Methylated C-glycosylflavones as taxonomic markers in orchids of the subtribe ornithocephalinae

Abstract

In a leaf flavonoid survey of 15 Brazilian orchid species from the subtribe Ornithocephalinae, flavone C-glycosides were found to be the major constituents in all except two Ornithocephalus taxa. In Zygostates cornuta a rare glycoside, apigenin 7,4′-dimethyl ether 6- C-glucoside- 2″- O-rhamnoside, was identified. This also occurred in Z. alleniana, Z. lunata, Z. pellucida and Z. pustulata. Two further similar apigenin 7,4′-dimethyl ether C-glycoside- O-rhamnosides, possibly with different C-linked sugars, were detected in Z. multiflora and Z. grandiflora. In Z. cornuta, apigenin 4′-methyl ether 6- C-glucoside-2″- O-rhamnoside was found to co-occur with the apigenin 7,4′-dimethyl ether derivative. Ornithocephalus myrticola was distinguished by the presence of an apigenin 8- C-methylpentoside and three apigenin 7-methyl ether 8- C-glycoside derivatives. In O. bicornis and O. kruegeri, another apigenin 7,4′-dimethyl ether C-glycoside was found, while from Phymatidum falcifolium, apigenin 7,4′-dimethyl ether and apigenin 7-methyl ether C-glycosides and their O-arabinosides were isolated. Chytroglossa marileoniae also contained an apigenin 7,4′-dimethyl ether C-glycoside, which appeared to be the same as that in P. falcifolium. Rauhiella silvana, on the other hand, could be distinguished from all the other taxa studied by the absence of methylated C-glycosylflavones and presence of isovitexin. 6-Hydroxyflavone O-glycosides, which are characteristic leaf constituents of some Oncidium species in the related subtribe Oncidinae were not detected in the present sample. However, the high frequency of methylated flavone C-glycosides suggests a chemically advanced evolutionary position for the Ornithocephalinae, which is in agreement with evidence from their complex floral and vegetative morphology.