Heat shock-stimulated germination in Mediterranean Basin plants in relation to growth form, dormancy type and distributional range

Heat shock-stimulated germination is frequently considered a trait restricted to a few plant families with impermeable seed coats in the Mediterranean Basin. We aimed to ascertain this general idea by using 68 eastern Mediterranean plant taxa differing in growth form, dormancy type and geographic distributional range. We conducted a laboratory experiment including heat shocks (60, 80, 100, 120 and 140 degrees C for 5 min) and a control treatment, and monitored germination for each species. Among the studied taxa, 24 (35%) showed a significant positive response for at least one of the treatments. Many Cistaceae and Fabaceae species showed a positive germination response to the heat shocks. However, we also observed heat shock-stimulated germination in the Brassicaceae, Malvaceae, Polygonaceae and Scrophulariaceae, and provided additional evidence for the Asteraceae, Lamiaceae, Liliaceae, Plantaginaceae and Rosaceae in the Mediterranean Basin. Distributional range and dormancy type were significant determinants of germination response to heat shocks whereas growth form and phytogeographic origin had lower and/or non-significant contribution. Species with wide or local distribution were less tolerant to moderate heat shocks than those with regional distributional ranges. Species with physically dormant seeds showed significantly more stimulated germination after heat shocks than those with permeable seeds. In conclusion, dormancy type is an important determinant of seed response to heat shock, and an association exists between distributional range and germination response to heat shocks. Our findings revealed that heat shock-stimulated germination is observed across a broader taxonomic spectrum in the Mediterranean Basin, including families not possessing physical dormancy.