Seed germination ecophysiology of two western North American species of Sambucus (Caprifoliaceae sensu lato), and comparisons with eastern North American and European congeners

Seeds of the western North American (wNA) species Sambucus caerulea in the Sambucus nigra species complex and Sambucus callicarpa in the Sambucus racemosa complex contain embryos that are 70-75% fully developed when freshly matured. These embryos must elongate approximately 15-25% before radicle emergence occurs. Embryos in seeds of both species grew better at 5 degrees C than at 25/15 degrees C. Dormancy was broken by cold stratification at 5 degrees C, which was also the optimal temperature for germination. Gibberellic acid substituted for cold stratification in both species. Thus, seeds of both species have intermediate complex morphophysiological dormancy (MPD). The seed dormancy characteristics of both species differ from those of congeners in eastern North America (eNA). In contrast, dormancy break in S. callicarpa is similar to its European congeners, but is unclear in S. caerulea. Our study represents the first to compare seed dormancy among wNA, eNA and European congeners. In North America, the geographic pattern of dormancy break for Sambucus is similar to that of Osmorhiza (Apiaceae) and Erythronium (Liliaceae): cold stratification is required in wNA taxa and warm + cold stratification is required in eNA taxa. Moreover, the pattern in S. callicarpa is similar to that in some species of Sanicula (Apiaceae): cold stratification in Europe and warm + cold stratification in eNA. The evolution of seed dormancy in these groups might have occurred under similar environmental circumstances, resulting in the sharing of dormancy-breaking mechanisms within geographical regions.