Is parietin a UV-B or a blue-light screening pigment in the lichen Xanthoria parietina?

Xanthoria parietina is a widespread lichen coloured by the orange cortical pigment parietin (= physcion). We studied the pigment content in 60 thalli sampled in 4 habitats along a sun-shade gradient from evergreen boreal forests through open deciduous stands to sea cliffs. The significant positive regression between contents of parietin per unit area and site factors (reflecting the openness of the canopy relative to an open sky) across sampled habitats suggested a photoprotective role of parietin at UV-B and/or blue wavelengths, the two absorbance maxima of parietin. UV-B susceptibility of X. parietina, measured as permanent reductions in photosystem II, decreased highly significantly with increasing parietin content per thallus area. However, as much as three-fold greater UV-B irradiances than ambient daily summer maxima, maintained continuosly for 240 h were required to cause UV-B damage even in thalli of shaded habitats. Since a previous study has documented a high PAR susceptibility of parietin-deficient X. parietina in the absence of UV-B, there are reasons to believe that the blue light screening of parietin is functionally more important than the UV-B screening. A strong positive relationship between parietin content per unit area and reflectance at 500 nm allows the parietin content in X. parietina thalli to be assessed non-destructively by reflectance measurements.