Chronic ozone fumigation induces alterations in thylakoid functionality and composition in two poplar clones

The effects of chronic O3 exposure (60 ppb for 5 h a day for 15 d) on photosynthetic electron transport activity and thylakoid polypeptide pattern has been studied in cuttings of two poplar clones (Populus deltoides × maximowiczii clone Eridano and P. × euramericana clone I-214). At the end of the O3-exposure period, there were no visible symptoms of injury on leaf surface in either clone. In comparison with the respective controls, plants of both treated clones showed a statistically significant decrease in the activity of the two photosystems (PSI and PSII), although to differing extents. In both clones, immunoblot analysis of the thylakoid proteins indicated a reduction, following O3 treatment, in the content of all polypeptides analysed, with a more marked reduction of PSII core complex D1 protein. A generalised decrease in leaf pigment content was shown by both O3-treated clones, and the activation of the xanthophyll cycle following ozone exposure was more evident in the Eridano clone. No alterations in the chloroplast structure were seen by electron microscope analysis in either clone. The only visible difference was a marked accumulation of starch grains in the Eridano clone. All these results suggest that chronic O3 treatment induces a generalised decrease in the activity of electron transport chain components and in the photosystem protein and pigment content, which could be interpreted as an adaptation of the poplar plants to the adverse environmental conditions.