POTASSIUM EFFLUX BY LICHEN THALLI FOLLOWING EXPOSURE TO AQUEOUS SULPHUR DIOXIDE*

Summary Potassium efflux is a useful experimental quantity for assessing damage to lichens exposed to SO 2 . This SO 2 ‐induced K + efflux is biphasic in response to increasing concentrations of aqueous SO 2 . The first phase can be extrapolated to identify the SO 2 concentration at which no K + release occurred above the control values. This threshold concentration was used to determine the relative sensitivities of various lichen species. Good agreement was observed between the total K + lost and the reduction in the total photosynthetic 14 C‐fixation rates. The magnitude of the SO 2 ‐induced K + efflux indicated that release occurred from intra‐cellular sources. The kinetics of the release process and the effects of SO 2 on membrane integrity are discussed.