Formation of UV-honey guides in Rudbeckia hirta.

The UV-honey guides of Rudbeckia hirta were investigated by UV-photography, reflectance spectroscopy, LC-MS analysis and studies of the enzymes involved in the formation of the UV-absorbing flavonols present in the petals. It was shown for the first time that the typical bull's eye pattern is already established at the early stages of flower anthesis on the front side of the petal surface, but is hidden to pollinators until the buds are open and the petals are unfolded. The rear side of the petals remains UV-reflecting during the whole flower anthesis. Studies on the local distribution of 19 flavonols across the petals confirmed that the majority are concentrated in the basal part of the ray flower. However, in contrast to the earlier studies, eupatolitin 3-O-glucoside (6,7-dimethoxyquercetin 3-O-glucoside) was present in both the basal and apical parts of the petals, whereas eupatolin (6,7-dimethoxyquercetin 3-O-rhamnoside) was exclusively found in the apical parts. The enzymes involved in the formation of the flavonols in R. hirta were demonstrated for the first time. These include a rare flavonol 6-hydroxylase, which was identified as cytochrome P450-dependent monooxygenase and did not accept any methylated flavonol as substrate. All enzymes were present in the basal and apical parts of the petals, although some of them clearly showed higher activities in the basal part. This indicates that the local accumulation of flavonols in R. hirta is not achieved by a locally restricted presence of the enzymes involved in flavonol formation.