Ozone damage in plants: 7 years of study in a free-air experimental facility (FO3X)

Ozone (O₃) is a toxic oxidative air pollutant with significant detrimental effects on natural vegetation and crop species. Free-air controlled exposure (FACE) facilities provide an ideal tool for O₃ effect studies, producing realistic results. The O₃ Free-Air Controlled Exposure (FO₃X - FACE) facility, located in Sesto Fiorentino, Italy, and established in 2015, is an AnaEE (Analysis and Experimentation on Ecosystems) European research platform. The facility permits the exposure of plants to three levels of O₃ concentrations (1.0, 1.5, and 2.0 times the ambient concentration, denoted as AA, x1.5AA, x2.0AA, respectively), with main environmental variables continuously monitored. Over the years, the accumulated exposure over 40 ppb hourly concentrations (AOT40) was calculated and used as an exposure-based O₃ index. The stomatal conductance (g_sto) model, based on the multiplicative algorithm, was used to parameterize the g_sto of 12 species (7 deciduous: Oxford poplar, Quercus robur, Quercus pubescens, Sorbus aucuparia, Alnus glutinosa, Vaccinium myrtillus, I-214 poplar, and 5 evergreens: Quercus ilex, Phillyrea angustifolia, Arbutus unedo, Pinus halepensis, Pinus pinea), in order to calculate the hourly stomatal O₃ flux (F_st), and thus the phytotoxic O₃ dose above an hourly threshold y of uptake (y = 1, POD₁) used as a flux-based O₃ index. Our studies have evaluated the effect of O₃ in gas exchange parameters as light-saturated photosynthesis (A_sat / R² POD₁ = 0.46 vs. AOT40 = 0.20) and stomatal conductance (g_sto /R² POD₁ = 0.18 vs. AOT40 = 0.18), as well as for the induction of specific visible foliar injury (VFI / R² POD₁ =0.40 vs. AOT40 = 0.32) and biomass loss (B_loss / R² POD₁ = 0.50 vs. AOT40 = 0.22). We demonstrate that species-specific flux-based O₃ index POD₁ is more relevant compared to the exposure-based O₃ index AOT40, showing the importance of comprehending the mechanism of O₃ damage to plants after the uptake through stomata. Our research has also been improving the derivation of experimentally based critical levels (CLs) for the protection of forests and crops vegetation from O₃ damage.