Summary The toxicity to lichens of dissolved sulphur dioxide (as measured by net photosynthetic 14C fixation) was studied as a function of pH and is linked with the oxidation‐reduction properties of sulphur dioxide. An examination of pigments from lichens and spinach exposed to aqueous sulphur dioxide in vivo and in vitro suggests that the increased toxicity observed at low pH values is associated in part with the destruction of chlorophyll by an irreversible oxidation process.

SUMMARY Photosynthetic 14C fixation by the lichen Umbilicaria muhlenbergii was reduced within 15 min of exposure to aqueous sulphur dioxide (75 ppm, pH 3.0), but no changes were observed in the spectra of extracted chlorophyll pigments. Exposed lichen samples maintained under moist, sulphur‐dioxide‐free conditions in the light for 24 h were able to fix 14C at pre‐exposure levels. Partial recovery was observed when samples were kept under moist dark conditions but little when the lichens were maintained dry in the light or dark. When samples were exposed repeatedly to sulphur dioxide, but with a recovery period between each exposure, they gradually lost the ability to regain their photosynthetic activity. After the third cycle of exposure and recovery, little 14C fixation was observed while changes in the spectra of extracted pigments was evident. Increased potassium levels were found in the medium during exposure. These levels reverted to control values after a suitable recovery period. In vitro studies showed that 10‐20 ppm sulphur dioxide was found to deactivate both staphylococcal nuclease and lysozyme. Possible explanations as to the mode of SO2 toxicity on lichen metabolism and suggestions as to recovery mechanisms are fully discussed.