Parietin is the predominant cortical pigment of lichens in the genus Caloplaca, a secondary product of the lichen Xanthoria parietina, and a pigment found in the roots of Curled Dock (Rumex crispus). It has an orangy-yellow color and absorbs blue light.
It is also known as physcion.
It has also been shown to protect lichens against UV-B light, at high altitudes in Alpine regions. The UV-B light stimulates production of parietin and the parietin protects the lichens from damage. Lichens in arctic regions such as Svarlbard retain this capability though they do not encounter damaging levels of UV-B, a capability that could be help protect the lichens in case of Ozone layer thinning.
It has also shown anti-fungal activity against barley powdery mildew and cucumber powdery mildew, more efficiently in the latter case than treatments with fenarimol and polyoxin B.
It reacts with KOH to form a deep, reddish-magenta compound.