Influence of Nitrogen Nutrition on the Utilization of L-Phenylalanine for Building Flavonoids in Buckwheat Seedling Tissues

Abstract

Summary In an attempt to elucidate metabolic mechanisms of the inhibitory effect of nitrogen feeding on the formation of phenolics, the influence of ammonium nitrate on the production of flavonoids, on the biosynthetic use of exogenous L-phenylalanine for their building, and on the functional capacity of the shikimic acid pathway in excised buckwheat cotyledons and hypocotyls was studied. In cotyledons the treatment caused, besides a marked decrease in the total production of flavonoids, a significant reduction of exogenous L-phenylalanine incorporation into these compounds while the activity of the shikimic acid pathway responsible for the primary synthesis of aromatic amino acids showed a slight increase. In hypocotyls, a decrease in the accumulation of flavonoids was accompained by an up to 30% inhibition of the shikimic acid pathway, but no change in the incorporation of L-phenylalanine exceeding the level of changes in its uptake could be observed. It was concluded that in buckwheat cotyledons, nitrogen-induced inhibition of flavonoid synthesis resulted from a shift in the distribution of L-phenylalanine in favour of protein biosynthesis, while in buckwheat hypocotyls a similar decrease in flavonoids was probably conditioned by an inhibition of the primary synthesis of L-phenylalanine via the shikimic acid pathway.