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Despite the huge biodiversity characterizing the Mediterranean environment, environmental constraints, such as high sunlight and high temperatures alongside with dry periods, make plant survival hard. In addition, high irradiance leads to increasing ozone (O3 ) concentrations in ambient air. In this era of global warming, it is necessary to understand the mechanisms that allow native species to tolerate these environmental constraints and how such mechanisms interact. Three Mediterranean oak species (Quercus ilex, Quercus pubescens and Quercus cerris) with different features (drought tolerant, evergreen or deciduous species) were selected to assess their biometrical, physiological and biochemical responses under drought and/or O3 stress (80-100 nl l(-1) of O3 for 5 h day(-1) for 77 consecutive days). Leaf visible injury appeared only under drought stress (alone or combined with O3 ) in all three species. Drought × O3 induced strong reductions in leaf dry weight in Q. pubescens and Q. cerris (-70 and -75%, respectively). Alterations in physiological (i.e. decrease in maximum carboxylation rate) and biochemical parameters (i.e. increase in proline content and build-up of malondialdehyde by-products) occurred in all the three species, although drought represented the major determinant. Quercus ilex and Q. pubescens, which co-occur in dry environments, were more tolerant to drought and drought × O3 . Quercus ilex was the species in which oxidative stress occurred only when drought was applied with O3 . High plasticity at a biochemical level (i.e. proline content) and evergreen habitus are likely on the basis of the higher tolerance of Q. ilex.

The phytotoxiticky of ozone is due to its high oxidant capacity and to its ability to generate toxic molecular species. It is well known that intracellular peroxidases play an important role in eliminating toxic forms of oxygen but little evidence has been reported on the role of peroxidases in the apoplastic compartment. The detoxification systems located in the foliar extracellular matrix and in the intracellular fluid of sensitive pumpkin plants (Cucurbita pepo L. cv. Ambassador) exposed to ozone (150 ppb. 5 days. 5 h day‐1) in a fumigation chamber, were analyzed. The analyses were carried out on both young and mature leaves. Ascorbate peroxidase (EC 1.11.1.11) was found in the extracellular matrix of the pumpkin tissues. Its activity increased in both young and mature leaves as a consequence of the treatment, while at intraeellular levels its effect was most prominent in mature leaves. Analysis of the ascorbie‐dehydroascorbic acid system revealed an enhancement of the pool content in the extracellular matrix of both kinds of leaves as a consequence of fumigation, while at the intracelluiar level small variations were found. Very little variation was observed in the glutathione pool as a consequence of fumigation. The analysis of a lipid peroxidation marker, malondi‐aldehyde. showed the significant effect of ozone on membrane lipids. Following fumigation, the free phenols in the extracellular matrix decreased in both young and mature leaves, while the free and glycoside‐bound phenols of the intracellular fluid showed little increase. The results support the hypothesis that ozone stimulates the an‐tioxidant systems mainly in the apoplast and that ascorbic peroxidase activity, ascorbic acid levels and cell wall stiffening are the most influenced parameters.

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