L’ozone est aujourd’hui considere comme le premier polluant atmospherique en termes de phytotoxicite. Bien que ses effets sur le metabolisme des arbres forestiers soient documentes, la reponse de la photorespiration et la relation entre le developpement des feuilles et leur sensibilite a l’ozone restent meconnues. Afin d’etudier ces aspects, de jeunes plants de peuplier (Populus x canescens [Aiton] Sm.) ont ete soumis en chambres de fumigation a une concentration de 120 ppb en ozone pendant 35 jours. Les arbres traites ont montre des symptomes foliaires (necrose, chlorose et abscission precoce) ainsi qu’une reduction de la croissance radiale de la tige. Ces effets ont ete accompagnes d’une diminution importante de la capacite photosynthetique (A, ?PSII) des feuilles matures, provenant de l’alteration de la capacite de fixation du CO2 par la rubisco (Vcmax et activite in vitro) et de la photochimie (chlorophylles, Fv/Fm). Dans ces feuilles, l’exposition a l’ozone a provoque une reduction importante de l’activite photorespiratoire in vivo et de l’activite glycolate oxydase, suggerant une inhibition coordonnee de la photosynthese et de la photorespiration due a la reduction de la capacite rubisco. Dans ce contexte, l’absence d’effet sur la sousunite H de la glycine decarboxylase et la stimulation de l’activite glutamine synthetase, deux enzymes pourtant liees a la photorespiration, suggerent une modification de leur role en reponse a l’ozone. Par ailleurs, une stimulation de la respiration mitochondriale et de l’activite phosphenolpyruvate carboxylase est induite par l’ozone et pourrait repondre a une demande accrue en pouvoir reducteur due a la mobilisation du systeme antioxydant. Le fait qu’aucun des parametres etudies n’ait ete modifie par l’ozone dans les feuilles en phase precoce d’expansion indique que la sensibilite a l’ozone apparait au cours du developpement foliaire, vraisemblablement au moment de la transition puits/source. / Ozone is considered the most important phytotoxic air pollutant and has well known effects on the metabolism of forest trees. However, the role of leaf development in ozone sensitivity remains unclear. In addition, the photorespiratory pathway has been scarcely investigated under ozone stress. To provide new insights into these issues, young poplar trees (Populus x canescens [Aiton] Sm.) were subjected to 120 ppb of ozone for 35 days in phytotronic chambers. Treated trees displayed foliar symptoms of injury (necrotic spots, chlorosis and precocious leaf senescence) and a reduced stem radial growth. In mature leaves, ozone reduced photosynthesis (A, ?PSII) by impairing CO2 fixation, mainly at the level of rubisco (Vcmax and in vitro activity). Photochemistry was also affected as shown by reduced chlorophyll content and Fv/Fm ratio. The rate of in vivo photorespiration and glycolate oxidase activity were reduced along with photosynthetic parameters, suggesting that photosynthesis and photorespiration were both inhibited by the ozone-induced impairment of rubisco. The amount of the H subunit of the glycine decarboxylase and total glutamine synthetase activity were respectively maintained and stimulated. As both enzymes are related to photorespiration, a potential shift in their metabolic involvement could have occurred under ozone stress. In addition, treated leaves showed a stimulation of mitochondrial respiration and phosphoenolpyruvate carboxylase activity which could be related to a higher demand for reducing power to sustain detoxification processes. As none of the parameters were modified by ozone in expanding leaves, ozone sensitivity may appear during leaf development, possibly when developing leaves undergo sink/source transition.