Germination niche of Sardinian endemic species in mountain riparian deciduous forests

The Supramontes region and Gennargentu massif are two of the most interesting territories of Central Eastern Northern Sardinia. Riparian vegetation among mountainous waterways of these territories are mainly constituted by Alnus glutinosa with other associated taxa such as Taxus baccata, Ilex aquifolium and Rhamnus persicifolia. Rare and threatened Sardinian endemic species such as Ribes multiflorum subsp. sandalioticum, Aquilegia barbaricina, Rhamnus persicifolia and Paeonia corsica grow under and close to the canopy of such riparian woods. Temperature is considered one of the major environmental factor governing seed germination in moist soil and, also is responsible for changes in dormancy states of seeds. Seed dormancy prevents germination in a specified period of time, under any combination of environmental factors that otherwise favour germination and it is mediated, at least in part, by the plant hormones abscisic acid and gibberellins. Dormancy can be broken by some environmental stimuli, such as a cold and/or warm stratification and gibberellic acid (GA) treatment. As dormancy is present throughout the higher plants in all major climatic regions, adaptation has resulted in divergent responses to the environment. Through this adaptation, germination is timed to avoid unfavourable weather for subsequent plant establishment and reproductive growth. In non-dormant seeds, the germination response to accumulated temperature could be modelled by a thermal time (θ) approach; in this model, seeds accumulate units of thermal time (°Cd) to germinate for a percentile g of the population. When seeds are subjected to temperatures (T) above a base temperature for germination (Tb), germination rate increases linearly with temperature to an optimum temperature (To), above which germination rate starts to decrease. In the sub-optimal range (To – Tb), germination occurs in the time tg, when the thermal time accumulated has reached the critical value (θg) for a percentile g of the population, and can be described as θg = (T –Tb)tg. In this work, the class of dormancy and thermal requirements for seed dormancy release and germination were investigate and/or confirmed for R. persicifolia, A. barbaricina, P. corsica and R. multiflorum subsp. sandalioticum; a thermal-time model, based on a soil heat sum approach, was developed in order to characterize its Thermal niche for germination and predict its seed germination phenology in the field.R. persicifolia showed physiological dormancy (PD), while the other three species highlighted morphophysiological dormancy (MPD); in particular, epicotyls MPD was found in P. corsica and confirmed in R. multiflorum subsp. sandalioticum. Thermal thresholds (Tb and θ50) requirements of seed germination were identified for all these species; in addition, the thermal thresholds for embryo growth was detected for A. barbaricina. The soil heat sum model developed in this work may have applicability to predictions of in situ regeneration of other species growing on Mediterranean mountain waterways. This work could confirmed that the studied species, belonging to different families placed in different phylogenetic clades, could have experienced a convergent evolution on their seed morphology and type of seed dormancy, as a response to similar environmental and climatic conditions due by the same habitat and ecosystem.