There is confusion in the ecological literature between seed dormancy and persistence in soil. Some ecologists seem to assume that dormancy is necessary for persistence, while others imply that dormancy and persistence are virtually synonymous. Here, we show that there is no close relationship between dormancy and persistence and, incidentally, that conventional methods of investigating soil seed banks underestimate the persistence of species with dormant seeds. The confusion appears to arise from the concept of ‘enforced dormancy’, which is not genuinely dormancy at all, and would be eliminated if ecologists adopted the definition of dormancy employed by physiologists. Dormancy is a characteristic of the seed, not of the environment, the degree of which defines the conditions required to make the seed germinate.

The ascorbic acid (ASC) system functions dynamically in seeds, although the strategies for ASC production and utilization may vary according to seed developmental and functional stages. In orthodox seeds, ASC content and ASC peroxidase activity increase during the early stages of development, then decrease during the desiccation stage, so that, at quiescence, seeds have neither ASC nor ASC peroxidase, but retain a small amount of dehydroascorbic acid (DHA) and significant activities of ASC recycling enzymes. ASC and ASC peroxidase activity re-start after a few hours from the onset of imbibition. In contrast, the ASC system is little affected during germination of recalcitrant seeds. Although the presence of the ASC system in seeds has often been considered only within the framework of seed antioxidant defences, ASC function in seeds is also likely to be related to its action as a specific co-substrate required for the activity of dioxygenases (e.g. 1-aminocyclopropane carboxylate oxidase, gibberellic acid hydroxylases and 9-cis-epoxycarotenoid dioxygenases) involved in the synthesis of ethylene, gibberellins and abscisic acid, respectively. The possible role of ASC in coordinating the activities of these key enzymes is discussed.

Nitrates have long been known to promote seed germination in many species, and various proposals have been made regarding the role of nitrates in this process. One hypothesis suggests a possible ecological role for nitrate, providing the seeds with a report of soil nitrogen status and the proximity of already established plants. However, nitrogen oxides (NO, NO2), which are present in the soil, also may be information carriers that indicate not only soil nitrate content, but also microbial activity and, therefore, soil quality. Because of annual variation of soil trace gas fluxes, seeds could be provided with information on seasonal and climate changes in their surroundings. Thus, nitrogen oxides would be the outer information carriers providing the seeds with integral data about many important factors required for successful germination and seedling establishment.

An abscisic acid (ABA)-induced cDNA fragment encoding a putative protein kinase (PK) was obtained by differential screening of a cDNA library from Fagus sylvatica seeds. The full-length clone, named FsPK1, was produced by 5′ rapid amplification of cDNA ends (RACE) extension. This clone contained the 11 catalytic domains present in all protein kinases, but displayed unusual characteristics found only in a few plant PKs. FsPK1 exhibits features of both serine/threonine and tyrosine protein kinases within the catalytic domain, a putative nuclear localization signal within the regulatory domain and the consensus sequence involved in binding of 14-3-3 proteins. The catalytic domain, expressed in Escherichia coli as a fusion protein, showed Ca2+-dependent in vitro kinase activity and dual serine/threonine and tyrosine specificity. Transcription of the FsPK1 gene was reduced by seed stratification at 4°C, and clearly increased when seeds were treated with 0.1 mM ABA, correlating with the inhibition of germination. Interestingly, FsPK1 transcripts were enhanced when ABA (0.1 mM) and calcium (1 mM) were added together, while the addition of EGTA (calcium chelator) and 3,4,5,-trimethoxibenzoic acid 8-(diethylamino) octyl ester (TMB-8, a calcium antagonist) decreased its expression. Furthermore, FsPK1 transcript expression was tissue specific and accumulated only in ABA-treated seeds, but not in any ABA-treated vegetative tissues examined. These results suggest that the expression of the corresponding protein could be related to the inhibition of germination mediated by ABA in a calcium-dependent pathway.

