Water Quality and pH Effects on Umbilicaria mammulata (Ach.) Tuck.

Abstract

The influence of water quality and pH on net C02 exchange over time in the lichen Umbilicaria mammulata was investigated. The experiments were set up to allow repeated exposure of thalli, over a two- to three-month period, to solutions with a predetermined chemistry and pH. The solutions used included Hoagland's nutrient solution, tap water, melted snow, distilled water, and distilled water adjusted to achieve pH levels from 2 to 10. The lichens showed reductions in net photosynthesis when exposed to the first two of the above solutions, but rates of decline over time were basically the same for all other treatments. This suggests that pH and water quality have relatively little control over rates of gas exchange in this species. In contrast, there was a large reduction in net C02 exchange over time for all treatments, suggesting that the frequency of watering and the nature of the environment in which the plants were growing had a greater effect than did any of the treatments the plants were exposed to. These results suggest that the duration and frequency of precipitation is of far greater consequence to this plant in the field than is the pH or water quality of the rain itself. The study of lichen physiology and ecology has grown to the point at which, for both carbon and nitrogen metabolism, the role of fluctuations in supplies of light, heat and water is at least partially understood (Kershaw & MacFarlane 1980; Larson 1980; Pearson & Benson 1977). We have equally a growing appreciation of the importance of morphology and anatomy in controlling the interaction between the organism and its environment (Larson & Kershaw 1976; Larson 1979a, 1981). Despite these advances, a number of aspects of the biology of lichens remain totally unknown. For the most part, these areas have not been dealt with because their importance to a general understanding of lichen biology has not been made clear. For example, little interest has been expressed in studying the response of either field or laboratory populations to variations in the quality of the aqueous solutions to which the plants are exposed. While Brodo (1973) quotes numerous field studies that purport to demonstrate a mineral and pH response in lichens, few of these studies actually involve the measurement of the water quality or pH of the aqueous solution in which the plants are bathed. Often water quality and pH are inferred from subjectively based field notes alone. In the laboratory, the situation is somewhat similar. Despite a general acceptance that lichens are sensitive to pH and dissolved minerals, there is surprisingly little concern about the nature of the water used to hydrate lichens in experimental situations. For example, MacFarlane and Kershaw (1977) hydrated three Peltigera species with distilled water while Dibben (1971) used both tap water and de-ionized water at pH 5.9-7.4. Le- chowicz (1978) used distilled water, Carstairs and Oechel (1978) water of an unspecified nature, Harris and Kershaw (1971) 12.5% Hoagland's solution, Larson (1979b) water, Larson (1978) distilled water, and Farrar (1978) tricine buffer (pH 6.6). This lack of concern 007-2745/82/431-437$0.85/0