Salinity tolerance of two critically endangered endemic species and its implications for distribution and conservation of model microinsular Mediterranean species

Medicago citrina and Euphorbia margalidiana are two microinsular species from the Western Mediterranean which are amongst the top 50 most threatened species of the Mediterranean. Conservation plans for both taxa involve translocation and ex situ seedling growth, yet salinity tolerance remains unknown for both species. The main objective of this work was to analyse how germination is modulated by salinity and to evaluate the seed buoyancy of both species to give light to their sea-dispersal capabilities. Salinity tolerance tests were performed using increasing concentrations (100, 200, 300, and 400 mM) solutions of different salts (NaCl, MgCl2, MgSO4, and Na2SO4). Recovery tests were carried out to analyse the potential ionic toxicity of salts. Seed buoyancy was also evaluated in sea and distilled water for each species. M. citrina was the most salt tolerant, germinating at values below 35.6 mS/cm, whilst E. margalidiana only germinated below 21.4 mS/cm. Na2SO4 and MgCl2 are the salts with the strongest inhibitory effect, whilst MgSO4 is the least inhibitory salt. In both species, buoyancy is very limited, being higher in M. citrina (up to 15 days) compared to E. margalidiana (4 days). The higher salinity tolerance and buoyancy allows M. citrina to colonise a broader range of islets than E. margalidiana, which requires moderate altitudes to keep populations away from salt impact. Both species cannot be considered as halophytes and so in situ and ex situ conservation measures should be carried out avoiding high salinity areas to ensure seedling development.