The sensitivity of Polygonum aviculare L. seeds to the dormancy-breaking effect of cycle-doses of fluctuating temperature changes as seeds lose dormancy due to storage under stratification temperatures. Sensitivity changes during seed stratification were characterized by a decrease in the number of cycles required to saturate the germination response, and by a progressive loss of the requirement for temperature fluctuations for dormancy breakage in increasing fractions of the seed population. The rate of these changes was dependent on the temperature at which seeds were stored for stratification; lower storage temperatures produced higher rates of change than higher storage temperatures. Germination curves, obtained in response to the effect of fluctuating temperature cycle-doses for seeds stratified at variable temperatures and times of storage, were brought to a common stratification thermal time (Stt) scale by accumulating thermal time units under a threshold temperature for dormancy loss to occur. Results showed that those seeds that had accumulated similar Stt units during stratification under different storage temperatures presented a similar germination response. Therefore, response-curve functions were adjusted to germination data of exhumed seeds that had accumulated similar Stt, obtaining a family of germination response curves in relation to Stt accumulation during storage. Based on these results, a simulation model was constructed relating dynamic changes in the parameters that determine germination response curves in relation to Stt accumulation. The model was tested against independent data, showing a good description of the dynamics of changes in the fraction of the seed population requiring temperature fluctuation for dormancy breakage as dormancy release progressed.

Since 1969, ten soil seed bank classification systems have been published. Among these systems, the number of recognized seed bank categories varies from three to twelve. Seed longevity is the main factor used for distinguishing categories, but dormancy and germination types are also important. Systems considering relatively few seed bank categories have been the most commonly proposed in contemporary plant ecology. In contrast, systems involving high numbers of categories have received limited interest because the detailed ecological knowledge of individual species required for their successful categorization is usually missing. A comprehensive table on the main features of seed bank classification systems is provided.

The search for germination-specific genes has been a laborious and unrewarding task, since many of the genes expressed during germination are also expressed in embryogenesis or in other developmental stages. By using mRNA differential display of transcript populations from maize (Zea mays L.) embryo axes, germinated for different times with or without a previous osmopriming treatment, a 682 bp cDNA was isolated that was present only after 24 h germination, and absent during osmopriming or during early germination. Screening of a cDNA library using the 682 bp probe yielded a 1554 bp cDNA that contained an open reading frame coding for 436 amino acids. This gene, referred to as ZmAA9-24, was expressed in root tissues, but was not detected in shoot or leaf tissues. Expression of ZmAA9-24 occurred earlier during germination (by 15 h) if embryo axes were imbibed in the presence of cytokinins or if seeds were previously osmoprimed. The predicted protein sequence of ZmAA9-24 is 39.6% identical to the product of the recently identified Arabidopsis gene COBRA (54.5% in the central region), which appears to participate in the regulation of cell expansion, particularly in roots, and belongs to the glycosylphosphatidylinositol (GPI)-anchored protein family. ZmAA9-24 expression might be regulated by both cell expansion and the cell cycle, processes that have a central role during seed germination.

Germinated seeds of Medicago truncatula Gaertn. with a protruded radicle length of 2.7 mm did not survive drying below 0.2 g H2O g–1 dw, as indicated by vital stain assays and the absence of growth resumption after rehydration. The re-establishment of desiccation tolerance was achieved using an osmotic treatment with polyethylene glycol (PEG), combined with a cold treatment. The ability to regain desiccation tolerance after germination was restricted to a period of growth characterized by radicle lengths between 1 and 3 mm. After PEG treatment of germinated seeds with 2.7 mm long radicles at –1.7 MPa at 10°C for 3 d and subsequent drying to 0.04 g H2O g–1 dw, 90% survived and developed into normal seedlings after rehydration. Desiccation tolerance could also be re-established in excised radicles, demonstrating that cotyledons were not essential for this process. Upon PEG incubation, sucrose accumulated rapidly prior to the re-establishment of desiccation tolerance in germinated radicles, regardless of the presence of cotyledons. Induction of MtDHN (a dehydrin) gene expression was correlated with the re-establishment of desiccation tolerance. Furthermore, the PEG-induced expression of MtDHN was repressed when fluridone was added to the PEG solution.

The cell cycle is the series of molecular events that allows cells to duplicate and segregate their chromosomes to form new cells. The finding that a protein kinase, the product of the yeast cdc2 gene, was fundamental in the regulation of the G2/M and G1/S transitions, associated with unstable proteins named cyclins, opened a very exciting and dynamic research area. The number of gene products that participate in the development and regulation of the cell cycle may be in the hundreds, and there is a high degree of conservation in protein sequences and regulatory pathways among eukaryotes. Although there are clear differences between plants and animals in cell structure, organization, growth, development and differentiation, the same types of proteins and very similar regulatory pathways seem to exist. Seed germination appears to be an excellent model system for studying the cell cycle in plants. Imbibition will reactivate meristematic cells – most initially with a G1 DNA content – into the cell cycle in preparation for seedling establishment. Early events include a thorough survey of DNA status, since the drying process and seed storage conditions reduce chromosomal integrity. The initiation of cell cycle events leading to G1 and S phases, and of the germination process itself, may depend on a G1 checkpoint control. Most, if not all, cell cycle proteins appear to be already present in unimbibed embryos, although there is evidence of protein turnover in the early hours, suggesting the need for de novo protein synthesis. Regulation also may occur at the level of protein modification, because existing G1, S and G2 cell cycle proteins appear to be activated at precise times during germination. Thus, cell cycle control during seed germination may be exerted at multiple levels; however, knowledge of cell cycle events and their importance for germination is still scarce and fragmentary, and different species may have developed unique control mechanisms, more suited to specific germination characteristics and habitat.

