Christine A. Williams

Advances in flavonoid research since 1992.

Some of the recent advances in flavonoid research are reviewed. The role of anthocyanins and flavones in providing stable blue flower colours in the angiosperms is outlined. The contribution of leaf flavonoids to UV-B protection in plants is critically discussed. Advances in understanding the part played by flavonoids in warding off microbial infection and protecting plants from herbivory are described. The biological properties of flavonoids are considered in an evaluation of the medicinal and nutritional values of these compounds.

A chemotaxonomic study of flavonoids from european teucrium species

Abstract A survey of aerial tissues of 42 European taxa of the genus Teucrium has revealed the widespread presence of five surface flavonoids: cirsiliol, cirsimaritin, cirsilineol, salvigenin and 5-hydroxy-6,7,3′,4′-tetramethoxyflavone. The latter two compounds are useful taxonomic markers in that salvigenin is characteristic of species of section Polium, while 5-hydroxy-6,7,3′,4′-tetramethoxyflavone is completely confined to species of the other five sections surveyed. Eleven flavone glycosides, four flavonol glycosides and the glycoflavone vicenin-2 were found to occur as vacuolar constituents. One of the flavone glycosides, cirsimaritin 4′-glucoside, only occurs in the species T. arduini, while two others, hypolaetin and isoscutellarein 7-acetyl-allosylglucosides, are characteristic of the closely related T. chamaedrys and T. webbianum. 6-Hydroxyluteolin is widely present as the 7-glucoside and 7-rhamnoside, the latter compound being a new glycoside. In general, the chemical results are correlated with sectional classification and usefully indicate that at least one taxon, T. compactum, is misplaced within the genus. Phyletically, the restriction of flavonol glycosides mainly to section Teucrium suggests that this may be the basic group within the genus.

Chrysin and other leaf exudate flavonoids in the genus Pelargonium

In a chemotaxonomic survey of 57 Pelargonium species, leaf exudate flavonoids were detected in 35% of the sample, mostly in trace amounts. However, chrysin and a related C-methylflavanone were identified as major leaf surface constituents of P. crispum, and a mixture of quercetin and kaempferol mono-, and di- and trimethyl ethers of P. quercifolium. In two other species, P. fulgidum and P. exstipulatum, methylated flavones were the only lipophilic flavonoids present. This is the first report of leaf surface flavonoids from the genus Pelargonium.

6-Hydroxyluteolin and scutellarein as phyletic markers in higher plants☆

Abstract A survey of some twelve highly specialized herbaceous families has shown that 6-hydroxyluteolin is present in the majority as a leaf constituent. It has been identified in the Plantaginaceae ( Plantago ), Globulariaceae ( Globularia and Lytanthus ), Labiatae ( Amethystea, Hemigenia, Isanthus, Trichostema and Westringia ), Buddleiaceae ( Buddleia, Chilianthus ) and the Valerianaceae ( Valerianella ). It is accompanied most frequently by luteolin, occasionally by scutellarein and by methylated derivatives. In Globularia , 6-hydroxyluteolin was found in leaves of all of the eleven species surveyed; in G. cordifolia it was accompanied by its 6-methyl and 6,4′-dimethyl ethers, and by scutellarein and its 6,4′-dimethyl ether (pectolinarigenin). Pectolinarigenin was also identified in Kerria (Rosaceae). These distribution patterns are very different from those of 8-hydroxylated flavones and flavonols and indicate that the ability to hydroxylate flavones in the 6-position arose relatively late in evolutionary time.

Flavonoid patterns in leaves of the gramineae

In a leaf survey of 274 species from 121 genera of the Gramineae, flavone C-glycosides and tricin were found to be the major flavonoids in 93% of the samples. By contrast, apigenin and luteolin O-glycosides were comparatively rare, as were the flavonols, kaempferol and quercetin. In only one species, Rottboellia exaltata were flavonols the sole flavonoids. 7.3′.4′-Trihydroxyflavone, which has been detected in the Juncaceae, was found in 3 of 5 samples of the species Bothriochloa bladhii. Flavonoid sulphates were present in 16% of the species examined. While in most of these plants tricin glycosides were conjugated with sulphate, in Paspalum convexum quercetin mono- and di-sulphates and 1-caffeylglucose sulphate were identified. Flavonoid sulphates are present in the tropical-subtropical subfamilies: Panicoideae (in 18% of species). Chloridoideae (15%) and Arundinoideae (40%) but were not found at all in tribes of the cool temperate regions. Proanthocyanidins were found in only 3% of the species surveyed. The flavan-4-ol, luteoforol and its apigenin analogue were detected only in the subfamilies Panicoideae and Chloridoideae, where they occured in 12 and 6% of species respectively.

Flavonoid patterns in the fruits of the umbelliferae

Abstract Fruit flavonoids have been surveyed in some 100 species representing all the major tribes of the Umbelliferae. Of the 25 flavone and flavonol glycosides detected, by far the most common were luteolin 7-glucoside and quercetin 3-rutinoside. Isorhamnetin was found to occur regularly in two tribes, Peucedaneae and Apieae and it was isolated for the first time as the 3-glucuronide from Anethum sowa . The distribution of flavones vs. flavonols in the subfamily Apioideae was correlated closely with the results of a previous survey of the same plants for their leaf flavonoids. There was also some correlation with morphology in that species with spines on the fruit (tribe Caucalideae) contained a much richer variety of flavonoids than species in other tribes. In these plants, luteolin is present with O -methylation (as chrysoeriol), with complex O -glycosylation (e.g. as 7-glucuronosylglucoside and 7-diglucoside) and with alterations in the usual position of sugar attachment (as the 5- and 4′-glucosides). The results add further chemical support to the view that Benthams circumscription of the tribe Caucalideae is a more natural one than that of Drude and also suggest that the Caucalideae is a highly specialised group within the family. The discovery of apigenin and luteolin 7-glucuronosylglucosides in Cuminum cyminum supports its removal from the Apieae and transfer to the Caucalideae.

