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.