Bidens frondosa is an annual weed occurring on mudflats with heteromorphic achenes. Central achenes are brown and elongated, and peripheral achenes are black and thicker. To identify differences in dormancy between the two morphs, germination of dry-stored achenes, and achenes stratified for 2, 4 and 13 weeks at 5°C, were tested at 15, 25 and 20/10°C in light for two populations. None of the achenes germinated after dry storage or when stratified for 2 or 4 weeks. Significant differences in dormancy between the two morphs were obvious at 20/10°C and 25°C after 13 weeks of stratification. Periphery achenes were less dormant as they germinated to 75% (Fürst-Pußckler-Park population, FPP) and 65% (Spreewald population, SPR) at 25°C, while only 18% (SPR) and 45% (FPP) of the central achenes germinated. Germination was completed within 3 days. At 20/10°C, the germination rate of periphery achenes was higher than those of central achenes, but nearly all achenes of both morphs had germinated after 21 days. No germination occurred at 15°C. Central achenes are morphologically better-adapted for dispersal by animals. Their deeper dormancy and slower germination might be a strategy to reduce the risk of germination in new, unfavourable habitats. In contrast, less dormant and fast-germinating peripheral achenes with a lower dispersal capability remain near the mother plant and thus can replace it in the following spring.
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