ACCELERATION OF LEAF SENESCENCE IN FAGUS-SYLVATICA L BY LOW-LEVELS OFTROPOSPHERIC OZONE DEMONSTRATED BY LEAF COLOR, CHLOROPHYLL FLUORESCENCE AND CHLOROPLAST ULTRASTRUCTURE

From April 1988 to October 1991 3-year-old seed propagated beech (Fagu s sylvatica L.) trees were exposed in open-top chambers to four differ ent levels of air pollution: (1) charcoal filtered air, (2) ambient ai r, (3) ambient air plus 30 nl 1(-1) ozone during the summer, and (4) a mbient air plus 30 nl 1(-1) ozone during the summer and 20 nl 1(-1) SO 2 and NO2 during the winter. Leaf colour was studied in the autumns of 1989 and 1991 and a close relationship between ozone dose and prematu re senescence was found. A correlation also exists between the colour groups and chlorophyll fluorescence (F-v/F-m). Ozone fumigation increa ses the size and speeds up the development of the plastoglobules. This is described using an index based on the volume of plastoglobules as a percentage of chloroplast volume. The index was significantly higher for ozone fumigated plants than for control plants during August to N ovember 1989. According to all three methods it is concluded that low levels of ozone accelerate leaf senescence processes in F: sylvatica. There are indications that leaves of the first and the second flush re act differently to the ozone treatment. Irrespective of the ozone trea tment a special cell wall structure, probably a local suberization, is confined to the subsidiary cells in leaves of the first flush.