Ozone Flux-Based Response Functions for Visible Foliar Injury and Photosynthetic Traits in a Bioindicator Species, Viburnum lantana L.

Tropospheric ozone (O³) is a phytotoxic air pollutant that can impair visible foliar injury (O³ VFI) and reduces photosynthesis in sensitive forest species. Viburnum lantana L. has been widely used as an in situ bioindicator of O³ pollution in mountainous areas of Europe; however, field-observed O³-induced VFI as well as critical levels (CLs) established to protect forests, have not been validated. This study validated field-observed O³ effects in V. lantana through experiments carried out in a Free-air O³ eXposure infrastructure (FO₃X) and determined which O³ metric (exposure-based—AOT40 or flux-based—POD₁) best explains O³ effect on leaf physiology and VFI. V. lantana saplings were subjected to ambient air (AA) conditions and elevated O³ levels at 1.5× and 2.0× AA. Throughout the experimental period (T1: 2-month and T2: 3.5-month O³ exposure) measurements were taken for the Plant Injury Index (PII), light-saturated net photosynthetic rate (A_sat), stomatal conductance (g_s), leaf color index (SPAD), and the maximum photochemical efficiency of photosystem II (Fv/Fm). O³ VFI was first observed in 2.0× after 16 days. As a result, O³ treatment influenced PII, which was significantly higher in the 2.0× (9.06 ± 3.24) than in the 1.5× and AA treatments (1.31 ± 0.62 and 1.29 ± 0.71) at T2. The A_sat, SPAD, and Fv/Fm were significantly affected by O³ treatments; no significant difference in g_s was found. POD1 better explained variability in O³ VFI and physiological parameters, with CLs proposed for V. lantana of 1.61 mmol m^–2 and 1.22 mmol m^–2 for a 4% reduction of A^sat and g_s, and a CL of 7.82 mmol m^–2 for the onset of O³ VFI.