Visible Foliar Injury and Ecophysiological Responses to Ozone and Drought in Oak Seedlings

To verify the responses of visible foliar injury (VFI), we exposed seedlings of three oak species for 4.5 months in an open air facility, using differing ozone (O₃) and drought treatments: O₃ (three levels from ambient to ×1.4 ambient), and drought (three levels of irrigation from 40% to 100% field capacity). We related the accumulated phytotoxic O₃ dose (POD₁) and cumulative drought index (CDI) to the O₃ and drought VFI and assessed growth increment (height, diameter, leaf number), biomass (of all organs), and physiological parameters: net photosynthesis per plant (P_n), photosynthetic nitrogen (PNUE) and phosphorus use efficiency (PPUE)). The results indicated that an increase in POD₁ promoted O₃ VFI in Quercus robur and Quercus pubescens, while Quercus ilex was asymptomatic. The POD₁-based critical level at the onset of O₃ VFI was lower for Q. robur than for Q. pubescens (12.2 vs. 15.6 mmol m^-2 POD₁). Interestingly, drought reduced O₃ VFI in Q. robur but increased it in Q. pubescens. Both O₃ and drought were detrimental to the plant biomass. However, Q. robur and Q. pubescens invested more in shoots than in roots, while Q. ilex invested more in roots, which might be related to a hormetic mechanism. P_n, PNUE and PPUE decreased in all species under drought, and only in the sensitive Q. robur (PPUE) and Q. pubescens (PNUE) under O₃. This study confirms that POD₁ is a good indicator to explain the development of O₃ VFI and helps a differential diagnosis of co-occurring drought and O₃ VFI in oak forests.