No abstract

& 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 (O 3 ) on the vegetation will remain a lasting concern during the twenty-first century, and deeper understanding of functional and structural responses to O 3 in plant foliage in a changing environment is needed. & Aims Comprehensive analysis of the O 3 injury spectrum, with a view to functional understanding of cellular processes in response to varying O 3 doses. & Methods Characterization of macro-and microscopic symptoms in the sun crown foliage of adult trees exposed to ambient and twice ambient O 3 levels in a Free Air O 3 Enrichment (FACE) experiment using light and electron microscopy. & Results Visible injury triggered by O 3 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.

For several decades, southern California experienced the worst ozone pollution ever reported. Peak ozone concentrations have, however, declined steadily since 1980. In this study, the structural injuries underlying ozone symptoms in needles of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) collected in summer 2006 from one of the most polluted sites in the San Bernardino Mountains were investigated using serial sections examined by light and electron microscopy. Ozone-specific light-green diffuse mottling was observed in the currentyear needles, whereas older foliage showed brownish mottling similar to winter fleck injury. Especially, within the outer layers of mesophyll, many markers of oxidative stress, typical for ozone, were observed in both apoplast and symplast. Altogether within cells of mottles, these markers were indicative of hypersensitive-like response, whereas degenerative structural changes were diagnosed in the surrounding mesophyll. Evidence of drought stress and frost injury to older needles was also detected. Hence, mottling injury appeared to be primarily caused by ozone stress, however, other environmental stressors also determined the symptom morphology and distribution, especially within the older foliage. Keywords Ponderosa pine Á Los Angeles basin Á Ozone visible injury Á Microscopic diagnosis Á Needle histochemistry Á Mesophyll ultrastructure Communicated by R. Matyssek.

European deciduous oaks are closely related and are known for their strong emission of volatile isoprenoids. They are chemo-taxonomically diverse, but hybridise frequently. Four-year-old oak seedlings growing together in a model ecosystem facility under near-natural conditions were studied. The leaves were morphologically classified in the three oak species Quercus robur, Q. pubescens and Q. petraea (with four provenances each) and further investigated by a molecular-genetic approach. Q. robur was morphologically and genetically clearly different from Q. pubescens and Q. petraea, whereas Q. pubescens and Q. petraea individuals used in this study were morphologically and genetically more similar. There was a minor impact of among and within species variability on isoprene synthesis, isoprene emission and photosynthesis. Isoprene emission rates normalised to 25 °C leaf temperature ranged from 5.78 to 10.66 nmol m(-2)  s(-1) , whereas photosynthesis ranged from 12.8 to 17.6 μmol m(-2)  s(-1) . On cloudy days, among the provenances of each species, only net photosynthesis of the Q. robur provenance Hünenberg was reduced and isoprene synthase activity of the Q. pubescens provenance Promotogno increased. On sunny days, photosynthesis did not differ among the provenances. Over all provenances, gas exchange on cloudy days did not differ significantly from sunny days. In the combined data of cloudy and sunny days, no differences between the studied provenances and oak species were detected in isoprene emission and photosynthesis. Thus, isoprene emission and photosynthesis rates were remarkably stable among oak species and provenances. The results indicate that taxonomic differences in the studied oak species are not reflected in isoprene emission and photosynthesis, probably because of the high plasticity of gene expression resulting in high phenotypic flexibility.

No abstract