The impact of ozone fumigation on chlorophyll a fluorescence parameters and chlorophyll content of birch trees grown at high and low fertilization were studied for 6-, 8-, and 12-week old leaves. Fluorescence parameters were measured with a portable fluorometer with its fibre optics tightly inserted in a gas exchange cuvette at light intensities from 0 to 220 μmol photons m-2 s-1. Ozone caused significant changes of primary photosynthetic reactions: a decrease of the quantum yield of photosystem II and an increase of non-photochemical quenching. In all leaves a biphasic light response of nonphotochemical quenching was observed. Ozone fumigation shifted the onset of the second phase from a PFD of about 60 μmol m-2 s-1 to about 30 μmol m-2 s-1. While the fertilizer concentration had no influence on this character, high fertilization supply of plants partially reduced O3-induced damage. The light responses of Ft, Fm' and NPQ observed in birch leaves grown in O3-free air indicate the existence of at least two different processes governing energy conversion of the photosynthetic apparatus at PS II in the range of PFD 0-200 μmol photons m-2 s-1. The first phase was attributed to a rather slowly relaxing type of non-photochemical quenching, which, at least at low PFD, is thought to be related to a state 1-2 transition. The further changes of the fluorescence parameters studied at higher PFD might be explained by an increase of energy-dependent quenching, connected with the energization of the thylakoid membrane and zeaxanthin synthesis. A major effect of ozone treatment was a lowering of PS II quantum yield. This reflects a reduction of PS II electron transport and corresponds to the reduction of CO2-fixation observed in ozonated leaves.The impact of ozone fumigation on chlorophyll a fluorescence parameters and chlorophyll content of birch trees grown at high and low fertilization were studied for 6-, 8-, and 12-week old leaves. Fluorescence parameters were measured with a portable fluorometer with its fibre optics tightly inserted in a gas exchange cuvette at light intensities from 0 to 220 μmol photons m-2 s-1. Ozone caused significant changes of primary photosynthetic reactions: a decrease of the quantum yield of photosystem II and an increase of non-photochemical quenching. In all leaves a biphasic light response of non-photochemical quenching was observed. Ozone fumigation shifted the onset of the second phase from a PFD of about 60 μmol m-2 S-1 to about 30 μmol m-2 s-1. While the fertilizer concentration had no influence on this character, high fertilization supply of plants partially reduced O3-induced damage. The light responses of Ft, Fm′ and NPQ observed in birch leaves grown in O3-free air indicate the existence of at least two different processes governing energy conversion of the photosynthetic apparatus at PS II in the range of PFD 0-200 μmol photons m-2 s-1. The first phase was attributed to a rather slowly relaxing type of non-photochemical quenching, which, at least at low PFD, is thought to be related to a state 1-2 transition. The further changes of the fluorescence parameters studied at higher PFD might be explained by an increase of energy-dependent quenching, connected with the energization of the thylakoid membrane and zeaxanthin synthesis. A major effect of ozone treatment was a lowering of PS II quantum yield. This reflects a reduction of PS II electron transport and corresponds to the reduction of CO2-fixation observed in ozonated leaves.We gratefully acknowledge data on chlorophyll content from Dr. W. Landolt and the technical assistance of Mrs. C. Rhiner, Mr. U. Bühlmann, Mr. P. Bleuler and Mr. A. Bur-kart in tending the plants and operating the O3 fumigation. We also thank Mrs. M.J. Sieber for correcting the English text. The study was financed through the ‘EUREKA 447 EUROSILVA’ program of the Swiss ‘Bundesamt für Bildung und Wissenschaft’ and through a joint agreement between The Swiss Federal Institute for Forest, Snow and Landscape Research and The Ural State Forestry Engineering Academy for a sabbatical of the first author in Switzerland, and INTAS (Brussels, Proj. Nr. 93–1645) for financial support of S.S. and W.B
