Stress markers and physiochemical responses of the Mediterranean shrub Phillyrea angustifolia under current and future drought and ozone scenarios

Mediterranean plants are particularly threatened by the exacerbation of prolonged periods of summer drought and increasing concentrations of ground-level ozone (O3). The aims of the present study were to (i) test if selected markers (i.e., reactive oxygen species, ROS; malondialdehyde, MDA; photosynthetic pigments) are able to discriminate the oxidative pressure due to single and combined stress conditions, and (ii) elucidate the physiochemical adjustments adopted by Phillyrea angustifolia (evergreen woody species representative of the maquis, also known as narrow-leaved mock privet) to perceive and counter to drought and/or O3. Plants were grown from May to October under the combination of two levels of water irrigation [i.e., well-watered (WW) and water-stressed (WS)] and three levels of O3 [i.e., 1.0, 1.5 and 2.0 times the ambient air concentrations, i.e. AA (current O3 scenario), 1.5 × AA and 2.0 × AA (future O3 scenarios), respectively], using a new-generation O3 Free Air Controlled Exposure (FACE) system. Overall, this species appeared relatively sensitive to drought (e.g., net CO2 assimilation rate and stomatal conductance significantly decreased, as well as total chlorophyll and carotenoid contents), and tolerant to O3 (e.g., as confirmed by the absence of visible foliar injury, the unchanged values of total carotenoids, and the detrimental effects on stomatal conductance, total chlorophylls and terpene emission only under elevated O3 concentrations). The combination of both stressors led to harsher oxidative stress. Only when evaluated together (i.e., combining the information provided by the analysis of each stress marker), ROS, MDA and photosynthetic pigments, were suitable stress markers to discriminate the differential oxidative stress induced by drought and increasing O3 concentrations applied singly or in combination: (i) all these stress markers were affected under drought per se; (ii) hydrogen peroxide (H2O2) and MDA increased under O3 per se, following the gradient of O3 concentrations (H2O2: about 2- and 4-fold higher; MDA: +22 and + 91%; in 1.5 × AA_WW and 2.0 × AA_WW, respectively); (iii) joining together the ROS it was possible to report harsher effects under 2.0 × AA_WS and 1.5 × AA_WS (both anion superoxide and H2O2 increased) than under 2.0 × AA_WW (only H2O2 increased); and (iv) MDA showed harsher effects under 2.0 × AA_WS than under 1.5 × AA_WS (increased by 49 and 18%, respectively). Plants activated physiological and biochemical adjustments in order to partially avoid (e.g., stomatal closure) and tolerate (e.g., increased terpene emission) the effects of drought when combined with increasing O3 concentrations, suggesting that the water use strategy (isohydric) and the sclerophyllous habit can further increase the plant tolerance to environmental constraints in the Mediterranean area.