SO2-induced decrease in photosynthetic activity in two barley cultivars. Evidence against specific damage at the protein-pigment complex level

This research investigated the SO2-induced effects on photosynthetic apparatus in two barley (Hordeum vulgare L.) cultivars (cv), Panda and Express. Following a chronic treatment with SO2 (80 ppb, 75 d) neither cv showed any visible signs of injury or chlorosis on leaf surfaces, while a significant reduction in Amax and Gw was detected in both cvs, although to different extents. Thylakoids of SO2-treated plants showed a decrease in the electron transport activity at the whole chain, photosystem II (PSII) and photosystem I (PSI) level in both cvs. The high performance liquid chromatography (HPLC) analysis of leaf pigments revealed a significant decrease in both cvs, more pronounced in Panda than in Express. Deriphat-polyacrilamide gel electrophoresis (Deriphat-PAGE) and two dimensional (2-D) electrophoretic analyses of the pigment-protein complexes did not show differences in SO2-treated samples of either cv. HPLC analysis of the green bands also showed no differences in the pigment content of fumigated samples of either cv, except for a decrease in β-carotene content and xanthophyll cycle pigment (VAZ) content respectively at band 1 (PSI) and band 5 (minor light-harvesting polypeptides of PSII) level in cv Panda, where the de-epoxidation index (DEP) significantly increased, while in Express, an increase in VAZ content and DEP value of band 5 was observed. These results suggest that the decrease in the photosynthetic activity can be ascribed, in addition to stomata closure induced by SO2, to a generalised, rather than specific, pollutant effect on photosynthetic apparatus, which could be interpreted as an adaptation to the adverse environment.