Cuttings of birch (Betula pendula Roth), poplar (Populus Xeuramericana (Dode) Guinier cv. Dorskamp), and alder (Alnus glutinosa (L.) Gaertn.) were exposed in the open field to ambient ozone (O₃), in both full sunlight and shade conditions, and in field fumigation chambers to filtered air (FA) or FA plus added O₃ (75 nL.L-1) from 07:00 to 19:00, 19:00 to 07:00, or for 24 h. Appearance of O₃-induced leaf symptoms was related to changes at the cellular level, especially in the cell wall. Changes were analyzed by light, fluorescence, transmission electron, and low-temperature scanning electron microscopy. Visible leaf symptoms appeared after 23-39 days of fumigation in chambers at all three elevated O₃ regimes. Shaded birch showed increased sensitivity to ambient O₃ compared with birch grown in sunlight. In the presence of visible O₃ injury, mesophyll cell walls of birch and poplar leaves were thickened, and droplet-like exudates (projections) formed on cell walls adjacent to intercellular air spaces and increased as symptoms worsened. The main exudate constituents were esterified pectins (calcium pectate), a small amount of protein (more than in the cell wall proper), and callose (in poplar leaves only). These exudates indicate that O₃ has caused a slow intercellular oxidative process to occur at the cell walls.