β-1,3-Glucanase (βGlu) expression in seeds plays important roles in the regulation of seed germination, dormancy and in the defence against seed pathogens. A thick β-1,3-glucan layer is typical for the seed envelope of cucurbitaceous species, confers seed semipermeability and is degraded during germination. In many species with coat-imposed dormancy, the seed envelope confers a physical constraint to radicle emergence. In the solanaceous species, the micropylar endosperm and testa have this function, and endosperm weakening appears to be a prerequisite for germination. Class I βGlu is transcriptionally induced in the micropylar endosperm of tobacco, tomato and other solanaceous seeds just prior to radicle emergence. βGlu induction and germination are tightly linked in response to plant hormones and environmental factors, e.g. they are both promoted by gibberellins and inhibited by abscisic acid (ABA). Sense and antisense transformation of tobacco reveals two sites of βGlu action: after-ripening-mediated release of testa-imposed dormancy and endosperm rupture during germination. The use of an ABA-inducible chimeric sense-transgene resulted in overexpression of class I βGlu in seeds and provided direct evidence that βGlu contributes to endosperm rupture. A model integrating βGlu, seed dormancy, after-ripening and germination is presented, and possible mechanisms for βGlu action are discussed. It is proposed that βGlu not only helps defend seeds against pathogens, but is also a key factor in regulating coat-imposed dormancy and germination of seeds in response to environmental and hormonal cues.

Seeds of sugar beet (Beta vulgaris L.) were collected at weekly intervals from 3 weeks before to 1 week after commercial harvest time, dried and stored at room temperature (18–22°C). Laboratory germination tests and flow cytometric analyses were performed immediately after harvest (fresh seeds) and five times at weekly intervals during storage (dry seeds). After 6 months of storage, seeds were exposed to a controlled deterioration treatment (CD). The proportion of G2 nuclei in the embryo was constant in the fresh seeds, regardless of their maturity. It decreased, however, after drying and CD, especially in those seeds harvested before maturation drying had commenced. The proportion of endosperm cells in the seed decreased with maturation, and a further decrease was observed after drying and CD. These observations suggest that nuclei with a higher nuclear DNA content were more sensitive to water stress caused by premature desiccation and to deterioration than nuclei with a lower DNA content. Fresh seeds exhibited some germination, but this increased after drying, suggesting that desiccation induced a switch from the developmental to the germination mode. Germination percentages were the highest in dry seeds collected at the commercial harvest time and a week after. This high germinability coincided with the highest proportion of G2 cells in the embryo. It is concluded that flow cytometry provides information about the status of sugarbeet seed maturation, seed quality and storage potential, and can be used for estimation of optimal harvest time.

Osmotic priming reduces the longevity of Vigna radiata (mung bean) seeds during subsequent dry storage. The present study has investigated the relationship between volumetric changes, modified water sorption properties and the decrease in seed longevity after priming. Volumetric analysis of control embryos revealed two major dehydration-related volumetric contractions in the biopolymer matrix, reflecting structural reorganization in biopolymer matrix during drying. These contractions occurred at hydration from 0.29 to 0.23 g g–1 dw and from 0.18 to 0.11 g g–1 dw, respectively. Volumetric contractions were reduced significantly after priming and also occurred at higher water contents (i.e. 0.48–0.32 g g–1 dw and 0.21–0.11 g g–1 dw). Consequently, at the same water content, primed seed embryos had higher specific biopolymer volume and lower specific density than control embryos. Water sorption study showed that priming did not change the monolayer hydration value, but altered the properties of multilayer water sorption in seed axes. The analysis of water clustering function suggested that priming enhanced the water–water association in seed axes, but not in cotyledon tissues. Solid-state 1H-NMR (nuclear magnetic resonance) investigation confirmed that priming did not cause a significant change of water dynamic properties in cotyledon tissues at water contents below 0.20 g g–1 dw. The relevance of volumetric modifications of the biopolymer matrix and changes in water clustering function to the reduced seed longevity after priming is discussed. It is proposed that priming-induced increases in surface reactivity may enhance deteriorative chemical reactions in re-dried seed tissues.

