Role of temperature in dormancy break and/or germination of autumn-maturing Achenes of eight perennial Asteraceae from Texas, U.S.A.

Achenes of Aster ericoides, Baccharis neglecta, Brickellia dentata, Eupatorium havavense, Gymnosperma glutinosum, Helianthus maximilianii, Verbesina virginica and Viguiera dentata were collected at maturity in autumn in southcentral Texas and tested for germination over a range of thermoperiods (15/6, 20/10, 25/15, 30/15 and 35/20°C) simulating those in the habitat from September to May. Achenes of Aster, Baccharis, Brickellia, Eupatorium and Gymnosperma were nondormant, thus cold stratification did not change the range of thermoperiods over which they germinated. However, cold stratification increased the percentage of Brickellia achenes germinating at 15/6 and 20/10°C. Achenes of Helianthus, Verbesina and Viguiera were conditionally dormant at maturity, and cold stratification decreased the minimum thermoperiod for germination of Helianthus, Verbesina and Viguiera achenes to 15/6°C and increased the maximum thermoperiod for germination of Verbesina achenes to 35/20°C. Thus, achenes of Helianthus and Viguiera exhibited a type 2 germination response pattern during dormancy break and those of Verbesina a type 3. Germination percentages of Helianthus, Verbesina and Viguiera achenes increased during 6-8 wk of incubation at 20/10 and 15/6°C, which simulate habitat temperatures from November to February. Therefore, winter field temperatures are favorable for dormancy loss as well as for germination. If the soil is too dry for germination of conditionally-dormant achenes in autumn, a reduction of the minimum temperature for germination during winter would allow them to germinate in mid- to late winter, several months before the onset of the high temperature and drought conditions of early summer.