The role of temperature in the regulation of dormancy and germination of two related summer-annual mudflat species

Abstract

Dormancy and germination requirements were investigated in seeds (achenes) of the closely-related annuals Bidens cernua and Bidens tripartita (Asteraceae). They showed clear differences in their temperature requirements for germination, their dormancy and seed longevity in soil. In B. cernua seeds, primary dormancy was relieved when stratified at 3, 8, and 12 °C, as seeds germinated to a maximum of more than 80% (3 °C and 8 °C) and 20% (12 °C), while temperatures of 15 °C and 18 °C had no effect. Secondary dormancy was induced after 20 weeks at 12 °C. Temperatures between 3 and 18 °C were effective in relieving dormancy in B. tripartita seeds as they germinated to more than 60% regardless of stratification temperature. With prolonged incubation time, dormancy was induced at 18 °C. Both species exhibited an annual dormancy cycle. B. cernua seeds came out of dormancy in spring at temperatures <15 °C and dormancy was induced in summer (>15 °C). In late summer, seeds were completely dormant before temperatures of <7 °C led to a relief of secondary dormancy. B. tripartita seeds came out of both primary and secondary dormancy when ambient temperatures were <12 °C in autumn. At temperatures >7 °C in spring and summer, dormancy was induced. Nevertheless, B. tripartita seeds could germinate (>40%) during the whole year at high and fluctuating temperatures. B. tripartita had the potential to accumulate a persistent seed bank, while buried B. cernua seeds were all dead after 20 months. Germination tests with dry-stored seeds showed that a fluctuating-temperature amplitude of ≥8 °C (mean 22 °C) led to a significantly higher germination (40%) of B. tripartita seeds, while B. cernua seeds did not germinate even at high amplitudes. Stratified B. tripartita seeds germinated at high constant temperatures >21 °C, while B. cernua seeds showed a nearly absolute requirement for fluctuating temperatures.