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).

Atmospheric CO2 enrichment effect on the Cu-tolerance of the C4 cordgrass Spartina densiflora.

A glasshouse experiment was designed to investigate the effect of the co-occurrence of 400 and 700ppm CO2 at 0, 15 and 45mM Cu on the Cu-tolerance of C4 cordgrass species Spartina densiflora, by measuring growth, gas exchange, efficiency of PSII, pigments profiles, antioxidative enzyme activities and nutritional balance. Our results revealed that the rising atmospheric CO2 mitigated growth reduction imposed by Cu in plants grown at 45mM Cu, leading to leaf Cu concentration bellow than 270mgKg-1 Cu, caused by an evident dilution effect. On the other hand, non-CO2 enrichment plants showed leaf Cu concentration values up to 737.5mgKg-1 Cu. Furthermore, improved growth was associated with higher net photosynthetic rate (AN). The beneficial effect of rising CO2 on photosynthetic apparatus seems to be associated with a reduction of stomatal limitation imposed by Cu excess, which allowed these plants to maintain greater iWUE values. Also, plants grown at 45mM Cu and 700ppm CO2, showed higher ETR values and lower energy dissipation, which could be linked with an induction of Rubisco carboxylation and supported by the recorded amelioration of N imbalance. Furthermore, higher ETR values under CO2 enrichment could lead to an additional consumption of reducing equivalents. Idea that was reflected in the lower values of ETRmax/AN ratio, malondialdehyde (MDA) and ascorbate peroxidase (APx), guaiacol peroxidase (GPx) and superoxide dismutase (SOD) activities under Cu excess, which could indicate a lower production of ROS species under elevated CO2 concentration, due to a better use of absorbed energy.