Antioxidative responses of oak species under ozone and water stress conditions

The Mediterranean basin is considered a global biodiversity hotspot but Mediterranean plants are threatened by natural and anthropogenic factors (e.g., tropospheric ozone, O3) which are expected to be harsher in the near future. The objective of this work was to give a thorough description of the detoxification mechanisms at the basis of the high plasticity of Mediterranean oak species. Two-year-old seedlings of Quercus ilex, Q. pubescens and Q. robur were grown under the combination of three levels of O3 (1.0, 1.2 and 1.4 times the ambient O3 concentration; AA, 1.2×AA and 1.4×AA) and water irrigation [1.2, 0.6 and 0.12 l day-1; well-watered (WW), moderate drought (MD) and severe drought (SD)] from June to October 2015. In Q. ilex, no symptoms due to O3 and/or drought were observed. This was confirmed by the minor propagation of reactive oxygen species (ROS) and the subsequent reduced cellular oxidative burst. Q. pubescens exposed to 1.4×AA and subjected to SD developed O3 injuries, consisting in minute roundish browning necrosis scattered among the leaf veins of both surfaces of mature leaves. An induction of the phenylpropanoid biosynthetic pathway by both stressors leaded to an increase of phenolic acids (+61% in comparison to AA). This enhanced antiradical ability of cell wall components partially balanced the ROS production but did not prevent foliar damage. In Q. robur, O3 symptoms were induced by both 1.2×AA and 1.4×AA concentrations under WW or MD conditions. Increasing O3 levels induced a significant rise of total flavonoids (+43 and +16% in 1.2×AA and1.4×AA). This species also displayed higher constitutive level of ascorbate. These mechanisms did not preserve the PSII photochemistry from impairment, as confirmed by the decrease of total chlorophylls under O3. A different species-specific degree of tolerance to single and combined stress was observed