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Soil salinity diminishes the dominance of species and affects their distribution. Phragmites australis is a dominant ecosystem engineer with broad distribution, high intraspecific variation and great socio-economic importance. Coastal ecosystems inhabited by P. australis are threatened by salinization. Here, we investigated salinity tolerance of freshwater and salt marsh population of the species, grown under two soil salinities and in two common gardens. Salinity significantly affected the growth, biomass, leaf physiological parameters and ion concentration. Climate altered morphology and ion concentration of P. australis. In contrast to our hypothesis, the salt marsh population was not more salt tolerant than the freshwater population, and both showed a low degree of salt stress, maintaining their photosynthesis and chlorophyll concentration, and only showing small decreases in biomass and height when salt-treated. We therefore ruled out local adaptation to soil salinity. Instead, both populations acclimated by phenotypic plasticity of biomass, root: shoot ratio, stomatal conductance and ion content. The salinity tolerance strategy of both populations was ion concentration by tissue desiccation, which was most efficient in the drier climate common garden. Hence, plants utilizing tissue desiccation as salinity tolerance strategy will have an advantage in areas where climate change leads to drier air humidity. [ABSTRACT FROM AUTHOR]

In the Mississippi River Delta, the common wetland grass, Phragmites australis, displays high genetic diversity, as several genetically distinct populations are co-occurring. Differences in salinity tolerance may be an important factor determining these populations' distribution in the delta. Our study investigated the salt tolerance of four genotypes exposed to 0, 10, 20, 30, and 40 ppt salinity. The growth rate, biomass, and the light-saturated photosynthetic rate were stimulated at 10 ppt salinity and inhibited at salinities higher than 20 ppt, compared to controls. Increased concentrations of Cl and Na were found in the roots and older leaves of plants exposed to high salinities. Salt tolerance levels differed between genotypes. High salinity tolerance was mainly achieved by reduced water uptake and vacuole compartmentalization of toxic ions. The most tolerant genotype sustained biomass and photosynthesis even at 40 ppt, whereas the most sensitive genotype did not survive salinities higher than 20 ppt. Our findings show that the observed occurrence of different genotypes in the Mississippi River Delta is correlated to genetically determined differences in salinity tolerance. Further investigations are needed to better understand the role that salinity tolerance plays in the invasion of certain introduced P. australis genotypes. [ABSTRACT FROM AUTHOR]