driving changes in seed dormancy during burial. METHODS: Seeds were collected in summer, buried in soil in mesh bags in autumn and exhumed every 6 months for 24 months. Germination of exhumed and laboratory-stored (15 degrees C) seeds was assessed at 20 degrees C in water or smoke water. Germination response to light or dark conditions, incubation temperature (10, 15, 20, 25 and 30 degrees C), nitrate and gibberellic acid were also examined following burial or laboratory storage for 24 months. In the laboratory seeds were also stored at various temperatures (5, 15, 37 and 20/50 degrees C) for 1, 2 and 3 months followed by germination testing in water or smoke water. KEY RESULTS: The two species exhibited dormancy cycling during soil burial, producing low levels of germination in response to smoke water when exhumed in spring and high levels of germination in autumn. In autumn, seeds germinated in both light and dark and at a broader range of temperatures than did laboratory-stored seeds, and some Actinotus leucocephalus seeds also germinated in water alone. Dormancy release of Actinotus leucocephalus was slow during dry storage at 15 degrees C and more rapid at higher temperatures (37 and 20/50 degrees C); weekly wet/dry cycles further accelerated the rate of dormancy release. Cold stratification (5 degrees C) induced secondary dormancy. By contrast, no Tersonia cyathiflora seeds germinated following any of the laboratory storage treatments. CONCLUSIONS: Temperature and moisture influence dormancy cycling in Actinotus leucocephalus seeds. These factors alone did not simulate dormancy cycling of Tersonia cyathiflora seeds under the conditions tested.     

DORMANCY AND GERMINATION IN SEEDS OF COMMON RAGWEED WITH REFERENCE TO BEAL'S BURIED SEED EXPERIMENT

Common ragweed (Ambrosia artemisiifolia L.) was one of 19 herbaceous weedy species used by Beal in his buried viable seed experiment started in 1879. No seeds germinated during the first 35 years of the experiment when germination tests were performed in late spring, summer or early autumn. Germination did occur in seeds buried for 40 years when seeds were exhumed and tested for germination in early spring. Data obtained in more recent research provide the probable explanation for these results. Seeds of common ragweed that do not germinate in spring enter secondary dormancy by mid to late spring and will not germinate until dormancy is broken the following late autumn and winter. Thus, during the first 35 years of the experiment seeds were dormant when tested for germination, whereas seeds buried for 40 years were nondormant. Seeds buried 50 years or longer did not germinate when tested in spring, probably because they had lost viability and/or seeds germinated during burial and seedlings died.     

Seed dormancy release and germination characteristics of <i>Corispermum lehmannianum</i> Bunge,an endemic species in the Gurbantunggut desert of China

Seed dormancy release and germination of Corispermum lehmannianum Bunge were tested using various treatments: temperature, cold stratification, gibberelins (GA3), dry storage and sand burial. Results showed that temperature and light did not affect the germination of fresh seeds, cold stratification and GA3 could improve seed germination, whereas dry storage and sand burial did not. The germination percentage was highest at 35/20 °C after the cold stratification and GA3 treatments. Corispermum lehmannianum seeds were classified as non-deep, Type-2, physiological dormancy (PD), whose seed dormancy could be released by cold stratification and GA3.     

Seed after-ripening is a discrete developmental pathway associated with specific gene networks in Arabidopsis

After-ripening (AR) is a time and environment regulated process occurring in the dry seed, which determines the germination potential of seeds. Both metabolism and perception of the phytohormone abscisic acid (ABA) are important in the initiation and maintenance of dormancy. However, molecular mechanisms that regulate the capacity for dormancy or germination through AR are unknown. To understand the relationship between ABA and AR, we analysed genome expression in Arabidopsis thaliana mutants defective in seed ABA synthesis (aba1-1) or perception (abi1-1). Even though imbibed mutant seeds showed no dormancy, they exhibited changes in global gene expression resulting from dry AR that were comparable with changes occurring in wild-type (WT) seeds. Core gene sets were identified that were positively or negatively regulated by dry seed storage. Each set included a gene encoding repression or activation of ABA function (LPP2 and ABA1, respectively), thereby suggesting a mechanism through which dry AR may modulate subsequent germination potential in WT seeds. Application of exogenous ABA to after-ripened WT seeds did not reimpose characteristics of freshly harvested seeds on imbibed seed gene expression patterns. It was shown that secondary dormancy states reinstate AR status-specific gene expression patterns. A model is presented that separates the action of ABA in seed dormancy from AR and dry storage regulated gene expression. These results have major implications for the study of genetic mechanisms altered in seeds as a result of crop domestication into agriculture, and for seed behaviour during dormancy cycling in natural ecosystems.     

Gibberellin requirement for Arabidopsis seed germination is determined both by testa characteristics and embryonic abscisic acid

The mechanisms imposing a gibberellin (GA) requirement to promote the germination of dormant and non-dormant Arabidopsis seeds were analyzed using the GA-deficient mutant ga1, several seed coat pigmentation and structure mutants, and the abscisic acid (ABA)-deficient mutant aba1. Testa mutants, which exhibit reduced seed dormancy, were not resistant to GA biosynthesis inhibitors such as tetcyclacis and paclobutrazol, contrarily to what was found before for other non-dormant mutants in Arabidopsis. However, testa mutants were more sensitive to exogenous GAs than the wild-types in the presence of the inhibitors or when transferred to a GA-deficient background. The germination capacity of the ga1-1 mutant could be integrally restored, without the help of exogenous GAs, by removing the envelopes or by transferring the mutation to a tt background (tt4 and ttg1). The double mutants still required light and chilling for dormancy breaking, which may indicate that both agents can have an effect independently of GA biosynthesis. The ABA biosynthesis inhibitor norflurazon was partially efficient in releasing the dormancy of wild-type and mutant seeds. These results suggest that GAs are required to overcome the germination constraints imposed both by the seed coat and ABA-related embryo dormancy.     

