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.

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 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.

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.

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.

The influence of the storage environment on seed viability and antioxidant potential was examined for four species native to Western Australia: Acacia bivenosa DC., Anigozanthos manglesii D. Don, Banksia ashbyi E.G. Baker, and Mesomelaena tetragona (R. Br.) Benth. Seeds were stored at four water contents (at c. 5%, 11–15%, 20–23% and 50% relative humidity) at each of five temperatures (–196, –18, 5, 23 and 50°C), and seed germination and seedling vigour monitored over an 18-month period. Deterioration was apparent in all species (except A. bivenosa) stored at 50°C, with 11% RH maximizing longevity for B. ashbyi and M. tetragona seeds, and 5% or 11% RH preventing deterioration for A. manglesii seeds. Seed viability generally remained high for all species stored at 23°C or less. Notably, however, germination and seedling vigour of A. manglesii and M. tetragona seeds gradually declined when stored at –18°C, suggesting that storage at this temperature was detrimental. The antioxidant activity of lipid extracts of seeds after 18 months storage at 5, 23 and 50°C was also examined to determine whether the seed viability decline was associated with a loss of antioxidants. Antioxidant activity varied between storage treatments and was not related to seed viability.

Chitinase is often produced in higher plants as a general defence response after wounding or pathogenic attack. Since germinating seeds are exposed to soil pathogens, the activity and expression of chitinase in muskmelon (Cucumis melo L.) seeds was investigated. One acidic and three basic chitinase isoforms were detected, beginning 40 d after anthesis in developing and fully mature seeds. Both acidic and basic chitinase isoforms were found in endosperm tissue during imbibition and after radicle emergence. Basic chitinase isoforms, but not acidic isoforms, were detected in the embryonic axes of imbibed seeds and in seeds before germination, indicating that chitinases are developmentally regulated in specific seed tissues. Two complete cDNAs, Cmchi1 and Cmchi2, were cloned from germinated muskmelon seeds and are predicted to encode chitinases that show 95% identity to a class III chitinase from cucumber (Cucumis sativus L.) and 61% identity to a class II chitinase from soybean (Glycine max L.), respectively. Southern blotting indicated that Cmchi2 was present only once in the muskmelon genome, while Cmchi1 may be present in one or two copies. Cmchi1 and Cmchi2 mRNAs were only detected in radicles of germinating seeds and in roots of mature plants, so additional genes other than Cmchi1 and Cmchi2 must be responsible for the chitinase activity in developing seeds. Salicylic acid and benzothiadiazole stimulated the expression of Cmchi1, but not Cmchi2, after radicle emergence. A putative role for chitinase in muskmelon seeds is defence against fungal pathogens.

Relationships between desiccation tolerance and dry matter, water and sugar contents were studied throughout the development of oil palm (Elaeis guineensis Jacq.) zygotic embryos and in immature embryos cultured on a sucrose-enriched medium. Embryo dry weight during in planta development increased between 80 and 140 d after pollination (DAP) and was then stable until maturity. Embryos underwent dehydration until 120 DAP, but their moisture content remained high at maturity (c. 2 g H2O g-1 DW). Desiccation tolerance was acquired between 83 and 104 DAP, and was positively correlated with embryo age and dry weight, and negatively correlated with initial water content during this period. Sucrose, the main soluble sugar present throughout embryo development, accounted for an average of 24% of the dry weight. Glucose and fructose contents decreased to less than 1 mg g-1 DW in embryos at maturity. At 117 DAP, as embryos became tolerant to desiccation, the monosaccharides/sucrose ratio fell to 0.015 and raffinose was detected. Stachyose appeared later in 147-day-old embryos and accumulated until shedding. In vitro culture of immature embryos in the presence of high sucrose concentrations (350 and 700 mM) resulted in an increase in their dry weight and a decrease in their water content, and induced the acquisition of desiccation tolerance. Under these conditions, sucrose accumulated in embryos to 30–40% on a dry weight basis, but neither raffinose nor stachyose was detected. Acquisition of desiccation tolerance by oil palm immature embryos was associated both in planta and in vitro with an accumulation of dry matter, a reduction of moisture content, and a fall in the monosaccharides/sucrose ratio. In planta, survival to dehydration was also related with the deposition of oligosaccharides whereas in vitro, it was related with high sucrose accumulation. The role of sugars in the acquisition of desiccation tolerance in oil palm embryos is discussed.