The pH dependence of SO₂ damage to lichens

In order to determine the influence of the hydrogen-ion concentration on the damage exerted to lichens by sulfur dioxide gas, thalli of the species Hypogymnia physodes and Xanthoria parietina were submerged in buffer solutions of pH 2 to pH 8 and subsequently exposed to SO₂ gas. Net photosynthesis was employed as a criterion of vitality.The degree of damage to the lichens after exposure to 4 mg of SO₂/m³ air for 14 hrs is dependent on the pH of the buffer solutions. Hypogymnia physodes shows the least damage at pH 7, Xanthoria parietina at pH 5 to pH 7. The degree of damage increases with increasing acidity. After exposure to SO₂ gas at pH 3 no apparent photosynthesis is achieved.The different degrees of impairment are due to the fact that the concentration of the toxic products, resulting from the reaction of SO₂ and water, is dependent on the pH value.The mere shift of the pH into the strong acid range of pH 3 and pH 2 damages the lichens with Xanthoria parietina being more seriously damaged than Hypogymnia physodes. Basicity, however, is already harmful to the lichens under test in the low range of pH 8. Here Xanthoria parietina is less affected than Hypogymnia physodes.