Ozone symptoms in leaves of woody plants in open top chambers: ultrastructural and physiological characteristics

The 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 (FE), Prunus avium (PA) and Viburnum lantana (VL) seedlings grown in charcoal filtered (CF) (~50% of the ambient O3) and non-filtered (NF) air open-top chambers (~ 92% of the ambient O3). 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 FE symptoms consisted of punctiform stipples with necrotic cells (hypersensitive response, HR), whereas in PA and VL reddening developed, revealing the accumulation of anthocyanins. In VL, symptoms appeared earlier than in the other species; in FE, symptoms developed more rapidly and led to premature leaf abscission. In FE, at least at the beginning, the onset of symptoms was combined with an enhanced photosynthetic efficiency (compensation mechanism), while in PA and VL this efficiency progressively decreased. Fluorescence parameters which are more closely connected to ozone stress were: reduction of performance index (PIABS) and of the active reaction centres (RC/CSo); increase in variable fluorescence relative to 30 ms (VI) and of the dissipation processes. Dissipation is a form of a defence mechanism against oxidative stress and is related to the role of the deactivated reaction centres (the silent centres) as well as to the anthocyanins. Symptom development correlates in all three species with the reduction of reaction centres. Symptomatic leaves had a lower net photosynthesis rate (Pn). Net photosynthesis correlates with the reduction of 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.