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 (O(3)) and drought treatments: O(3) (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 O(3) dose (POD(1)) and cumulative drought index (CDI) to the O(3) 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 POD(1) promoted O(3) VFI in Quercus robur and Quercus pubescens, while Quercus ilex was asymptomatic. The POD(1)-based critical level at the onset of O(3) VFI was lower for Q. robur than for Q. pubescens (12.2 vs. 15.6 mmol m(−2) POD(1)). Interestingly, drought reduced O(3) VFI in Q. robur but increased it in Q. pubescens. Both O(3) 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 O(3). This study confirms that POD(1) is a good indicator to explain the development of O(3) VFI and helps a differential diagnosis of co-occurring drought and O(3) VFI in oak forests.