UV-induction of sun-screening pigments in lichens

Induction of the pigments parietin and melanin by specific wavelength ranges of solar radiation was studied in two widespread foliose lichens: the ubiquitous sun-tolerant Xanthoria parietina , from which the orange parietin can be extracted in living thalli by acetone, and naturally melanin-free shade-adapted specimens of the old forest species Lobaria pulmonaria . Parietin-free X. parietina and melanin-free L. pulmonaria were exposed to solar radiation in the field under filter foils transmitting or absorbing natural UV-radiation (< 400 nm). Synthesis of both pigments depended on the presence of UV-radiation. A growth chamber experiment showed that the induction of parietin and melanin synthesis required the presence of UV-B (280-320 nm). UV-A (320-400 nm) induced a very small synthesis only, whereas photosynthetically active radiation (PAR) was not inductive. Melanins and parietin were only synthesized in thalli being hydrated during UV-exposures, suggesting that induction required active metabolism. The photosystem II (PSII) of X. parietina was not adversely affected by any applied radiation treatment, regardless of the hydration status during exposure, whereas L. pulmonaria was susceptible both to high PAR- and UV-irradiances in the desiccated state.