ROLE OF TEMPERATURE IN THE GERMINATION ECOLOGY OF THE SUMMER ANNUAL BIDENS POLYLEPIS BLAKE (ASTERACEAE)

Achenes (seeds) of Bidens polylepis were dormant at maturity in autumn , and they came out of dormancy while buried in moist soil in a nonhea ted greenhouse during winter or at 5 or 15/6 degrees C for 12 weeks. A s seeds came out of dormancy, they first germinated at 30/15 and 35/20 degrees C in light and at 30/15 degrees C in darkness. With additiona l loss of dormancy, the minimum temperature for germination in light a nd darkness decreased to 15/6 degrees C. Nondormant seeds re-entered d ormancy (secondary dormancy) when (1)kept in the greenhouse during sum mer, (2) subjected sequentially to simulated March-October temperature s in incubators or (3) exposed to 25/15 or 30/15 degrees C for 24 and 12 weeks, respectively. As seeds entered dormancy, they first lost the ability to germinate at 15/6 degrees C and then at higher temperature s. Buried seeds exposed to seasonal temperature changes for 28 months in the nonheated greenhouse exhibited an annual dormancy/nondormancy c ycle, with full dormancy occurring in September-November. Seeds came o ut of dormancy in winter (December-March), and they germinated to 85-9 6% in light at simulated habitat temperatures from mid-April until Jul y. Although 20-50% of the seeds germinated in darkness at simulated ha bitat temperatures in spring (March-June), only 2% of them germinated while buried in soil in the nonheated greenhouse. Thus, seeds have the potential to form a persistent seed bank.