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 eastern Denmark, resulting in a 23% difference in biologically effective UV‐B (UV‐BBE) irradiance. The maximum UV‐Bbe given was 8.61 kJ m−2 day−1. The ozone levels were either the ambient (average 32 nl l−1) or ambient with ozone addition (average resulting concentration 71 nl l−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/Fm) 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 Fv/Fm occurred in the order: control

Seedlings of 12 provenances of European beech (Fagus sylvatica) were exposed to ambient, non-filtered air (NF) or NFj50 nl l −" ozone (NF50) for 8 h d −" in open-top chambers (OTCs), from 1 June to 4 October 1995. In 1996 exposure was continued from 31 May to 1 October at four levels : charcoal-filtered air (CF), NF, NF50 and NFj100 nl l −" ozone (NF100). Provenances were grown for both seasons in outside reference plots. All treatments were replicated. Ozone did not affect gas exchange in the provenances until late in the second season. NF100 reduced photosynthesis by 18% in August 1996 compared to CF. In September, photosynthesis was reduced by 22% in NF50 and by 29% in NF100. After two seasons, ozone reduced the root : shoot ratio by 24% when comparing CF and NF100 ; this was caused by reductions in the root biomass. Ozone did not affect height growth or stem diameter, and there were no ozoneiprovenance interactions for any growth parameter. There was, however, a significant ozoneiprovenance interaction for photosynthesis, showing northwest European provenances to be more sensitive to ozone than southeast European provenances when comparing dose-response estimates. This is interpreted in terms of genetic adaptation of the photosynthetic apparatus to regional growing conditions. Seedlings in the chambers grew 45% taller, and had 28% more shoot biomass and 29% smaller root biomass, resulting in a 44% reduction of root : shoot ratios compared to seedlings outside. Increased temperature and decreased PAR inside the chambers relative to the outside were probably the main causes for the differences. The magnitude of the chamber effects in OTCs raises doubts about conclusions drawn from ozone exposures in such chambers. This and previous ozone experiments with OTCs may have reached inaccurate conclusions concerning the size of ozone responses due to chamber effects.

Seedlings of four deciduous tree species maple (Acer pseudoplatanus), beech (Fagus sylvatica), horse chestnut (Aesculus hippocastanum) and lime (Tilia cordata) were exposed to de-icing salt (NaCl) either through the soil or applied to the above ground plant parts. A soil solution of 1.65 g l-1 NaCl was maintained from the start of the experiment in January 1999 until termination in June 1999. The main effects caused by salt treatment through the soil were a reduction in photosynthesis of up to 50% and the development of leaf chlorosis or necrosis covering up to 50% of the total leaf area for the most sensitive species (lime and beech); maple and horse chestnut were relatively tolerant. There was no significant correlation between Cl or Na concentration in leaves and the relative sensitivity of the species. Saturated salt solution was applied to bark, buds or leaf scars on two occasions three weeks apart during the winter season. This affected the timing of bud break with delays of up to eight days compared with the controls. In the most sensitive species the above ground salt treatments partly prevented bud break (beech) or reduced photosynthesis (lime). Uptake through the bark was most important for the development of stress effects, compared with uptake through the other above ground plant parts.

We tested a hypothesis that elevated ozone was an eliciting or contributing factor in outbreaks of the ' top dying ' (or ' subtop dying ') syndrome in Norway spruce (Picea abies). Progeny were used from open-pollinated trees within a stand with the ' top dying ' syndrome. The mother trees were classified in relation to the expression of the ' top dying ' syndrome, and progeny from the healthiest and least healthy thirds of the population were exposed to high and low concentrations of ozone for three seasons. Elevated ozone did not affect height growth of the trees. It did not measurably affect net photosynthesis, stomatal conductance or instantaneous water use efficiency. Chlorophyll and carotenoid contents were also not significantly affected by ozone concentration. In the first year, instantaneous water use efficiency was lower in the progeny of the unhealthy mother trees than in the healthy mother trees. Furthermore, the unhealthy mother trees tended to produce longer annual shoots and showed more winter damage at the end of the experiment. None of these parameters were related to ozone concentration in the atmosphere. These results do not support a hypothesis that elevated ozone is a significant contributory factor or an eliciting factor in the development of the syndrome.

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.