SULPHUR DIOXIDE: ITS EFFECT ON PHOTO‐SYNTHETIC 14C FIXATION IN LICHENS AND SUGGESTED MECHANISMS OF PHYTOTOXICITY

S ummary The toxicity to lichens of dissolved sulphur dioxide (as measured by net photosynthetic 14 C 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.