Seed germination requirements and salt stress tolerance of coastal rare species in Sardinia
To survive to adverse factors that characterize coastal environments, plant species often require special physiological or metabolic adaptations to overcome environmental stresses. Stress may be physical (e.g. temperature) or chemical (e.g. salinity). Many communities comprise highly specialized species, which have comparatively restricted geographical distributions. The coastal species investigated in this Ph.D. program were chosen accordingly to their habitat: Phleum sardoum (Hackel) Hackel and Rouya polygama (Desf.) Coincy for sandy dunes; Brassica insularis Moris and Lavatera triloba L. ssp. pallescens (Moris) Nyman for coastal cliffs; Lavatera triloba L. ssp. triloba and Halopeplis amplexicaulis (Vahl) Ces., Pass. & Gibelli for ultra-saline environments. Moreover, seed germination ecology of L. agrigentina Tineo, a species growing in clayey-chalky plains of South Italy, was also investigated for a comparative study within the Lavatera genus. For all the studied species, light and temperature requirements for seed germination were evaluated; their germination responses to salt stress (NaCl) and their germination recovery. Interpopulation variability on germination patterns was also evaluated for R. polygama, B. insularis and L. triloba ssp. triloba. Salt spray tolerance on the vegetative growth and biomass production during the early seedling developmental stages was evaluated for B. insularis, L. triloba ssp. pallescens, L. triloba ssp. triloba and H. amplexicaulis. Light did not affect germination percentages in any of the studied species enabling seed germination also under soil surface and highlighting that seeds were not photo-inhibited for germination. Seed germination of P. sardoum and R. polygama, as well as that of L. agrigentina, L. triloba ssp. pallescens and L. triloba ssp. triloba, reflected the optimal range of temperatures of “typical” Mediterranean species, suggesting germination in autumn-winter, when water availability, soil moisture and rainfalls are high, and temperatures are not excessively prohibitive for germination and consequent seedlings establishment. B. insularis differed from other “typical” Mediterranean plants, for which germination at low temperatures is a widely extended trait, demonstrating that germination of this species may occur in a wide time window during the year. H. amplexicaulis seed germination was highly promoted by the daily fluctuation of temperatures, while germination at constant temperatures was sensibly lower. Salinity tests showed higher germination percentages in the non-saline conditions, with seed mortality increasing proportionally with NaCl concentrations and temperatures. Salt tolerance limits varied among species, from a minimum of 100 mM NaCl for P. sardoum to 600 mM for H. amplexicaulis and L. triloba ssp. pallescens, without a clear habitat related pattern. The species for which salt spray experiments were conducted showed different responses on seedling growth to salt aerosol tolerance, with these differences being related to the habitat of each species and their distance from the sea. Populations of B. insularis and L. triloba ssp. pallescens, growing in coastal cliffs highly influenced by wind and salt spray, showed the lowest seedling mortality. High interpopulation variability in salt spray tolerance was detected for B. insularis, between a coastal and an inland population, with the latter resulting not adapted to this abiotic environmental factor. Seedling survival of the two inland species (L. triloba ssp. triloba and H. amplexicaulis) was inversely proportional to the increase of nebulization frequency, demonstrating a low adaptation to salt spray, likely due to their distance from the sea coast and/or to interposed vegetation that may determine a lower impact of marine aerosol. The results of this study lead to a better knowledge on the autoecology of the investigated species and to their limits of tolerance to abiotic factors such as temperature, soil salinity and salt spray.
Coastal environments, Mediterranean flora, NaCl, ambienti costieri, dry after-ripening, flora mediterranea, recovery, recupero, salt spray, spray salino, stratificazione secca, Settore BIO/03 - Botanica Ambientale e Applicata
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