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The objective of the present study was to investigate whether peak concentrations of ozone can deplete the apoplastic ascorbate pool of needles from Norway spruce trees (Picea abies L. Karst.) and, thereby, contribute to damage to forest trees. Twigs of forest trees grown at high altitude (1950m above sea level; Mt Patscherkofel, Austria) were enclosed in situ in chambers and fumigated for 5‐5 or 17 h with ozone concentrations ranging from 60 to 798 nmol mol−1. Adjacent branches were fumigated with filtered air. Ozone influx into the foliage ranging from 1‐7 to 17nmolm−2s−1 had little effect on whole‐needle ascorbate or glutathione contents. However, apoplastic ascorbate decreased by about 30% when the needles were exposed to environmentally relevant ozone concentrations and increased about 3‐fold at higher ozone concentrations. This response suggests the induction of ascorbate as a protective system and may also be important under field conditions. Needles of spruce trees from high altitude that were exposed to chronically increased ozone concentrations contained significantly higher apoplastic ascorbate concentrations than needles from spruce trees from lower altitudes with lower mean atmospheric ozone concentrations. The results show that peak concentrations of ozone do not act in spruce via a depletion of the apoplastic ascorbate pool.

Summary. The uptake of air pollutants depends both on pollutant concentration and on stomatal conductance. This paper deals with the uptake of ozone (03) from the air into the needles of Norway spruce [Picea abies (L.) Karst.] under ambient climatic conditions. Regulation of 03 uptake by the stomata is shown and also the difference b~-tween the "physiologically active 03 concentration" and the 03 concentration of the ambient air. Data from the sun and shade crown of spruce trees at 1000 m a. s. 1. are presented. Analysis of data from three vegetation periods has shown that at low ambient 03 concentrations the 03 uptake is largely regulated by stomatal conductance. Water vapour pressure deficit (VPD) of the atmosphere is the climatic factor which showed the highest positive correlation with 03 concentration. However, a high leaf-air VDP led to stomatal closure, thus reducing the 03 uptake in the needles despite high 03 concentrations in the ambient air. The potential 03 stress caused by high 03 concentrations can be strongly mitigated by this natural closing of the stomata and the simultaneous occurrence of moderate drought stress.

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