Ecophysiology of Seed Germination in Asarum sieboldii Miq. (Aristolochiaceae) with Underdeveloped Embryos

Asarum sieboldii is a perennial herb of the Aristolochiaceae, and has a trans-Atlantic disjunct distribution. The aims of this study were to determine the requirements for breaking dormancy and germination of A. sieboldii seeds and to compare its dormancy characteristics with those of the congener in eastern North America. Ripe seeds of A. sieboldii had an underdeveloped embryo and were permeable to water. The embryos elongated at medium temperatures in September after high temperature period in summer, and germination occurred between mid-September and early-October. After germination, seedling emergence was delayed until February, and most seedlings emerged in late March of the following year. Therefore, radicles emerge from the seeds during autumn, and cotyledons emerge the following spring. The underdeveloped embryos elongated to the critical length for germination when the seeds were moved from high (25^oC) to medium (20^oC) temperatures. Radicles did not emerge from the seeds incubated at 25^oC until the seeds were transferred to 20^oC. When the seeds were warm-stratified at 25^oC for 4, 8, and 12 weeks and then moved to spring temperatures (15^oC), more than 80% of the seeds germinated within 6 weeks, whereas less than 20% of the seeds germinated without warm stratification. Germinated seeds required subsequent chilling period for seedling emergence. When seeds with emergent radicles were cold-stratified and then moved to spring temperatures (15^oC), the cotyledons emerged. Thus, radicle dormancy is broken by high summer temperatures, and epicotyl dormancy is broken by low winter temperatures. These results suggest that the seeds of A. sieboldii have deep simple epicotyl morphophysiological dormancy caused by both underdeveloped embryos and a physiological dormancy. This dormancy type is similar to that of seeds of the eastern North American species A. canadense.