Differential sensitivity to ozone in two poplar clones. Analysis of thylakoid pigment-protein complexes

The effect of acute O-3 exposure (5 h at 150 ppb) on photosynthetic electro n transport and the thylakoid polypeptide pattern has been studied in 2-mon th-old cuttings of two poplar clones (Populus deltoides x maximowiczii clon e Eridano and P. x euramericana clone I-214), known for their differential response to O-3 in terms of visible symptoms. The more sensitive Eridano de veloped severe blackish bifacial necrosis concentrated in the interveinal a reas of the basal and intermediate leaves, while in the less sensitive I-21 4 clone, visible symptoms were slight or entirely absent. A generalized, si gnificant decrease in leaf pigment content was shown by both of the stresse d clones, although the decline in p-carotene content was much more marked i n Eridano than in I-214, Activation of the xanthophyll cycle was not detect ed in either clone. In both clones, immunoblot analysis of the thylakoid pr oteins showed a decrease in the levels of all the PSII polypeptides tested following O-3 exposure, whereas the PsaA/B protein in the PSI core was more severely affected in Eridano than in I-214, No changes in the chloroplast structure could be seen on electron microscopic study in either clone. The only visible difference was a great accumulation of starch grains in Eridan o, All of these results suggest that O-3 exposure induces alterations in th e photosynthetic apparatus in both poplar clones, Eridano being affected to a greater extent than I-214.