The leaf flavonoids of the orchidaceae |
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Authors: | Christine A. Williams |
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Affiliation: | Botany Department, Plant Science Laboratories, The University, Whiteknights, Reading, Berks., RG6 2AS, U.K. |
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Abstract: | In a leaf survey of 142 species from 75 genera of the Orchidaceae, flavone C-glycosides (in 53%) and flavonols (in 37 %) were found to be the most common constituents. However, since these compounds are not found uniformly and their distribution shows a strong correlation with plant geography, it is not possible to represent the Orchidaceae by a single flavonoid profile. Thus, flavone C-glycosides are most common in tropical and subtropical species of the Epidendroid and Vandoid tribes (in 63%) and flavonol glycosides are more characteristic of temperate species of the Neottioid tribes (in 78%). By contrast 6-hydroxyflavones (in 6 species), luteolin (in 2 species) and tricin as the 5-glucoside (in 1 species) are all rare. Three new glycosides were characterised: scutellarein 6-methyl ether 7-rutinoside from Oncidium excavatum and O. sphacelatum, pectolinarigenin 7-glucoside from 0. excavatutn and Eria javanica, and luteolin 3′,4′-diglucoside from Listera ovata. The xanthones, mangiferin and isomangiferin were found in Mormolyca ringens, Maxillaria aff. luteo-alba and 5 Polystachya species and a mangiferin sulphate tentatively identified in P. nyanzensis. Other unusual phenolic constituents include 6,7-methylenedioxy- and 6,7-dimethoxycoumarins from Dendrobium densiflorum and D. farmeri, formed by the rearrangement during the extraction process from the corresponding O-glucosyloxycinnamic acids. The origin and relationship of the Orchidaceae to other monocot groups are discussed in the light of the flavonoid evidence. |
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Keywords: | Orchidaceae flavonols 6-hydroxyflavones luteolin 3′,4′-diglucoside scutellarein 6-methyl ether 7-rutinoside pectolinarigenin 7-glucoside mangiferin sulphate dihydroquercetin 3-glucoside biochemical systematics. |
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