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 SO2. This SO2‐induced K+ efflux is biphasic in response to increasing concentrations of aqueous SO2. The first phase can be extrapolated to identify the SO2 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 14C‐fixation rates. The magnitude of the SO2‐induced K+ efflux indicated that release occurred from intra‐cellular sources. The kinetics of the release process and the effects of SO2 on membrane integrity are discussed.