Ozone-induced cytochemical and ultrastructural changes in leaf mesophyll cell walls

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.