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 reduce photosynthesis in sensitive forest species. Viburnum lantana L. has been used as an in situ bioindicator of O3 pollution in mountainous areas of Europe; however, flux-based response functions and critical levels (CLs) for this species have not yet been established. This study validated field-observed O3 effects in V. lantana through experiments carried out in a Free-air O3 eXposure infrastructure and determined which O3 metric (exposure-based AOT40 or flux-based-POD1) best explains O3 effects on leaf physiology and VFI. Throughout the experimental period (T2: 3.5-month O3 exposure), V. lantana saplings were subjected to ambient air (AA) conditions and elevated O3 levels (1.5× and 2.0× AA). O3 VFI appeared after 16 days in 2.0× and increased progressively during the growing season, reaching the highest Plant Injury Index (PII) values in the 2.0× (9.06 ± 3.24) compared with 1.5× and AA treatments (1.31 ± 0.62 and 1.29 ± 0.71). Elevated O3 also significantly reduced net photosynthetic rate (Asat), relative chlorophyll content (SPAD), and the maximum photochemical efficiency of photosystem II (Fv/Fm); no significant difference in stomatal conductance (gs) was found. The flux-based metric POD1 better explained variability in O3 VFI and physiological parameters. Based on the best-fitting models, CLs for V. lantana were estimated at 1.61 mmol m−2 and 1.22 mmol m−2 for a 4% reduction in Asat and gs, and a CL of 7.82 mmol m−2 for the O3 VFI-onset.