Seed dormancy and germination niches of Mediterranean species along an altitudinal gradient in Sardinia

The most crucial environmental factor for the failure of seedling emergence / establishment is temperature stress; consequently, seed germination should be triggered by favourable temperature conditions for the further development of the seedling. Therefore, temperature is the primary environmental factor regulating germination, and it is also considered responsible for changes in dormancy states of seeds. Seed dormancy has probably evolved differently across species, through adaptation to various habitats and prevailing environments. Under a Mediterranean climate, characterised by a considerable unpredictability of temperature and precipitation, with hot dry summers and cold wet winters, the plant growth and reproduction must occur in a window of favourable conditions that may vary in length and in which environmental cues and constraints play a central role. In this work, the effect of environmental factors on the thermal requirements for seed dormancy loss and germination of Mediterranean species was evaluated. Target species, located in Sardinia along an altitudinal gradient, have been chosen giving priority to endemic species as well as to the taxa with a high phytogeographical interest. In total, seeds of 26 provenances of 18 species were collected and the effect of temperature variation with altitude on the thermal requirements for dormancy loss and germination of the different species, as well as among species belonging to the same genus and also between different populations of the same species was investigated. The possible differences in germination strategies between Mediterranean and temperate species were also evaluated. In addition, the effect of different pre-treatments (cold and warm stratification and dry after ripening) and hormones (gibberellins) on the thermal requirements for seed dormancy release and germination of the target taxa was assessed. To study and monitor the annual trend of local soil temperatures, close to the study species, 24 data-loggers were buried along the altitudinal gradient. Finally, lipid thermal profile and water activity of target taxa was studied by Differential Scanner Calorimetry analysis to determine their biophysical and physiological characteristics. Through this study, the Mediterranean climate in Sardinia was characterized and seed germination responses of different Mediterranean species, as well as among populations of the same species, in relation with the altitude and different treatments were detected. Thermal thresholds (the base temperature “Tb” and thermal constant for 50% germination “S”) requirements of seed germination for all taxa were identified and significant differences on Tb and S of Mediterranean species compared with those temperate species were also detected. The thermal time for 50% germination (θ50) was calculated and compared for three Rhamnus species, and for Digitalis purpurea, Scrophularia trifoliata, Santolina insularis and Ptilostemon casabonae. In conclusion, this work has highlighted inter- and intra-specific differences on thermal thresholds for seed dormancy loss and germination of Mediterranean species; in addition, through the identification of lipid thermal profile and water activity of the target taxa, has provided further knowledge on the interactions between seeds and the natural environment in which they are located.