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Sclerophylly and synthesis of phenolic compounds are active responses of plants subjected to environmental stress (drought, low nutrient supply, u.v.-B radiation, ozone). Here we describe the morphological and histochemical alterations occurring in field-grown leaves of Fagus sylvatica L. from three sites located along an ecological gradient : from a site in cool and protected conditions to one located on a mountain ridge, where the trees grow on a thin layer of soil and are exposed to the wind and to intense solar radiation in summer. The morphological data show that, as the ecological conditions of the stand worsen, individual leaf surface decreases, while the thickness of the leaves and their specific d. wt (i.e. d. wt per unit leaf area) increases. Histochemical and ultrastructural tests show a marked increase of phenolics during the course of the year. These substances, present primarily in the leaves of trees growing in stress conditions, have been identified mainly as tannins. They accumulate in the vacuoles, especially those of the upper epidermal layer and the palisade mesophyll ; at a later stage they appear to be solubilized in the cytoplasm and retranslocated, eventually impregnating the outer wall of the epidermal cells amidst the cellulose fibrils, where they cluster together and form an electron-opaque layer between the wall and the cuticle. Observation of the epidermal cells also reveals that the outer cell wall is thicker. The paper discusses the roles of secondary metabolites in protection and detoxification processes ; the possible ecological significance of these alterations in the ecophysiology of beech trees.

This study was carried out in the summer of 2001 at the Lattecaldo open‐top chamber research facility (Canton Ticino, southern Switzerland). The aim of this research study was to examine the behaviour of Fraxinus excelsior, Prunus avium and Viburnum lantana seedlings grown in charcoal‐filtered (CF) (∼ 50% of the ambient O3) and non‐filtered (NF) (∼ 92% of the ambient O3) air open‐top chambers. Investigations included the assessment of visible foliar symptom development, anatomical and ultrastructural analysis of symptoms, measurements of direct chlorophyll a fluorescence (fluorescence findings were processed by means of the JIP test) and leaf gas exchange measurements. The three species displayed different foliar symptoms. In F. excelsior, symptoms consisted of punctiform stipples with necrotic cells (hypersensitive response, HR), whereas, in P. avium and V. lantana, reddening developed, revealing the accumulation of anthocyanins. In V. lantana, symptoms appeared earlier than in the other species; in F. excelsior, symptoms developed more rapidly and led to premature leaf abscission. In F. excelsior, at least at the beginning, the onset of symptoms was combined with an enhanced photosynthetic efficiency (compensation mechanism), whereas, in P. avium and V. lantana, this efficiency progressively decreased. The fluorescence parameters most closely connected to ozone stress were as follows: a reduction in performance index (PIABS) and active reaction centres (RC/CS0), and an increase in the variable fluorescence relative to 30 ms (VI) and of the dissipation processes. Dissipation is a form of defence mechanism against oxidative stress and is related to the role of the deactivated reaction centres (the silent centres) as well as anthocyanins. Symptom development correlated in all three species with the reduction in reaction centres. Symptomatic leaves had a lower net photosynthetic rate (Pn). Net photosynthesis correlated with the reduction in VI, which suggests an accumulation of reduced plastoquinone, produced in the luminous phase of photosynthesis, which was not capable of reaching the dark phase reactions.

Summary• This paper compares the responses to ozone in five woody species: Fagus sylvatica (FS), Acer pseudoplatanus (AP), Fraxinus excelsior (FE), Viburnum lantana (VL) and Ailanthus altissima (AA). The hypothesis being tested was that the strategies that plants adopt to resist oxidative pressure are species-specific.• The study was carried out on field grown plants in an area in Northern Italy characterized by elevated levels of ozone pollution. The observations were made both at ultrastructural (using light and electronic microscopy) and physiological (using chlorophyll a transient fluorescence and microspectral fluorometry) level.• Common responses were: the hypersensitive response (i.e. the death of palisade mesophyll cells) and the formation of callose layers separating injured from healthy cells. FS and AP were capable of thickening the palisade mesophyll cell walls. This thickening process involved changes in cell wall chemical structure, evidenced by the accumulation of yellow autofluorescence compounds. Species-specific behaviours were observed with the fluorescence analysis, with special reference to the photochemical de-excitation constant (Kp). This value increased in FE and AP, and decreased in AA.• The observed responses are interpreted as adaptative strategies against the ozone stress. The increase of Kp indicates that the reaction centres were working as more effective quenchers.

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