Ecophysiology of seed germination in composites inhabiting fire-prone Mediterranean ecosystems

Within the framework of a research project concerning adaptive mechanisms of postfire regeneration in Mediterranean ecosystems, the ecophysiology of seed germination was studied in the following composites: Dittrichia viscosa (L.) Greuter, Helichrysum stoechas (L.) Moench ssp. barrelieri (Ten.) Nyman and Phagnalon graecum Boiss. & Heldr. All three species are common in the Mediterranean region; the former is a colonizer while the latter two are usually present in phrygana (low-shrub, fire-prone Mediterranean vegetation). As the result of their numerous, anemochorous dispersal units the three species are endowed with the potential to invade disturbed areas, in general, and burned ones, in particular. Germination in Dittrichia was generally very low in the dark (0-10%). Moreover, an absolute light requirement was revealed and germination could be promoted even by green safelight. In Helichrysum, final dark germination reached its highest value (ca. 50%) at relatively low temperatures (10 degrees C, 15 degrees C). Light and nitrates (optimal concentration 20 mM) promoted germination dramatically above dark control. Nevertheless, full induction of germination required the presence of both these factors. Phagnalon germinated optimally (70-100%) over a broad temperature range (10-25 degrees C), in the dark. However, light proved beneficial at suboptimal temperatures. In the three species studied, light and, in the case of Helichrysum, nitrate availability are particularly important for seed germination and eventual seedling recruitment in the postfire environment.