Role of dispersal date and changes in physiological responses in controlling timing of germination in achenes of Geum canadense

Abstract

This study examines the importance of the date of dispersal, and of the changes in the physiological responses of achenes before and after dispersal, in regulating timing of germination in Geum canadense Jacq. The effect of dispersal date on germination was tested by collecting achenes from the parent plants from September through June and sowing them on soil in an unheated greenhouse. Changes in germination responses were monitored by testing achenes of each collection in light and darkness over a range of thermoperiods. Regardless of test conditions, achenes required light for germination. At maturity in September, 90% of the achenes germinated at the September thermoperiod (30:15 °C (maximum:minimum)), but none germinated in November at the November thermoperiod (15:6 °C). Achenes sown in September germinated that autumn and the following spring, but none of those sown in November germinated until spring. As achenes overwintered on plants in the field, germination percentages decreased at 20:10, 25:15, and 30:15 °C and increased at 15:6 and 5 °C. Thus, achenes sown in early spring germinated immediately, while those sown in early summer did not germinate until later. Germination requirements of the achenes also changed after dispersal. Stratification lowered the temperature requirement for germination, and thus achenes sown in autumn germinated in March at temperatures that had inhibited germination in the autumn. Achenes dispersed in May and June did not regain the ability to germinate at high temperatures during summer, and they lost the ability to germinate at low temperatures. Therefore, most of these latter achenes did not germinate until the following spring, after they had been stratified during the winter.