Seed characteristics, germination preferences and seed dormancy patterns have been proposed as tools for understanding evolutionary patterns (Martin, 1946; Baskin and Baskin, 2004; Nikolaeva, 2004). Because of bio-molecular methods, knowledge of phylogenetic relationships is increasing (e.g. APG, 2003), and we regard it as meaningful to study differences and similarities among documented, closely related taxa in order to increase understanding of adaptations and changes in seed dormancy and germination preferences. One difficulty when comparing seed dormancy and germination between taxa is the intra-taxon variation (Probert et al., 1985; Schütz and Milberg, 1997; Andersson and Milberg, 1998; Keller and Kollman, 1999; Pezzani and Montaña, 2006). Variation within a taxon may depend on genetic differences, local weather during growth of mother plants and maturation of seeds, seed position on the mother plant, soil quality, or other naturally occurring factors. To be able to draw conclusions on a general level, for example for modelling or predicting changes in emergence pattern following climate change, knowledge about a taxon, including its variation, is needed. The general taxon performance within an area can be studied by using seed batches from different populations as replicates of the taxon.
From an ecological perspective, germination can be viewed as being dependent on seed dormancy, germination preferences, and the interaction between these two characteristics and local climate and weather. We define seed dormancy here as a seed character that prevents germination, even if suitable germination conditions prevail. For ecological interpretations we consider that seed dormancy should be regarded as a continuous property of a seed batch (even though it is not known whether it is a continuum or an on–off property for a single seed); the degree of dormancy for a seed batch can be reduced, and, for some species, induced again, in response to environmental events and/or time. In this paper, we use ‘dormancy strength (weak–strong)’ for a general description of a seed batch or taxa, and ‘degree of dormancy (low–high)’ for describing any specific moment on the continuous scale.
The germination timing of the four Papaver taxa occurring as weeds in southern Sweden was investigated: P. argemone, P. rhoeas, P. dubium ssp. dubium and P. dubium ssp. lecoqii. The origin of these taxa is probably within south-eastern Europe, Anatolia and the eastern Mediterranean region (Kadereit, 1990). Papaver rhoeas is a close relative of the two P. dubium subspecies, while P. argemone is more distantly related; it is closer to Roemeria (which is not established in Sweden; Jonsell, 2001) than to the other Papaver studied here (Kadereit et al., 1997; Carolan et al., 2006). The four taxa often co-occur in various combinations, including all together, both in Sweden (Jonsell, 2001) and in England (McNaughton and Harper, 1960). Hybrids are rare in nature, and artificial inter-specific crosses either fail completely or reproduction of the hybrids usually fails (McNaughton and Harper, 1964). Even though all four taxa occur as weeds (e.g. Roberts and Boddrell, 1984; Jonsell, 2001), P. rhoeas is the only one regarded as being among the most serious weeds on a global scale (Holm et al., 1997). Nowadays, P. argemone is, due to sensitivity to herbicides, a minor weed (Jonsell, 2001), often found only on the edge of agricultural fields (personal observation).
Papaver rhoeas emerges in both autumn and spring in England (e.g. Roberts and Feast, 1970) as well as in Sweden (Baskin et al., 2002). In England, P. dubium is reported as being mostly a summer annual by McNaughton and Harper (1964), in contradiction to Roberts and Boddrell (1984) who found P. argemone, P. dubium ssp. dubium and P. dubium ssp. lecoqii emerging throughout the year, when seeds were mixed with soil and the soil was disturbed three times a year. However, seedlings were most common in autumn and infrequent in the middle of summer and winter. The four taxa were compared here by subjecting seeds from southern Sweden to three different artificial climates and two different times for dispersal.