Photosynthetic responses to elevated CO2 and O3 in Quercus ilex leaves at a natural CO2 spring

Abstract: Photosynthetic stimulation and stomatal conductance (Gs) depression in Quercus ilex leaves at a CO₂ spring suggested no down-regulation. The insensitivity of Gs to a CO₂ increase (from ambient 1500 to 2000μmolmol⁻¹) suggested stomatal acclimation. Both responses are likely adaptations to the special environment of CO₂ springs. At the CO₂-enriched site, not at the control site, photosynthesis decreased 9% in leaves exposed to 2× ambient O₃ concentrations in branch enclosures, compared to controls in charcoal-filtered air. The stomatal density reduction at high CO₂ was one-third lower than the concomitant Gs reduction, so that the O₃ uptake per single stoma was lower than at ambient CO₂. No significant variation in monoterpene emission was measured. Higher trichome and mesophyll density were recorded at the CO₂-enriched site, accounting for lower O₃ sensitivity. A long-term exposure to H₂S, reflected by higher foliar S-content, and CO₂ might depress the antioxidant capacity of leaves close to the vent and increase their O₃ sensitivity. [Copyright &y& Elsevier]