Disentangling the effect of atmospheric CO2 enrichment on the halophyte Salicornia ramosissima J. Woods physiological performance under optimal and suboptimal saline conditions

A mesocosm experiment was designed to assess the effect of atmospheric CO2 increment on the salinity tolerance of the C3 halophyte Salicornia ramosissima. Thus, the combined effect of 400 ppm and 700 ppm CO2 at 0, 171 and 510 mM NaCl on plants growth, gas exchange, chlorophyll fluorescence parameters, pigments profiles, antioxidative enzyme activities and water relations was studied. Our results highlighted a positive effect of atmospheric CO2 increment on plant physiological performance under suboptimal salinity concentration (510 mM NaCl). Thus, we recorded higher net photosynthetic rate (AN) values under saline conditions and 700 ppm CO2, being this effect mainly mediated by a reduction of mesophyll (gm) and biochemical limitation imposed to salt excess. In addition, rising atmospheric CO2 led to a better plant water balance, linked with a reduction of stomatal conductante (gs) and an overall increment of osmotic potential (Ѱo) with NaCl concentration increment. In spite of these positive effects, there were no significant biomass variations between any treatments. Being this fact ascribed by the investment of the higher energy fixed for salinity stress defence mechanisms, which allowed plants to maintain more active the photochemical machinery even at high salinities, reducing the risk of ROS production, as indicated an improvement of the electron flux and a rise of the energy dissipation. Finally, the positive effect of the CO2 was also supported by the modulation of pigments profiles (mainly zeaxhantin and violaxhantin) concentrations and anti-oxidative stress enzymes, such as superoxide dismutase (SOD) and ascorbate peroxidase (APx).