The physiological basis for ethylene-induced dormancy release in three Echinacea species with special reference to the influence of the integumentary tapetum

Seeds of Echinacea species have endogenous physiological dormancy. Dormancy release is induced by moist chilling stratification, but seeds treated with ethylene can show increased germination comparable to stratification. The primary aims of this work were to discover whether endogenous ethylene production was required for dormancy release and germination in Echinacea seeds and to investigate the physiological basis for stratification and ethylene-induced dormancy release. There were no significant differences in ethylene production in untreated versus stratified seeds. Seeds subjected to a dormancy release treatment showed reduced sensitivity to exogenous abscisic acid (ABA). Isolated embryos were completely released from dormancy when the outer envelope surrounding the embryo was removed. Isolated embryos with the envelope intact were also induced to germinate when treated with ACC (1-aminocyclopropane-1-carboxylic acid) to increase ethylene production. It was determined that the covering envelope was derived from enlargement of the endothelium layer surrounding the egg sac (integumentary tapetum) plus several crushed layers of the outer integument. Ethylene does not appear to be required for dormancy release and germination in Echinacea seeds. Two possible physiological mechanisms were discovered to explain stratification and ethylene-induced dormancy release. These included a change in seed sensitivity to ABA and changes in the tissues covering the embryo. The data suggests that ethylene-induced dormancy release is independent of stratification and possibly acts by inducing physiological events that are normally downstream of stratification.