Key message The diversity of structural injury underlying visible symptoms by ozone stress resulted from the succession of degenerative processes and programmed-cell death events, depending on the ozone uptake and varying on a year-toyear basis.
Context The effects of tropospheric ozone (O₃) on the vegetation will remain a lasting concern during the twenty-first century, and deeper understanding of functional and structural responses to O₃ in plant foliage in a changing environment is needed.
Aims Comprehensive analysis of the O₃ injury spectrum, with a view to functional understanding of cellular processes in response to varying O₃ doses.
Methods Characterization of macro- and microscopic symptoms in the sun crown foliage of adult trees exposed to ambient and twice ambient O₃ levels in a Free Air O₃ Enrichment (FACE) experiment using light and electron microscopy.
Results Visible injury triggered by O₃ resulted from (i) degenerative processes of varying severity (photobleaching, accelerated  cell senescence, ACS), (ii) programmed cell death with disruption of cell content (hypersensitive reaction–like, HR-like) and occasional leakage of cellular debris into the apoplast, (iii) overlapping degenerative and disruptive processes, primarily in the upper mesophyll and within organelles prone to oxidative stress (chloroplasts and mitochondria) and (iv) necrosis in lower mesophyll with leakage of cellular debris in the intracellular space.
Conclusion Especially the degenerative and disruptive traits showed contrasting structural features. In the case of stippling symptoms, the structural variability was particularly high, as a consequence of interactions between early degenerative and late disruptive processes. These findings thus confirmed the close dependency of processes—and a further spectrum of ozone injury—on rates of ozone uptake. Such relationships and development of injury, as observed in the case of beech (Fagus sylvatica L.) foliage, are expected to be basically similar in other broadleaved tree species.