Catalyzes the 3'-hydroxylation of the flavonoid B-ring to the 3',4'-hydroxylated state. Catalyzes in vitro 3'-hydroxylation of different flavonoids. Catalyzes the conversion of apigenin to luteolin, naringenin to eriodictyol, and kaempferol to quercetin. Possesses specific 5'-hydroxylase activity toward chrysoeriol (a 3'-methoxylated flavone) and is indispensable for tricin formation. Converts chrysoeriol to selgin, a precursor of tricin, suggesting that chrysoeriol, instead of tricetin, is an intermediate in tricin biosynthesis. a 3'-unsubstituted flavone + O2 + reduced [NADPH--hemoprotein reductase] = a 3'-hydroxyflavone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Secondary metabolite biosynthesis; flavonoid biosynthesis. Belongs to the cytochrome P450 family.