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Cumulative ozone (O 3 ) uptake and O 3  flux were related to physiological, morphological and biochemical characteristics of Norway spruce ( Picea abies  (L.) Karst.) trees of different ages. Under ambient CO 2  conditions, photosynthetic capacity ( A  max ) declined in mature trees when cumulative O 3  uptake into needles, which provides a measure of effective O 3  dose, exceeded 21 mmol m −2  of total needle surface area. A comparable decline in  A  max  of seedlings occurred when cumulative O 3  uptake was only 4.5 mmol m −2 . The threshold O 3  flux causing a significant decline in  A  max  ranged between 2.14 and 2.45 nmol m −2  s −1  in mature trees and seedlings subjected to exposure periods of ≥ 70 and ≥ 23 days, respectively. The greater O 3  sensitivity of young trees compared with mature trees was associated with needle morphology. Biomass of a 100-needle sample increased significantly with tree age, whereas a negative correlation was found for specific leaf area, these changes parallel those observed during differentiation from shade-type to sun-type needles with tree ontogeny. Age-dependent changes in leaf morphology were related to changes in detoxification capacity, with area-based concentrations of ascorbate increasing during tree ontogeny. These findings indicate that the extent of O 3 -induced injury is related to the ratio of potentially available antioxidants to O 3  influx. Because this ratio, when calculated for ascorbate, increased with tree age, we conclude that the ratio may serve as an empirical basis for characterizing age-related differences in tree responses to O 3 .

Age effects on Norway spruce ( Picea abies ) susceptibility to ozone uptake: a novel approach relating stress avoidance to defense