Kinetin-Mediated Stimulation of Accumulation of Buckwheat Flavonoids in the Dark

A short treatment o f excised buckwheat cotyledons with a solution o f kinetin lead to an up to 9-fold stimulation o f anthocyanin biosynthesis, to an about 50 percent increase in the accum ula­ tion o f rutin, and to an about 30 percent increase, on the average, in the accumulation o f C-glycosylflavones in the treated material during its posttreatment incubation in the dark. When the treated cotyledons were incubated in a solution o f L-phenylalanine anthocyanin accumulation in the dark practically attained the same high level as it was observed in the illuminated cotyledons fed with exogenous L-phenylalanine. In experiments with 14C-labelled L-phenylalanine kinetin induced a sharp rise in the labelling (resp. in the utilization o f exogenous substrate for biosynthesis) o f anthocyanins and rutin in the dark and a slight increase in the radioactivity o f C-glycosylflavones. Similar labelling changes occurred in the illuminated cotyledons. However, both kinetin and light still more effectively promoted biosynthetic use o f the endogenous sub­ strate. As a result the relative portion o f flavonoids formed from exogenous L-phenylalanine under these conditions showed a decrease as compared with the ratio o f precursor use in the un­ treated cotyledons. The results show that low accumulation rates o f anthocyanins and other flavo­ noids in the dark are conditioned by the limited access o f substrate (L-phenylalanine) molecules to the flavonoid enzymes lending further support to the idea that flavonoid biosynthesis is normally controlled at the substrate rather than at the enzymic level.