The etr1-2 mutation in Arabidopsis thaliana affects the abscisic acid, auxin, cytokinin and gibberellin metabolic pathways during maintenance of seed dormancy, moist-chilling and germination

In Arabidopsis thaliana, the etr1-2 mutation confers dominant ethylene insensitivity and results in a greater proportion of mature seeds that exhibit dormancy compared with mature seeds of the wild-type. We investigated the impact of the etr1-2 mutation on other plant hormones by analyzing the profiles of four classes of plant hormones and their metabolites by HPLC-ESI/MS/MS in mature seeds of wild-type and etr1-2 plants. Hormone metabolites were analyzed in seeds imbibed immediately under germination conditions, in seeds subjected to a 7-day moist-chilling (stratification) period, and during germination/early post-germinative growth. Higher than wild-type levels of abscisic acid (ABA) appeared to contribute, at least in part, to the greater incidence of dormancy in mature seeds of etr1-2. The lower levels of abscisic acid glucose ester (ABA-GE) in etr1-2 seeds compared with wild-type seeds under germination conditions (with and without moist-chilling treatments) suggest that reduced metabolism of ABA to ABA-GE likely contributed to the accumulation of ABA during germination in the mutant. The mutant seeds exhibited generally higher auxin levels and a large build-up of indole-3-aspartate when placed in germination conditions following moist-chilling. The mutant manifested increased levels of cytokinin glucosides through zeatin-O-glucosylation (Z-O-Glu). The resulting increase in Z-O-Glu was the largest and most consistent change associated with the ETR1 gene mutation. There were more gibberellins (GA) and at higher concentrations in the mutant than in wild-type. Our results suggest that ethylene signaling modulates the metabolism of all the other plant hormone pathways in seeds. Additionally, the hormone profiles of etr1-2 seed during germination suggest a requirement for higher than wild-type levels of GA to promote germination in the absence of a functional ethylene signaling pathway.     

Involvement of ABA in induction of secondary dormancy in barley (Hordeum vulgare L.) seeds

At harvest, barley seeds are dormant because their germination is difficult above 20 degrees C. Incubation of primary dormant seeds at 30 degrees C, a temperature at which they do not germinate, results in a loss of their ability to germinate at 20 degrees C. This phenomenon which corresponds to an induction of a secondary dormancy is already observed after a pre-treatment at 30 degrees C as short as 4-6 h, and is optimal after 24-48 h. It is associated with maintenance of a high level of embryo ABA content during seed incubation at 30 degrees C, and after seed transfer at 20 degrees C, while ABA content decreases rapidly in embryos of primary dormant seeds placed directly at 20 degrees C. Induction of secondary dormancy also results in an increase in embryo responsiveness to ABA at 20 degrees C. Application of ABA during seed treatment at 30 degrees C has no significant additive effect on the further germination at 20 degrees C. In contrast, incubation of primary dormant seeds at 20 degrees C for 48 and 72 h in the presence of ABA inhibits further germination on water similarly to 24-48 h incubation at 30 degrees C. However fluridone, an inhibitor of ABA synthesis, applied during incubation of the grains at 30 degrees C has only a slight effect on ABA content and secondary dormancy. Expression of genes involved in ABA metabolism (HvABA8'OH-1, HvNCED1 and HvNCED2) was studied in relation to the expression of primary and secondary dormancies. The results presented suggest a specific role for HvNCED1 and HvNCED2 in regulation of ABA synthesis in secondary seed dormancy.     

Role of endogenous hormones,glumes, endosperm and temperature on germination of Leymus chinensis (Poaceae) seeds during development

Aims Leymus chinensis is an original dominant plant in the Songnen grassland, and it has great value for restoration of severely degraded land. However, seeds are dormant, and low germination percentage is a problem for restoring L.chinensis grassland. The mechanism of seed dormancy is not been well understood. The primary aims of the present study were to investigate the dormancy mechanism of L.chinensis seeds (caryopses) with reference to the role of embryo-covering layers, endogenous hormones and temperature.Methods Changes in concentration of the endogenous hormones GA 3, indoleacetic acid (IAA), zeatin riboside (ZR) and abscisic acid (ABA) in L.chinensis seeds from anthesis to maturity were measured by the enzyme-linked immunosorbent assay method. Germination at different stages of maturity were tested at 16/28°C, 5/28°C and 5/35°C for intact seeds with glumes (control), intact seeds with glumes removed (naked-whole seeds) and intact seeds with glumes and one-half of the endosperm removed (naked-half seeds).Important findings Of the four endogenous hormones monitored, only the concentration of ZR differed significantly between the beginning and the end of seed development (increased); the GA 3 /ABA ratio also did not differ. Rank of germination percentage of control at the three temperature regimens was 5/28°C> 16/28°C> 5/35°C. Germination percentage of the naked-half seeds reached 100% under the three temperature regimens. We concluded that dormancy of L.chinensis seeds is not mainly controlled by endogenous hormones. Germination temperature, mechanical resistance of glumes and inhibition of endosperm are the main factors controlling dormancy and germination of L.chinensis seeds.