Effects of increased UV-B radiation and elevated levels of tropospheric ozone on physiological processes in European beech (Fagus sylvadca)

Abstract

Introduction Zeuthen, J., Mikkelsen, T N., Paludan-Miiller, G. and Ro-Poulsen, H. 1997. Effects of increased UV-B radiation and elevated levels of tropospheric ozone on physiological processes in European beech (Eagus sylvatica). -Physiol. Plant. 100: 281-290.As a consequence of the ongoing reduction of the stratospheric ozone layer, 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 and in combination, in open-top chambers equipped with lamps. The simulated UV-B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastem Denmark, resulting in a 23% difference in biologically effective UV-B (UV-BBE) irradiance. The maximum UV-BBE given was 8.61 kJ m"^ day"'. The ozone levels were either the ambient (average 32 nl 1"' ) or ambient with ozone addition (average resulting concentration 71 nl 1"'). Compared to the control treatment (ambient UV-B, ambient O3) the elevated levels of UV-B and O3 affected the trees negatively, expressed as declines in net photosynthesis (Pn), stomatal conductance (gs), chlorophyll fluorescence (Fv/F^) and acceleration of senescence, measured as yellowing of the leaves. The UV-B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in Pn and F^fF^ occurred in the order: control < UV-B < O3 < UV-B and O3. Compared to the control, the combination treatment with high levels accelerated the visual senescence processes by ca 27 days, while for high UV-B and O3 alone, there was an acceleration by 14 and 21 days, respectively. UV-B and O3 in combination enhanced the negative effects compared with UV-B and O3 alone. The Pn and F^IF^ results could be related to this acceleration process. The chamber effect was investigated by comparing the control plots with a plot without open-top chamber. The trees in the chambers showed a higher Pn and Fy/Fn, and a 14-day delayed senescence compared to the trees outside, probably caused by higher temperatures, a more protected environment and altered conditions inside the chambers.