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 (O-3), in both full sunlight and shade conditions, and in field fumigation chambers to fil tered air (FA) or FA plus added O-3 (75 nL.L-1) from 07:00 to 19:00, 1 9:00 to 07:00, or for 24 h. Appearance of O-3-induced leaf symptoms wa s related to changes at the cellular level, especially in the cell wal l. Changes were analyzed by light, fluorescence, transmission electron , and low-temperature scanning electron microscopy. Visible leaf sympt oms appeared after 23-39 days of fumigation in chambers at all three e levated O-3 regimes. Shaded birch showed increased sensitivity to ambi ent O-3 compared with birch grown in sunlight. In the presence of visi ble O-3 injury, mesophyll cell walls of birch and poplar leaves were t hickened, and droplet-like exudates (projections) formed on cell walls adjacent to intercellular air spaces and increased as symptoms worsen ed. The main exudate constituents were esterified pectins (calcium pec tate), a small amount of protein (more than in the cell wall proper), and callose (in poplar leaves only). These exudates indicate that O-3 has caused a slow intercellular oxidative process to occur at the cell walls.