Catalyzes the intramolecular cyclization of bicyclic chalcones into tricyclic (S)-flavanones. Responsible for the isomerization of 4,2',4',6'-tetrahydroxychalcone (also termed chalcone) into naringenin. a chalcone = a flavanone. Secondary metabolite biosynthesis; flavonoid biosynthesis. Mostly expressed in siliques and flowers, and, to a lower extent, in leaves. By light. Part of the biosynthetic pathway for all classes of flavonoids, a large class of secondary plant metabolites, many of which are brightly colored. Belongs to the chalcone isomerase family.