The yellow colour of Chrysanthemum segetum petals is due to the presence of the 7-O-glucosides of quercetin and particularly gossypetin (8-hydroxyquercetin). In petal extracts of C. segetum an enzyme was demonstrated which catalyzes the transfer of the glucosyl moiety of uridine 5'-diphosphoglucose (UDPG) to the 7-hydroxyl group of flavonols with gossypetin and quercetin as the best substrates. Besides flavonols flavanones and flavones were found to be glucosylated in the 7-position. The pH-optimum of the reaction highly depended on the substrate used. With quercetin as substrate, maximal enzyme activity occurred at a pH of 8.25 and a temperature of 25 degrees C, but 7-O-glucosylation also proceeded at low temperatures. Studies on temperature stability revealed, that there was no influence on the glucosylation reaction up to 40 degrees C. Higher temperatures led to a loss of enzyme activity. Using gossypetin as a substrate a similar course of temperature stability was observed. Addition of Mg2+, Ca2+ and KCN slightly stimulated 7-O-glucosylation, whereas Co2+ Cu2+, Fe2+, Hg2+ p-hydroxymercuribenzoate and N-ethylmaleimide showed a strong inhibitory effect. Additional enzymatic studies were performed with the commercial strain ''Stern des Orients'' where gossypetin 7-O-glucoside is restricted to the inner parts of the petals. For enzyme extracts from both parts of the petals gossypetin was found to be the most attractive substrate. In comparison to quercetin (133.4 mu kat/kg protein) an about three times higher specific activity of the 7-O-glucosyltransferase(s) was determined with gossypetin (382.1 mu kat/kg protein) as substrate, indicating that hydroxylation of quercetin in 8-position to gossypetin precedes 7-O-glucosylation.

Yellow colouration of Dahlia variabilis is mainly provided by isoliquiritigenin 4′-glucoside and butein 4′-glucoside. Incubation of petal extracts with uridin 5′-diphosphoglucose and isoliquiritigenin or butein led to the formation of one product which was identified as the respective 6′-deoxychalcone 4′-glucoside. Glucosylation of hydroxyl groups in other positions was not observed. Naringenin chalcone and eriodictyol chalcone were not accepted as substrates. The 4′-glucosylation of isoliquiritigenin and butein showed a broad pH optimum ranging from pH 7 to 8 and was stimulated by Mg2+, Ca2+ and Mn2+. N-Ethylmaleimide, p-hydroxymercuribenzoate, Cu2+, Zn2+, and Fe2+ clearly reduced the activity of the enzyme. The apparent Km values for UDP-glucose, isoliquiritigenin and butein were 90, 26 and 276 [μᴍ respectively.

Flavonoid analysis and supplementation experiments with dihydroflavonols and leucocyanidin on two cyanic, two acyanic and one white/red-variegated flowering strain of Dianthus caryophyllus (carnation) showed that in the acyanic strains recessive alleles (aa) of the gene A interrupt the anthocyanin pathway between dihydroflavonols and leucoanthocyanidins. The instability in the variegated strain involves the same step and is obviously caused by the multiple allele a (var) . In confirmation of these results, dihydroflavonol 4-reductase activity could be demonstrated in enzyme extracts from cyanic flowers and cyanic parts of variegated flowers but not in preparations from acyanic flowers or acyanic parts. The enzyme catalyzes the stereospecific reduction of (+)dihydrokaempferol to (+)-3,4-leucopelargonidin with NADPH as cofactor. A pH optimum around 7.0 and a temperature optimum at 30° C was determined, but the reduction reaction also proceeded at low temperatures. (+)Dihydroquercetin and (+)dihydromyricetin were also reduced to the respective flavan-3,4-cis-diols by the enzyme preparations from carnation flowers, and were even better substrates than dihydrokaempferol.

Flavonol synthase (FLS) was demonstrated in crude extracts from flower buds of Dianthus caryophyllus (carnation). The enzyme catalyzed the conversion of dihydrokaempferol and ihydroquercetin to kaempferol and quercetin, respectively. The reaction required 2-oxoglutarate, ferrous ion and ascorbate as co-factors and had a pH optimum at about 7.4. The demonstration of FLS activity allowed comparative studies on flavonol and anthocyanin biosynthesis during bud and flower development. Besides FLS the flavonoid enzymes chalcone synthase (CHS), flavanone 3-hydroxylase (FH T) and dihydroflavonol 4-reductase (DFR) were measured. DFR is specifically involved in anthocyanin synthesis, while CHS and FHT provide dihydroflavonol, the common substrate for both FLS and DFR . Maximum expression of CHS, FHT and FLS activity was already observed in small buds, whereas DFR activity started to increase much later and reached its highest level in opened flowers. A substantial correlation was observed between the time courses of FLS and DFR activity and the accumulation of flavonols and anthocyanins, respectively. The competition of FLS and DFR for dihydroflavonols was found to be largely circumvented by different substrate specificities and by the sequential expression of the two enzymes. Both flavonols and anthocyanins are obviously not, or only to some extent, subject to degradation.