Water Relations in Living, Dead, and Cellulose Models of the Lichen Umbilicaria papulosa

Abstract

relations of living lichens, dead lichens, and cellulose model lichens were studied at different relative humidities. An air flow system allowed instantaneous weight measurement while water uptake or loss was occurring. In all cases, living lichens had the greatest uptake rates, followed by dead lichens and cellulose models. Nonaqueous staining techniques showed that in living, but not in dead lichens, most of the water is held in the algal layer. The processes of water vapor uptake and water loss are apparently physical, and the differences in rates can be explained by differences in the physical properties of the test materials. The water relations of lichens have puzzled biologists for many years (Stocker, 1956; Ried, 1960; Smith, 1962). Lichens generally have no special organs for water uptake and no obvious mechanism for water conservation, yet they can survive in some of the driest habitats on earth. In such inhospitable environments, lichens obtain much of their water in the form of atmospheric water vapor (Heatwole, 1966; Lange et al., 1970). Scofield and Yarman (1943) studied the phenomenon of water vapor absorption in the field and laboratory, and found that the water content of a thallus varied with the ambient atmospheric humidity. Similar results have been obtained for numerous lichens in our laboratory. Scofield and Yarman (1943) concluded that the water relations of lichens do not differ from those of a hydrophilic gel, and also concluded, as others have, that the processes of absorption and evaporation are purely physical. Metabolic water uptake has been ruled out by use of the metabolic inhibitors sodium azide and sodium fluoride, which had no effect on the rate of liquid water uptake by Peltigera polydactyla (Smith, 1961). loss is also considered to be a physical process, and the current feelings are summarized very appropriately by Smith (1962): Water loss appears to be entirely a physical process, and its rate is affected by those factors which affect the evaporation of water. Our purpose in studying water relations in the lichen Umbilicaria papulosa (Ach.) Nyl. (= Lasallia papulosa (Ach.) Llano) was to use a newly devised quantitative technique for measuring water uptake and loss over short periods of time and to try to determine the relationships of the symbionts to water retention and loss. Many others have compared lichen thalli to hydrophilic gels; however, we felt that it would be of interest to compare them to cellulose-fiber models that more closely resembled lichens in texture and shape. 1 Department of Botany, The Ohio State University, Columbus, Ohio 43210. (Contr. 806.) 2Present address: Department of Medicine, University Hospital, 410 W 10th Avenue, Columbus, Ohio 43210. This content downloaded from 157.55.39.104 on Wed, 28 Dec 2016 18:25:04 UTC All use subject to http://about.jstor.org/terms 1971] SHOWMAN ET AL.: WATER RELATIONS OF UMBILICARIA 445