The lichens are poikilohydric organisms which are very
resistant against the stress caused by radiation, especially
against UV-B radiation. UV-B radiation has a negative effect on
physiological functions of lichens. However, the lichnes
developed some adaptive mechanisms to protect own thalli
against harmful effects of UV-B radiation. I analyzed the
content of UV-absorbing compounds and pigments by
spectrophotometric methods in my Master thesis. In my
experiment with controlled conditions with supplemental UV-B
radiation, I found that the response of photosynthetic
apparatus of different species of lichens, their photobionts,
respectively, is species-specific. According to my experimental
findings the different species of lichens in natural state
contain some amount of UV-absorbing compounds. Synthesis of
these compounds is induced by currently incident UV-B
radiation. The amount of UV-absorbing compounds is increased by
higher doses of UV-B radiation. Therefore, these compounds have
a photoprotective function. The best known UV-absorbing
compounds are the parietin and the usnic acid. In my
experiments, supplemental UV-B radiation had a negative impact
on the photosynthetic processes in lichens. I found that the
parametres of chlorophyll fluorescence derived from Kautsky
curve supplemented by saturation pulses and Chlorophyll
Fluorescence Imaging technique (especially effective quantum
yield ΦII), and fast induction kinetics decreased due to the
impact of UV-B radiation. Similarly to UV-B absorbing
compounds, the response of chlorophyll fluorescence parameters
was  species-specific.