Germination is an energy-demanding process that requires the operation of mitochondria, which must survive desiccation in the quiescent seed and become rapidly functional after imbibition to meet the ATP demand. The relationship between germination and mitochondrial performance was addressed by analysing the properties of mitochondria isolated from control, primed and aged pea (Pisum sativum L.) seeds. Mitochondria were isolated and purified at early stages of germination (before radicle protrusion), and their oxidative properties, membrane integrity and ultrastructure were examined. Mitochondria isolated after 12 h of imbibition readily oxidized exogenous NADH and Krebs cycle substrates at high rates. However, their phosphorylation efficiency was restricted by poor membrane integrity. After 22 h from the beginning of imbibition, purified seed mitochondria had intact outer membranes and oxidized the substrates at slightly lower rates, but with higher respiratory control (improved capacity for phosphorylation). Purified seed mitochondria were always found to be deficient in endogenous NAD, although the organelles were capable of importing and retaining the cofactor. While the priming treatment appeared to slightly increase the performance of mitochondria, seed deterioration by accelerated ageing strongly affected the oxidative properties of mitochondria, which were badly impaired in ATP production. Outer and inner membrane integrity was identified as the primary target for desiccation and ageing stress. A link between mitochondrial function and seed quality was also corroborated by respiration measurements of seed fragments at the onset of imbibition.

Development of the rudimentary embryo in mature carrot (Daucus carota) seed during imbibition was characterized. The small embryo in the carrot seed, located in the micropylar region, elongated into the lateral part during imbibition and attained about two-thirds the length of the seed before radicle protrusion. Developing embryos excised from imbibed seeds were only capable of germinating in both water and Murashige–Skoog (MS) medium when they reached maximum size. The corrosion cavity into which the embryo grew enlarged concomitantly with endosperm degradation. The expression of endo-β-mannanase (EC 3.2.1.78), which is assumed to be involved in endosperm degradation, was characterized. A cDNA encoding an endo-β-mannanase was obtained by reverse transcription polymerase chain reaction (RT-PCR) using total RNA extracted from 24-h-imbibed carrot seeds. The full-length cDNA (DcMAN1) exhibited 64% deduced amino acid sequence identity with tomato (Lycopersicon esculentum) seed germination-associated mannanase (LeMAN2). DcMAN1 mRNA and endo-β-mannanase activity were first detected in the micropylar-half seed and then in the lateral-half seed. The timing of the appearance of DcMAN1 mRNA and endo-β-mannanase activity in the lateral-half seed corresponded with that of embryo development into this region. These results suggest that the expression of DcMAN1 and endo-β-mannanase activity in imbibed carrot seeds is associated with the enlargement of the corrosion cavity, which accompanies embryogenesis.

Saline soils contain numerous salts with varying impact on seed germination. Seeds of three non-halophytic species found in Chinese sandy deserts (Artemisia ordosica, Aristida adscensionis and Bassia dasyphylla) were incubated in salt solutions (NaCl, KCl or MgCl2, each with or without CaCl2) at 20°C in the dark. The effects of each salt on the percentage of seeds from which visibly detectable radicles emerged, and the percentage of seeds with emerging radicles surviving to a length of at least 4 mm, were examined. NaCl, KCl and MgCl2 were toxic to emerging radicles in all three species, but before radicle emergence these salts reduced seed germinability only in A. ordosica. The toxic effects of each salt on the radicles were alleviated in all three species by the addition of low concentrations of CaCl2. MgCl2 was more toxic than NaCl and KCl, and the concentration of CaCl2 needed to alleviate the toxicity of MgCl2 was several times higher than that required for alleviation of NaCl or KCl toxicity. These results suggest that Ca2+ present in saline soils alleviates the toxic effects of other salt components on seed germination, and that the results of germination experiments with a single salt are not always applicable to field conditions.

