EFFECTS OF INCREASED UV-B RADIATION AND ELEVATED LEVELS OF TROPOSPHERIC OZONE ON PHYSIOLOGICAL PROCESSES IN EUROPEAN BEECH (FAGUS-SYLVATICA)

As a consequence of the ongoing reduction of the stratospheric ozone l ayer, the vegetation is exposed to increasing levels of UV-B radiation (280-320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study , 5-year-old European beech trees were exposed from 1 July to October 1993 to two levels of UV-B radiation and two levels of ozone, alone an d in combination, in open-top chambers equipped with lamps. The simula ted UV-B levels corresponded to either clear sky ambient level or a 14 % decrease in the stratospheric ozone column over eastern Denmark, res ulting in a 23% difference in biologically effective UV-B (UV-B-BE) ir radiance. The maximum UV-B-BE given was 8.61 kJ m(-2) day(-1). The ozo ne levels were either the ambient (average 32 nl l(-1)) or ambient wit h ozone addition (average resulting concentration 71 nl l(-1)). Compar ed to the control treatment (ambient UV-B, ambient O-3) the elevated l evels of UV-B and O-3 affected the trees negatively, expressed as decl ines in net photosynthesis (P-n), stomatal conductance (g(s)), chlorop hyll fluorescence (F-v/F-m) and acceleration of senescence, measured a s yellowing of the leaves. The UV-B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in P- n and F-v/F-m occurred in the order: control < UV-B < O-3 < UV-B and O -3. Compared to the control, the combination treatment with high level s accelerated the visual senescence processes by ca 27 days, while for high UV-B and O-3 alone, there was an acceleration by 14 and 21 days, respectively. UV-B and O-3 in combination enhanced the negative effec ts compared with UV-B and O-3 alone. The P-n and F-v/F-m results could be related to this acceleration process. The chamber effect was inves tigated by comparing the control plots with a plot without open-top ch amber. The trees in the chambers showed a higher P-n and F-v/F-m and a 14-day delayed senescence compared to the trees outside, probably cau sed by higher temperatures, a more protected environment and altered c onditions inside the chambers.