The goal of the present study was to examine the effects of slow and rapid changes of ozone (O3) concentrations on the physiological behaviour of current-year needles of Norway spruce (Picea abies (L.) Karst.). For this purpose five-year-old spruce seedlings were exposed in growth chambers for 49 days to either charcoal-filtered air, slowly increasing O3 concentrations from zero up to 100 nl l-Ü in weekly steps of 25 nl l-Ü, or immediately to 100 nl l-Ü of O3. During the investigation period gas exchange, carbohydrate and antioxidant contents of the current flush were measurd. In needles which experienced slowly increasing O3 concentrations, cumulative O3 uptake was approximately 30% lower than in needles continuously fumigated with 100 nl l-Ü of O3. The higher O3 uptake in the permanent 100 nl l-Ü O3 treatment caused a pronounced decline in net photosynthesis, in the efficiency of CO2 uptake and in the starch content of the seedlings. Initially the ascorbate pool increased, but after 5 weeks of exposure ascorbate concentrations declined and were comparable to values obtained in charcoal-filtered controls, while the thiol contens were enhanced during fumigation with permanent 100 nl l-Ü O3. On the contrary, slowly increasing O3 caused a significant increase in total needle ascorbate throughout the fumigation period, which probably prevented an O3-induced decline in the photosynthesis was not affected although the thiol contents were not enhanced. Furthermore, starch content was slightly higher than in O3-free controls. These results suggest that seedlings of Norway spruce have the possibility to acclimate to O3 stress, as slowly increasing O3 concentrations seemed to increase resistance and the seedlings were able to compensate.
