<p>The Earth continually receives natural sunlight consisting of photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) emitted from the Sun, which strongly impacts lichens. UVR tolerance in lichens has rarely been explored. Therefore, this current study sought to evaluate the physiological and biochemical effects of UVR in the epiphytic lichens <i>Xanthoria candelaria</i> and <i>Xanthoria elegans</i>. There was a significant rise in parietin content when subjected to PAR + UV-A + UV-B (PAB) irradiations. The lichens’ thalli exposed to PAB radiation substantially reduced photosynthetic performance and chlorophyll (Chl) concentration. In <i>X. candelaria</i> (thalli having less parietin content), UVR had a detrimental impact on the amount of photosynthetic pigment and Chl fluorescence. Thus, it was concluded that in <i>X. elegans</i>, the layer of parietin functions as a natural barrier, which reduces UVR exposure to algal cells. Furthermore, the ergosterol content decreased, and the thiobarbituric acid-reactive substances content increased. These findings suggest that a major contributing component of UVR-induced cell membrane damage is the generation of reactive oxygen species (ROS). Lichens develop robust antioxidant defences against ROS-induced oxidative stress, comprising several antioxidant substances such as carotenoids and enzymes, including catalase, peroxidase, superoxide dismutase, and ascorbate peroxidase. Based on the findings of this research, we can conclude that parietin might be essential for the sustainable existence of the studied lichens under harsh environmental conditions, including intense solar irradiance.</p>