Response of Quercus cerris to combined ozone and drought stress

Urban plants can help to improve air quality, and consequently human health. Due to the negative future prospects in 2050 for the urban environment caused by global climatic change [such as elevated ozone (O3) concentration and drought]. There is a need to monitor and proactively manage urban forests, adjusting them to change and using them to help cities to adapt to change. Special attention must be paid to the species that populate environments characterized by above-optimal temperatures in summer, such as the Mediterranean ones. Under field conditions, the study of the response to environmental stresses is difficult because environmental factors can be synergistically or antagonistically modified. For these reasons, the aim of this work is the analysis of the impact of a combined stress (O3 and drought) to simulate under controlled conditions the impact of a 2050 environmental scenario on the physiological performance of the Mediterranean Quercus cerris. Profiles related to leaf gas exchange, chlorophyll a fluorescence and leaf water potential were analyzed in 2-years old Q. cerris saplings exposed to O3 (80-100 ppb, 4 h day-1 for 12 consecutive weeks), to drought (daily irrigated with 30% of effective evapotranspiration) and to O3 combined with drought in order to improve the response to oxidative stress. At the end of the exposure, plants do not exhibit any foliar symptoms. Photosynthetic processes, however, were significantly changed. Since the first week of treatment, oxidative stress induced decrease in net photosynthesis (A), above all in drought-stressed plants (about 2-fold), in comparison to controls. This trend unvaried until the end of the exposure due to a reduction of the stomatal conductance (gs). gs was lower in drought and combined-stressed plants during the whole exposure period (until a minimum about 5-fold after 5 weeks when compared to controls). In O3-treated plants, intermediate A [8 vs 3-11 (other three thesis’ range) μmol CO2 m-2 s-1] and gs [0.10 vs 0.07-0.22 (other three thesis’ range) mol H2O m-2 s-1, after 4 weeks] values were observed in comparison to the other three thesis explained by a gas avoidance mechanism activated by a stomata regulation causing a medium decrease of the net photosynthesis. Combined stresstreated plants did not show significant changes in comparison to drought stressed individuals in terms of photosynthetic status. Although intercellular CO2 concentration (Ci) significantly changed after every treatment. Although the variable and maximal fluorescence ratio (Fv/Fm) did not show significant change during the exposure, all the plants showed values inside the optimal range. A regulatory adjustment of photosynthetic processes was highlighted during the exposure by the higher values of no-photochemical quenching (qNP) of all plants in comparison to controls and therefore suggests a tendency to increase the efficiency of thermal energy dissipation within PSII. Measurements of predawn leaf water potential revealed a leaf drought stress (around -2.0 vs -0.5 MPa in controls) in plants grown under water deficit conditions especially in combined stress plants suggesting a synergistic effect on the hydric status. On this basis drought stress (single and combined) should be considered more harmful to Q. cerris than O3.