L-Tyrosine as a Precursor of Flavonoids in Buckwheat Cotyledons

Exogenous L-tyrosine was incorporated 1 3 -1 4 times less effectively than exogenous L-phenylalanine into flavonoids in excised cotyledons o f 3 days old buckwheat seedlings but proved to be an only 5 times poorer precursor when the experiments were carried out with 5 days old material. Simultaneous administration of L-phenylalanine markedly reduced incorporation o f L-tyrosine into flavonoids. A similar decrease in the utilization o f exogenous L-tyrosine for flavonoid bio­ synthesis occurred in kinetin-treated cotyledons. However, in cotyledons treated with high doses o f glyphosate, an inhibitor o f the shikimic acid pathway, an increase in the formation of flavonoids from exogenously supplied L-tyrosine was observed. Under all conditions the relative incorporation rate o f exogenous L-tyrosine was highest for anthocyanins and lowest for C-glycosylflavones while within the latter class o f com pounds the luteolinic derivatives orientin and isoorientin incorporated more label than their apigeninic analogues vitexin and isovitexin. PAL and TAL activities were found to be present in the cotyledons in a ratio o f 50:1. The possible role o f L-tyrosine as an alternative natural precursor for the biosynthesis o f flavonoids and other related polyphenols is discussed.