FLAVONOID BIOSYNTHESIS IN PETALS OF RHODODENDRON-SIMSII

The results of a comparative in vivo study of flavonoid biosynthesis i n 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. Mor eover, supplementary in vitro experiments with enzyme preparations sho w that p-coumaroyl-CoA is a much better substrate for chalcone synthas e than caffeoyl-CoA. This suggests that in Rhododendron the 3'-hydroxy lation of the B-ring occurs at the stage of a C-15 intermediate. In ad dition, flavonoid 3'-hydroxylase activity was demonstrated by means of comparative in vivo experiments with, respectively, [4a,6,8-C-14]nari ngenin 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 ma y proceed via the sequence: naringenin, eriodictyol, dihydroquercetin (3'-hydroxylation at the flavanone stage), whereas the formation of th e anthocyanins may take place via naringenin, dihydrokaempferol and di hydroquercetin (3'-hydroxylation at the dihydroflavonol level).