Cycling of physically dormant (PY) seeds between states insensitive and sensitive to dormancy-breaking factors in the environment has recently been demonstrated in Fabaceae and Convolvulaceae, and it may be a common phenomenon in seeds with water-impermeable seed coats. In contrast to seeds of many species with physiological dormancy (PD), those with PY cannot cycle between dormancy and non-dormancy (ND). In this paper, we evaluate the role of sensitivity cycling in controlling the timing of germination of seeds with PY in nature, and show that sensitivity cycling in seeds with PY serves the same ecological role as dormancy cycling in seeds with PD. Thus, sensitivity cycling in seeds with PY ensures that germination in nature occurs only at (a) time(s) of the year when environmental conditions for growth are, and are likely to remain, suitable long enough for the plant to complete its life cycle or to form a perennating structure. Further, we describe the experimental procedures necessary to determine whether sensitivity cycling is occurring, and discuss briefly the possible relevance of sensitivity cycling to dormancy classification.

The quality of a seed (germination and vigour) is established during its development and maturation, but can be improved by post-harvest processing and pre-sowing treatments. During commercial seed production, maturity is usually estimated visually, relying on experience of the growers, but seed researchers are working to find molecular markers that can be applied easily to help in establishing optimal harvest time. One marker is cell cycle activity expressed as DNA replication in the seeds, analysed by flow cytometry. This fast and accurate method for the estimation of DNA content in plant nuclei allows detection of nuclei at different replication stages in different seed tissues and thus makes it possible to follow changes in the physiological state of a seed. DNA replication, as a late event during germination, can also be used to mark completion of germination and transition to early seedling growth. This information can be useful in the evaluation of seed quality and for following the advancement of priming. Flow cytometric analysis of ploidy can be also used as a basis for control of purity of some polyploid species seed lots.

Eragrostis is a cosmopolitan genus of the family Poaceae. Several wild species, including E. pilosa (L.) Beauv., are harvested for food, but the only cultivated crop-species is tef [E. tef (Zucc.) Trotter]. Despite its importance as a staple food and its plasticity to diverse environmental conditions, little is known about the structural and physiological strategies that adapt tef seeds to endure diverse and variable moisture regimes. Here, we report the presence of slime cells, a type of modified epidermal cell, covering the fruit of tef and its wild relative, E. pilosa. The slime produced by Eragrostis belongs to the ‘true’ slime type, since it is exclusively composed of pectins. Pectin forms uniform layers on the cell wall inner surface, which are confined by a thin cellulose layer to prevent release into the cell lumen. In the presence of water, pectins quickly hydrate, causing swelling of the slime cells. This is followed by their detachment, which may be controlled by a thin cuticle layer on the fruit surface. The ability of slime to absorb and maintain moisture around the grain is thought to be an adaptive feature for Eragrostis growing in dry habitats. This retention of water by slime may create conditions that are suitable for rapid germination.

Aerosol smoke and smoke-water can break dormancy and promote seed germination of many plant species. In this study we investigated changes in gene expression after imbibition of light-sensitive Lactuca sativa L. cv. ‘Grand Rapids’ achenes with dilute smoke-water compared to water control samples kept in the dark or continuous light, using the fluorescent differential display technique. Although no difference was detected in the smoke-water versus water control samples germinated in light, smoke-water treatment resulted in the differential display of several expressed sequence tags (ESTs) when compared to water control samples kept in the dark. The most pronounced fragments isolated correspond to known genes related to germination, with functions in cell wall expansion, regulation of translation, the cell division cycle, carbohydrate metabolism and abscisic acid (ABA) regulation. Real-time polymerase chain reaction (PCR) validation revealed that the transcript abundance of the genes, HVA22, short-chain dehydrogenase/reductase and late embryogenesis abundant protein, are upregulated after smoke treatment when compared to control achenes kept in the light. The results indicate that smoke has a dual effect. On the one hand, the smoke can induce genes that may be linked to ABA action, whereas, on the other hand, it elicits a faster germination rate by inducing a similar pattern in gene expression as light treatment. Smoke effects could be manifested mainly through the induction of the cell division cycle, cell wall extension and storage mobilization.

Evidence for effects of saprophytic fungi on buried seed demography is usually obtained from studies involving the simultaneous burial of fungicide-treated seeds and of untreated seeds. However, any potential influence of fungicide treatment on seed dormancy levels is generally ignored in these studies. Also, some studies assume that a combination of several fungicidal compounds provides better protection against a broader range of fungi, ignoring chemical interactions that may potentially occur between different compounds. To investigate these issues, we carried out a 6-month burial experiment using seeds of Anthriscus sylvestris (L.) Hoffm., Centaurea nigra L. and Daucus carota L., and three substrates differing in organic matter content. Three fungicidal compounds, captan, iprodione and mancozeb, were applied alone and in combination, including an untreated control. All fungicidal compounds and combinations thereof provided protection against fungal-induced seed mortality and, except for a low efficacy of iprodione in protecting seeds of Anthriscus, there were no pronounced differences in seed mortality between different fungicide treatments. Captan temporarily inhibited germination in Centaurea, whereas a similar inhibition in Daucus seeds caused by mancozeb was more long lasting, suggesting an induction of secondary dormancy. Organic matter content had only a negligible influence on these results. Our results suggest that the basic conclusions from most seed burial studies are robust with respect to their choice of fungicide. We conclude by discussing further implications of our findings for the design and interpretation of seed burial studies.