Breaking of Dormancy Towards a Threshold or Dormancy Nondormancy Cycle

Combined together, the results of the experiments reported above make it possible to outline the pattern of seed germination and survival in the soil in H. mantegazzianum. Seeds in the field start to germinate early in spring (March in the study area in the western part of the Czech Republic) and most

Fig. 5.6. Percentage of non-dormant seeds that germinated in the laboratory at 10/5°C, immediately after removal from the soil. Pattern recorded over two vegetation seasons is related to mean air temperature for months when seeds were tested. See Fig. 5.5 for details of data. Based on data from Moravcova et al. (2006).

Fig. 5.6. Percentage of non-dormant seeds that germinated in the laboratory at 10/5°C, immediately after removal from the soil. Pattern recorded over two vegetation seasons is related to mean air temperature for months when seeds were tested. See Fig. 5.5 for details of data. Based on data from Moravcova et al. (2006).

of them do so up to the end of April or beginning of May (Figs 5.5 and 5.6). Despite considerable effort, no new seedlings were found in invaded sites later in the season and also, no buried seeds germinated in the soil after then (Moravcova et al., 2006). However, the buried seeds removed from the soil started to germinate in the laboratory in November (Fig. 5.6). This corresponds to the fact that seeds need about 2 months of cold stratification below 8°C to break dormancy (Krinke et al., 2005; Moravcova et al., 2005); in November, this requirement is met for the seeds buried in the soil.

However, in the field seeds do not germinate in the autumn because of unsuitable conditions; in the laboratory they do. Obviously, buried seeds start to germinate in the laboratory approximately 2 months after the outside air temperature has dropped below 10°C (Fig. 5.6). But no buried seeds germinated before, i.e. in the course of the preceding late spring, summer and autumn when day temperature was above 10°C. This confirms the experimentally detected fact that although stratified seeds of H. mantegazzianum are able to germinate at a broad range of temperatures (Fig. 5.3), after being exposed to above 6°C for a longer time (up to 2 months) they cease germination and another cold period is needed to restart the germination process (Fig. 5.6).

It appears that seeds that have not germinated in spring (ca. 9% of seeds buried in the previous autumn) re-enter or retain dormancy during the high summer temperatures and break dormancy again during the following cold autumn and winter period (Fig. 5.5 and 5.6). It is hard to say whether the seeds in soil go through the annual dormancy/non-dormancy cycles which have been known for many annuals (Baskin and Baskin, 1998) and also for some perennials. The conditional dormancy/non-dormancy cycles are reported, for example, for a perennial Rumex obtusifolius L. (van Assche and Vanlerberghe, 1989), or biennials Verbascum blattaria L., V. thapsus L. (Vanlerberghe and van Assche, 1986; Baskin and Baskin, 1981) and Oenothera biennis L. (Baskin and Baskin, 1991) and dormancy/non-dormancy cycles are known also for perennials such as Lychnis flos-cuculi L. (Milberg, 1994a), Primula veris L. and Trollius europaeus L. (Milberg, 1994b), Rhexia mariana L. var. interior (Pennell) Kral & Bostick (Baskin et al., 1999) or in sedges (Schütz, 1998). In H. mantegazzianum this question seems to be more complex. In burial experiments, the majority of seeds found dormant after 1 year in the soil had a morphologically fully developed embryo (Fig. 5.8C), which means that such seeds are only physiologically dormant; morphological dormancy must have been broken at that time. However, some seeds with an underdeveloped (Fig. 5.8A) or partially developed embryo (Fig. 5.8B) that had kept morphological (or morphophysiological) dormancy were also found (Moravcova et al., 2006). Therefore, it seems more probable that dormancy in seeds of H. mantegazzianum is not broken completely in the first spring, but the breaking happens gradually and this process can take place only in months with sufficiently cold temperatures. This explains why the seeds staying dormant in soil have to wait until the next spring to germinate. That a small amount of seeds (about 1%) are able to survive in dormant state in the soil for at least 3 years (Fig. 5.7) suggests that the dormancy-breaking processes can take quite a long time in a small fraction of seeds and that the threshold is gradually achieved through accumulation of active temperatures during cold months.

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