Abstract
Abstract At 0‐ to 12‐week intervals, imbibed achenes of ten perennial Asteraceae from eastern North America were removed from 5°C (cold stratification) and tested for germination over a range of daily thermoperiods. The objective was to determine changes in the temperature requirements for germination as achenes came out of dormancy (after‐ripened). In Eupatorium fistulosum, Helenium autumnale, Kuhnia eupatorioides, Mikania scandens, Ratibida pinnata, Solidago altissima and Verbesina alternifolia, the minimum temperature for germination decreased during afterripening (Type 2 response), and in Aster divaricatus, Echinacea simulata and E. tennesseensis the minimum and maximum temperatures decreased and increased, respectively (Type 3 response). The majority of the 32 species of Asteraceae whose afterripening pattern has been investigated have a Type 2 response, and the family is one of many in eastern North America with species that have Type 2. We hypothesize that the ancestors giving rise to species with Type 2 were species with (1) nondormant seeds that required high temperatures for germination, or (2) dormant seeds that afterripened during warm (dry) winters and germinated only at high temperatures.
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