FLAVONOID BIOSYNTHESIS IN PETALS OF RHODODENDRON-SIMSII

The results of a comparative in vivo study of flavonoid biosynthesis in Rhododendron simsii petals, with [2-C-14]p-coumaric acid and [2-C-14]caffeic acid indicate that p-coumaric acid is the true precursor. Moreover, supplementary in vitro experiments with enzyme preparations show that p-coumaroyl-CoA is a much better substrate for chalcone synthase than caffeoyl-CoA. This suggests that in Rhododendron the 3'-hydroxylation of the B-ring occurs at the stage of a C-15 intermediate. In addition, flavonoid 3'-hydroxylase activity was demonstrated by means of comparative in vivo experiments with, respectively, [4a,6,8-C-14]naringenin and [4a,6,8-C-14]dihydrokaempferol as C-15 precursors. In both cases, the formation of radioactive 3',4'-hydroxylated co-pigments and anthocyanins was established. Furthermore, co-pigment biosynthesis may proceed via the sequence: naringenin, eriodictyol, dihydroquercetin (3'-hydroxylation at the flavanone stage), whereas the formation of the anthocyanins may take place via naringenin, dihydrokaempferol and dihydroquercetin (3'-hydroxylation at the dihydroflavonol level).