Effects of SO2 fumigations on photosynthetic CO2 gas exchange, chlorophyll a fluorescence emission and antioxidant enzymes in the lichens Evernia prunastri and Ramalina farinacea

The effects of elevated gaseous SO2 concentrations in the lichens Evernia prunastri (L.) Ach. and Ramalina farinacea (L.) Ach. were investigated by means of gas exchange, modulated chlorophyll fluorescence analysis and antioxidant enzyme assays. The response to SO2 of the studied species differed markedly. Net photosynthetic rates were more adversely affected in E. prunastri than in R. farinacea. In addition, processes dependent on thylakoid membrane integrity such as PSII-mediated electron flow and nonphotochemical quenching were reduced to a greater extent by exposure to SO2 in E. prunastri. Moreover, the ability to reoxidize the quinone pool was lower in this species. Finally, the activity of chloroplastidic and cytoplasmic antioxidant enzymes was decreased in E. prunastri in response to fumigations but increased in R. farinacea. The results suggest that the ability to process and deal with the SO2 once it has been absorbed must play a role in determining the sensitivity of these lichen species to this air pollutant.