The leaf flavonoids of the Zingiberales

A survey of flavonoids in the leaves of 81 species of the Zingiberales showed that, while most of the major classes of flavonoid are represented in the order, only two families, the Zingiberaceae and Marantaceae are rich in these constituents. In the Musaceae (in 9 species), Strelitziaceae (in 8 species) and Cannaceae (1 of 2 species) flavonol glycosides were detected in small amount and in the Lowiaceae no flavonoids were fully identified. In the Zingiberaceae kaempferol (in 22%), quercetin (72%) and proanthocyanidins (71%) are distributed throughout the family. The two subfamilies of the Zingiberaceae may be distinguished by the presence of myricetin (in 26%), isorhamnetin (10%) and syringetin (3%) in the Zingiberoideae and of flavone C-glycosides (in 86% of taxa) in the Costoideae. A number of genera have distinctive flavonol profiles: e.g. Hedychium species have myricetin and quercetin. Roscoea species isorhamnetin and quercetin and Alpinia species kaempferol and quercetin glycosides. A new glycoside, syringetin 3-rhamnoside was identified in Hedychium stenopetalum. In the Zingiberoideae flavonols were found in glycosidic combination with glucuronic acid, rhamnose and glucose but glucuronides were not detected in the Costoideae or elsewhere in the Zingiberales. The Marantaceae is chemically the most diverse group and may be distinguished from other members of the Zingiberales by the occurrence of both flavone O- and C-glycosides and the absence of kaempferol and isorhamnetin glycosides. The distribution of flavonoid constituents within the Marantaceae does not closely follow the existing tribai or generic limits. Flavonols (in 50% of species). flavones (20%) and flavone C-glycosides (40%) are found with similar frequency in the two tribes and in the genera Calathea and Maranta both flavone and flavonol glycosides occur. Apigenin- and luteolin-7-sulphates and luteolin-7,3′-disulphate were identified in Maranta bicolor and M. leuconeura var. kerchoveana and several flavone C-glycosides sulphates in Stromanthe sanguinea. Anthocyanins were identified in those species with pigmented leaves or stems and a common pattern based on cyanidin-and delphinidin-3-rutinosides was observed throughout the group. Finally the possible relationship of the Zingiberales to the Commelinales, Liliales, Bromeliales and Fluviales is discussed.

Negatively charged flavones and tricin as chemosystematic markers in the palmae

A survey of 125 species of the Palmae revealed a complex pattern of flavonoids in the leaf. C-Glycosylflavones, leucoanthocyanins and tricin, luteolin and quercetin glycosides were common, being present in 84, 66, 51, 30 and 24% of the species respectively. Apigenin and kaempferol were recorded in only a few species and isorhamnetin only once. Eighteen flavonoids were identified: the 7-glucoside, 7-diglucoside and 7-rutinoside of both luteolin and tricin, tricin 5-glucoside, apigenin 7-rutinoside, quercetin 3-rutinoside-7-galactoside, isorhamnetin 7-rutinoside, orientin, iso-orientin, vitexin, isovitexin and vitexin 7-O-glucoside. Many of the C- and O-flavonoid glycosides were present as the potassium bisulphate salts and negatively charged compounds were detected in 50% of the species. The distribution patterns are correlated with the taxonomy of the family in several ways. Thus, the Phoenicoideae and Caryotoideae have distinctive flavonoid patterns, there is evidence to support the separation of the subfamilies Phytelephantoideae and Nypoideae, and tricin is a useful marker at tribal level. At the generic level, Cocos is clearly separated from Butia, and other Cocoseae and Mascarena and Chamaedorea form well defined groups within the Arecoideae. A numerical analysis of these biochemical data, together with morphological characters, produces a new classification which suggests that the flavonoid data may have more systematic value than is indicated when they are applied to the traditional classification.

Leaf survey of flavonoids and simple phenols in the genus Rhododendron

Abstract A leaf survey of 206 Rhododendron species, subspecies and varieties showed that the genus possesses a relatively uniform flavonoid pattern. Three compounds which are generally rare in the angiosperms, gossypetin, azaleatin and caryatin, occur in 76, 34 and 10 per cent of the species respectively. Quercetin and leucoanthocyanidins are present in all taxa, while dihydroflavonols are present in 68 per cent, myricetin in 51 per cent and kaempferol in 23 per cent of the sample. Gossypetin is notably absent from the subgenus Pentanthera while caryatin characterises the single subgenus Hymenanthes. Of the three dihydroflavonols, dihydroquercetin and dihydromyricetin are reported in the genus for the first time, the former being isolated as the 3-arabinoside. The flavonols of Rhododendron leaf were found to be present as the 3-arabinosides, 3-rhamnosides, 3-galactosides and 3-glucosides. Simpler phenols were surveyed in leaves of 55 species with the following results: orcinol in 7 per cent, hydroquinone in 9 per cent, rhododendrol in 37 per cent, o -coumaric acid in 19 per cent, gentisic acid in 80 per cent and gallic acid in 84 per cent. Taxonomically, the results generally support the accepted sectional and subsectional classifications, although they suggest that on chemical grounds certain species might be misplaced. Phyletically, the data indicate that the genus still retains a wide range of primitive phenolic characters. Geographically, the separation of R. lochae , the only Australian species, from the rest of the genus in S.E. Asia is reflected in its chemistry.

The leaf flavonoids of the orchidaceae

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.