By the end of vegetation season in foliage of sensitive broadleaved trees, macro- and microscopic injury, as a consequence of ozone stress, results from several responses at cellular level. The symptom display depends on ozone uptake and environmental conditions, which spatial and temporal variability causes important variation in symptom morphology and severity.
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-to-year 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.
Ozone injury, Cell degeneration, Cell content disruption, Chloroplast degeneration, HR-like response, Accelerated cell senescence, Microscopic validation
Vollenweider, P., Günthardt-Goerg, M.S., Menard, T. et al. Annals of Forest Science (2019) 76: 71. https://doi.org/10.1007/s13595-019-0856-5
For the read-only version of the full text: https://rdcu.be/bJ0Ek
More than 1000 pictures from this study are part of the WSL database and are available from the corresponding author on reasonable request.