Seed dormancy and germination in an ecological context : comparative studies of annual weeds

Abstract: Germination ecology studies, i.e. studies of interactions between characteristics of the seeds and environmental circumstances, provide understanding of spatial and temporal patterns of emergence of wild species, as weeds in the field. A large number of species have seed dormancy, i.e. seeds respond to circumstances not directly evoking germination. Thus, germination may occur only if specific environmental events have occurred in the seeds' past life story, and/or if a suitable time period has passed since ripening on mother plants. In this thesis, three characteristics, i.e. dormancy pattern, germination preferences and dormancy strength, hypothesised as mutually independent seed characteristics, are used to describe dormancy and germination in an ecological context. This conceptual model may be used for descriptions of differences between species, to understand emergence patterns in the field from controlled germination tests, and probably for increasing the understanding of evolution of ecological important properties of seed dormancy and germination.Fifteen annual weedy taxa were investigated: two groups of temperate climate weeds, Lamium and Papaver, and one group of tropical weeds, co-occurring Asteraceae species of different genera. Intra-species variations in germination occurred for all species, but species-specific germination responses within all groups were revealed in analyses. All species showed some kind of species-specific dormancy pattern that was expressed as increased germination in response to one or more environmental factor not required for the actual germination.For Lamium and Papaver, the general dormancy pattern was genus-specific. Germination preferences varied slightly within genus. Dormancy strength was to some extent species-specific, but highly variable. The species belonging to Asteraceae showed differences in dormancy pattern. It is shown how such differences can be visualized and compared by plotting response time and achieved germination in two-dimensional graphs, giving species-specific pictures.Dormancy pattern and germination preferences explained how Papaver can perform as winter annuals in warmer climates, but substantially as summer annuals in colder climates, without local adaptations. For Lamium, the results indicate a local adaptation of increasing dormancy strength: in relatively cold climate in Sweden, a strong dormancy restricted a part of each cohort from germinating during autumn, thus preventing from winter mortality. For the Asteraceae, five of six species clearly responded to cold pretreatment, which is noteworthy considering the small seasonal temperature differences in the study area and in the areas of origin for the species. The six Asteraceae achieved similar emergence timing in the field by responding to different environmental factors.Together, the results from these fifteen species indicate that dormancy pattern is an evolutionary conservative characteristic, dormancy strength is relatively easily changed, and germination preference is intermediate.