Photosynthetic activity of the lichen Xanthoria elegans in relation to the daily course of temperature and humidity, a field study in the Piora valley, Ticino, Switzerland

At high altitudes in the absence of vegetation of woody and scrubby plants lichens are dominant primary producers on boulders and rocks. To live there, lichens are forced to be well adapted to alpine environmental stress factors including extreme temperature changes and high solar radiation, including UV. This often results in high rock surface temperatures, low air humidity and drought. To study the dependence of the cellular activity of lichen on temperature and humidity, the initial fast chlorophyll fluorescence rise, indicating photosystem II (PS II) activity, was measured on site with dry and hydrated samples of Xanthoria elegans. Xanthoria elegans is frequently found on calcareous-dolomitic and siliceous rock surfaces, typically also on stony walls and roofs of old agricultural buildings in the Piora Valley, Ticino, Switzerland. Although a complete desiccation of the lichen leads to a full loss of the photosynthetic activity (Brock 1975, Lange & Matthes 1981, Lange et al. 1994, Lange 2003, Ding et al. 2013, Phinney et al. 2019), the lichen Xanthoria elegans fully recovered within few minutes upon rehydration. As measure for photosynthetic activity, the optimal yield of photosystem II, Fv/Fm, was determined. Xanthoria elegans reached maximum values of up to 0.63-0.67 in the hydrated state. The photosynthetic activity decreased during water loss and dropped more rapidly at temperatures higher than 20oC. Water saturation was followed by the electrical conductivity at the thallus surface. Drought is the main environmental stress regulating photosynthesis and growth in lichens in alpine regions