The involvement of ethylene in determining the time to radicle protrusion was investigated in ethylene-insensitive gain-of-function (GOF) receptor mutants in tomato and Arabidopsis, as well as in single and double loss-of-function (LOF) receptor mutants in Arabidopsis. Because ethylene evolution from seeds is coincident with radicle protrusion, and the ability to convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene is diagnostic for seed vigour, it was hypothesized that ethylene-insensitive mutants would require more time to complete germination compared to wild-type seeds. Mutant Never Ripe (Nr) tomato seeds from two genetic backgrounds refuted this hypothesis, while experiments with wild-type seeds, treated with the ethylene action inhibitors, 2,5-norbornadiene or silver thiosulphate, supported it. However, reciprocal crosses between wild-type and Nr demonstrated that ethylene insensitivity during seed development determined subsequent time to complete germination, rather than the ability of the embryo/endosperm to perceive ethylene in the mature seed during germination. Additionally, seed quality, determined by standard vigour tests, was reduced in Nr compared to wild-type seeds, establishing a disconnection between rapid completion of germination and seed vigour. In Arabidopsis, all ethylene-insensitive GOF, and five of six single LOF mutants, required more time to complete 50% radicle protrusion, while double LOF mutants required the same, or less, time to complete germination compared to wild-type seeds. These findings support a role for ethylene perception in determining the length of time Arabidopsis seeds remain in the lag phase prior to radicle protrusion.

Aleurone layers of mature germinating barley (Hordeum vulgare, cv. Himalaya) grains respond to heat shock by synthesizing heat-shock proteins (HSPs) and by selectively suppressing the synthesis of proteins normally translated by endoplasmic reticulum (ER)-bound ribosomes. To determine if this also was the case during seed development, we investigated the synthesis of proteins translated by ER-bound ribosomes in heat-shocked aleurone layers isolated from developing barley grains. The optimal induction temperature for the heat shock response in developing aleurone layers was 37.5°C, and temperatures above 42°C inhibited translation. HSPs with apparent molecular masses of 71.1, 66.2, 57.8, 19.1 and 18.8 kDa were induced. Other studies have shown that, in gibberellic acid (GA)-induced aleurone layers from mature barley grains, these temperatures were 40°C and 45°C, respectively. Furthermore, in developing aleurone layers, mRNAs encoding proteins translated by ER-bound ribosomes (mRNAs for a lipid transfer protein and a putative amylase/protease inhibitor) remained stable during heat shock. The ER membranes themselves remained in stacks, but the lumen became distended with electron-dense material. Heat shock prevented the movement of proteins from the ER into the rest of the endomembrane pathway. In contrast, other studies show that in mature, GA-induced aleurone layers, heat shock dissociates ER stacks and blocks translation, but the processing of secretory proteins in the endomembrane pathway is not inhibited. The observation that the same tissue at different developmental stages may respond differently to heat shock indicates that components of the heat-shock response are developmentally regulated. This system provides an opportunity to better understand the nuances of the heat-shock response, especially the post-transcriptional gene regulatory mechanisms that occur.

The expression patterns of β-1,3-glucanases (βGlu) and chitinases (Chn) were investigated during the seed germination of members of the Cestroideae (three Nicotiana species, Petunia hybrida) and the Solanoideae (Capsicum annuum, Physalis peruviana) subgroups of Solanaceous species. Rupture of the micropylar testa (seed coat) and rupture of the micropylar endosperm, i.e. radicle emergence, were distinct and temporally separate events during the germination of Cestroideae-type seeds. βGlu accumulation in imbibed Cestroideae-type seeds, occurring after testa rupture but prior to endosperm rupture, was inhibited by abscisic acid (ABA) and promoted by gibberellins (GA) and light, in strict association with germination, and appeared to be caused by transcriptional regulation of the class I βGlu genes. The micropylar cap of Solanoideae-type seeds does not allow a distinction between testa and endosperm rupture, but βGlu accumulation occurred prior to radicle emergence of pepper and P. peruviana seeds. ABA inhibited endosperm rupture and βGlu accumulation in the micropylar cap of pepper seeds. In contrast to tomato, βGlu accumulation in pepper seeds was not only confined to the micropylar cap, was due to distinct, tissue-specific βGlu isoforms, and was not accompanied by Chn accumulation. In conclusion, ABA inhibition of germination and βGlu accumulation in the micropylar endosperm appears to be a widespread event during the seed germination of Solanaceous species. In contrast, accumulation of Chn and distinct βGlu isoforms in the embryo, prior to germination, appears to be a species-specific phenomenon within the Solanaceae. In addition, a post-germination co-induction of βGlu and Chn in the root of the emerged seedling was found in endospermic and non-endospermic species and could represent an evolutionarily conserved event during dicot seedling development.