Osmotic and ionic effects of NaCl and Na2SO4 salinity on Phragmites australis

Osmotic and ion-specific effects of NaCl and Na2SO4 on Phragmites australis (Cav.) Trin ex. Steud. were investigated in a laboratory experiment by examining effects of iso-osmotic solutions of NaCl and Na2SO4 on growth, osmolality of cell sap, proline content, elemental composition and gas exchange. Plants were supplied with a control standard nutrient solution (Ψ = −0.09 MPa) or solutions of NaCl or Na2SO4 at water potentials of −0.50, −1.09 or −1.74 MPa. Salt treatments increased root concentrations of Na and S or Cl, whereas P. australis had efficient mechanisms for exclusion of Na and S and partly Cl ions from the leaves. Incomplete exclusion of Cl from the leaves may affect aboveground biomass production, which was significantly more reduced by NaCl than Na2SO4. Stomatal conductance was negatively influenced by decreasing water potentials caused by NaCl or Na2SO4, implying that a non-significant photosynthetic depression observed in plants grown at −1.74 MPa was mainly due to osmotically induced stomatal closure. This was supported by decreasing internal CO2 concentrations. Saline conditions increased the intrinsic water use efficiency and did not alter photosynthetic parameters derived from light response curves, supporting the assumption of a well-functioning CO2 utilization in salt stressed plants. The leaf proline concentration increased equally in NaCl and Na2SO4-treated plants, and may play an important role as a compatible organic solute. P. australis possesses a range of mechanisms conferring tolerance to both NaCl and Na2SO4 stress and except in terms of growth the phytotoxicity of NaCl and Na2SO4 are comparable.