A comprehensive study of the complex polyphenol biosynthesis in developing leaves of apple (Malus domestica) was performed comprising gene expression, enzyme activities and polyphenol composition. During leaf development, an early increase in gene expression was observed for phenylalanine ammonia lyase (PAL, EC, chalcone synthase (CHS, EC, flavanone 3-hydroxylase (FHT, EC and dihydroflavonol 4-reductase/flavanone 4-reductase (DFR/FNR, EC Their enzyme activities showed a corresponding trend during the time course. A parallel set of experiments was carried out with leaves treated with prohexadione-Ca (ProCa), which is an enzyme inhibitor of 2-oxoglutarate dependent dioxygenases (2-ODDs). ProCa is known to induce changes in polyphenol biosynthesis, which are accompanied by a reduced incidence of fire blight and scab, the two major pome fruit diseases. The application of ProCa led to an increase in activities of PAL, CHS, FHT and DFR/FNR, which was based on an enhanced gene expression. In contrast, an inhibition of gene expression was detected for anthocyanidin synthase (EC These effects are interpreted as a feedback regulation by changed polyphenol levels. Because of the inhibition of the 2-ODDs FHT and flavonol synthase (EC, some pronounced changes in polyphenol composition were observed. Eriodictyol, the substrate of FHT, accumulated as eriodictyol-7-O-glucoside and 6″-O-trans-p-coumaroyleriodictyol 3′-O-glucoside. In addition, the 3-deoxycatechin luteoliflavan was formed which is not present in untreated apple leaves. Hence, beyond the redirection of polyphenol biosynthesis by the enzyme inhibitor, changed polyphenol levels obviously cause a distinct induction of gene expression by feedback regulation.