Germination ecology of two closely related taxa in the genus Oenanthe: Fine tuning for the habitat?

For two closely related amphibious plant taxa of the genus Oenanthe (Apiaceae) germination traits were examined. Habitats of the two taxa differ in hydroregime. The endemic Oenanthe conioides of the river Elbe estuary experiences daily tidal fluctuations whereas the widespread Oenanthe aquatica grows at the edge of ponds and in freshwater wetlands with rare and unpredictable fluctuations of water table.Seeds of both taxa could be characterized as non-dormant and light dependent. Under temperature fluctuations, germination percentage was higher than under constant temperatures. Salinity was tolerated to up to 3.3‰. At 10‰ there was a strong decrease in germination percentage, which did not result from ionic toxicity, as experiments with a gradient in osmolarity showed.Differences between the taxa were found concerning hydrological and temperature fluctuations. While O. aquatica showed no reduction in germination percentage under permanent 1 cm flooding, O. conioides did. On the other hand, germination under an artificial tidal hydroregime was better in O. conioides than in O. aquatica. During fluctuating temperatures of 3/22 and 6/22 °C O. conioides germinated much quicker and had a higher final germination percentage.Differences between the taxa could be correlated with differences of the hydrological regime in the specific habitat. Taking into account that O. conioides is phylogenetically a relatively young taxon, it may be hypothesized that a quick adaptation to the tidal conditions might have taken place.