Parietin, a photoprotective secondary product of the lichen Xanthoria parietina.

Secondary lichen products can be extracted from air-dry thalli of Xanthoria parietina, Xanthoparmelia conspersa and Parmelina tiliacea with 100% acetone without affecting either short-or long-term viability. In Xanthoria parientina damage by acetone started to occur as water content reached the critical lower limit for photosystem II (PSII) activity. Extraction of the blue-light absorbing cortical pigment parietin increased the susceptibility of both air-dry and hydrated thalli to high light. Damage by high light levels caused a permanent reduction in F v/Fm, quantum yield for photosynthetic O2 production and photosynthetic capacity measured after a 2-day recovery period at low light levels (20 μmol photons m-2 s-1). Parietin therefore protects the photosynthetic apparatus of Xanthoria parietina against damage by high light levels. Extraction of UV-absorbing pigments from Xanthoparmelia conspersa and Parmelina tiliacea did not increase photoinhibition after 24 h exposure to high light.