DIFFERENCES IN GROWTH, LEAF SENESCENCE AND INJURY, AND STOMATAL DENSITY IN BIRCH (BETULA-PENDULA ROTH) IN RELATION TO AMBIENT LEVELS OF OZONE IN FINLAND

The differences in growth, leaf senescence, visible ozone injuries and stomatal density between one coastal site (natural ozone) and two inl and sires (natural and elevated ozone) in Finland were deter mined for saplings of Betula pendula clones grown under open-field conditions d uring two glowing seasons. Responses in growth, leaf senescence, visib le injuries, and stomatal density were determined in relation to cumul ative ozone exposure accumulated over the thresholds of 30, 40 and 50 ppb (10(9)) during the exposure period. In addition, the effects of th e different ozone exposures on ultrastructure of chloroplasts were stu died. Increasing ozone exposure resulted in reduced shoot dry weight, stimulated (first year) or reduced (second year) height growth, accele rated autumn yellowing of leaves, increased stomatal density, visible symptoms and chloroplast injuries, and increased number and size of pl astoglobuli. Newly expanded mature leaves in midsummer were more sensi tive to ozone episodes than younger developing leaves in the early gro wing season. In most parameters, the best correlation was achieved wit h the exposure index AOT30. Ozone risk for birch is highest in the sou thern coastal area of Finland, where background ozone concentrations a re higher than in inland sites. (C) 1997 Elsevier Science Ltd.