Seed dormancy, after-ripening and light requirements of four annual Asteraceae in south-western Australia.

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The role of dormancy, temperature and light in the regulation of seed germination of four annual Asteraceae from south-western Australia was investigated. The experiments aimed to identify after-ripening patterns, and to relate these to climatic conditions of the habitat in which the species occur. Seeds of all species were strongly dormant at maturity and maintained high levels of dormancy for time periods corresponding to the duration of summer in south-western Australia. Dry after-ripening was promoted best by temperatures lower than those prevailing in the dry season, although differences among storage temperatures were mostly insignificant. Germination percentages were highest at average winter temperatures (15 degrees C). A logistic model revealed significant differences in germinability among species, but not between incubation temperatures or light and dark treatments across species. Three species with seeds >0.5 mg germinated better in darkness than in light, whereas germination in darkness was almost inhibited in the species with the smallest seeds (0.14 mg). The course of dormancy loss, tested over a range of fluctuating incubation temperatures (7-30 degrees C), showed that seeds of three species came out of dormancy first at temperatures that prevail in south-western Australia during the winter (10-15 degrees C). Seeds from one species, introduced from South Africa, first lost dormancy at the lowest temperature (7 degrees C). All species showed after-ripening patterns of Type 1, typical of species growing in Mediterranean climates. The germination characteristics of the investigated species can be interpreted as ensuring that initial growth and establishment occur during the winter growing season, thereby avoiding the hot and dry summer conditions that follow seed dispersal.