CHRONOLOGY OF CHANGES WITHIN THE PHOTOSYNTHETIC APPARATUS OF POPULUS-NIGRA UNDER OZONE STRESS

The sequence of changes within the photosynthetic apparatus of Populus nigra leaves, fumigated with ozone in open-top chambers, was investig ated. Besides two chambers with charcoal-filtered air (CF), two with n on filtered air (NF) and two with non filtered air +50 nl l(-1) ozone (NF+) were used. The first effect to be found on photosynthesis was a significant reduction in carboxylation efficiency (CE) in 10 day old N F- and NF+-leaves compared to CF-leaves. This effect was enhanced with the ageing of the leaves. On the other band photochemical capacity me asured by dark-adapted chlorophyll fluorescence (F-v/F-m) was not redu ced in NF-leaves compared to CF-leaves during the whole growing season . In NF+-leaves F-v/F-m showed a decrease in comparison to CF-leaves f rom 42 days onwards, when CE was already reduced by about 60% in the s ame leaves. In 34 and 75 day old leaves there was an increase in the r atio of the xanthophylls violaxanthin + antheraxanthin + zeaxanthin to chlorophyll a+b in leaves that were exposed to elevated ozone concent rations indicating enhanced light stress in these leaves.