Growth and photosynthetic responses of the cordgrass Spartina maritima to CO2 enrichment and salinity

Future climatic scenarios combine increasing concentrations of atmospheric CO2 and rising sea levels. Spartina maritima is a C4 halophyte that is an important pioneer and ecosystem engineer in salt marshes of the Atlantic coast of southern Europe. A glasshouse experiment investigated the combined effects on its growth and photosynthetic apparatus of approximately doubling CO2 concentration (from 380 to 700 μmol mol−1) at a range of salinity (0, 171 and 510 mM NaCl). We measured relative growth rates, gas exchange, chlorophyll fluorescence parameters, photosynthetic pigment concentrations, and total ash, Na+, K2+, Ca2+ and N concentrations. Elevated CO2 stimulated growth of S. maritima by c. 65% at all external salinities; this growth enhancement was associated with greater net photosynthetic rate (A) and improved leaf water relations. A increased despite a drop in stomatal conductance in response to 700 μmol mol−1 CO2. CO2 and salinity had a marked overall effect on the photochemical (PSII) apparatus and the synthesis of photosynthetic pigments. ΦPSII values at midday decreased significantly with external salinity in plants grown at 380 μmol mol−1 CO2; and Fv/Fm and ΦPSII values were higher at 700 μmol mol−1 CO2 in presence of NaCl. Plant nutrient concentrations declined under elevated CO2, which can be ascribed to the dilution effect caused by an increase in biomass. The results suggest that the productivity S. maritima and the ecosystem services it provides will increase in likely future climatic scenarios.