Flavonoids in the leaves of Hillebrandia and Begonia species (Begoniaceae)

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Highlights

  • Foliar flavonoids of and 126 Begonia taxa belonging to the Begoniaceae were surveyed.

  • Hillebrandia sandwicensis endemic to Hawaii was surveyed for flavonoids for the first time.

  • Ten flavonols and eleven C-glycosylflavones were isolated and characterized from their species.

  • Begonia species were divided into two chemotypes, flavonol and C-glycosylflavone types.

Abstract

The fresh leaves of Hillebrandia sandwicensis and 126 Begonia taxa were chemotaxonomically surveyed for flavonoids. Of their taxa, H. sandwicensis and 119 species, one variety and three hybrids were analyzed for flavonoids for the first time. Ten flavonols and eleven C-glycosylflavones were isolated and characterized as quercetin 3-O-rutinoside (1), kaempferol 3-O-rutinoside (2), isorhamnetin 3-O-rutinoside (3), quercetin 3-O-glucoside (4), quercetin 3-methyl ether 7-O-rhamnosylglucoside (5), quercetin 3,3'-dimethyl ether 7-O-rhamnosylglucoside (6), quercetin glycoside (13), quercetin glycoside (acylated) (14), kaempferol glycoside (17) and quercetin 3-O-rhamnoside (18) as flavonols, and isovitexin (7), vitexin (8), isoorientin (9), orientin (10), luteolin 6-C-pentoside (11), luteolin 8-C-pentoside (12), schaftoside (15), isoschaftoside (16), chrysoeriol 6,8-di-C-pentoside (19), apigenin 6,8-di-C-arabinoside (20) and isovitexin 2''-O-glucoside (21) as C-glycosylflavones. Quercetin 3-O-rutinoside (1) alone was isolated from H. sandwicensis endemic to Hawaii. Major flavonoids of almost Begonia species was also 1. Begonia species were divided into two chemotypes, i.e. flavonol containing type and C-glycosylflavone containing type. Of 14 section of the Begonia, almost species of many section, i.e. sect. Augustia, Coelocentrum, Doratometra, Leprosae, Loasibegonia, Monopteron and Ruizoperonia, were flavonol types. On the other hand, C-glycosyflavone type was comparatively most in sect. Platycentrum.

Introduction

The family Begoniaceae consists of 1800 more than species of two genera, Hillebrandia and Begonia (Mabberley, 2017). Of their genera, the genus Hillebrandia consists of only one species, H. sandwicensis Oliv. and is endemic to Hawaii. Other species belonging to the genus Begonia are distributed to the Tropical and Temperate zones in Asia, Africa, and Central and South America. Most species have a characteristic succulent, often jointed, stem, with large membranous stipules. Many also have a thick rhizome or tubers, and some climb with the aid of aerial roots. A few have woody stems and are fibrous-rooted. Begonia species provide many very popular ornamentals (Heywood, 1978).

Flavonoids in the Begonia species have been reported by some authors. Five flavonols and two flavones were isolated from the leaves of B. erythrophylla, and identified as kaempferol 3-methyl ether, quercetin 3-methy ether, quercetin and its 3-O-rutinoside and 3-O-rhamnoside, and luteolin and its 7-O-glycoside (Vereskovskii et al., 1987). From the same species, four C-glycosylflavones, vitexin, isovitexin, orientin and isoorientin, were also found (Vereskovskii et al., 1987). Flavonols and C-glycosylflavones were isolated from the leaves of B. grandis Dryand (Karpova et al., 2016). and B. fischeri Schrank var. palustris (Hartw. ex Benth.) Irmsch. (Karpova et al., 2019). In Japan, two Begonia species, B. formosana (Hayata) Masam. and B. fenicis Merr. are growing in more southern area, Okinawa. Of their species, two flavonols, quercetin 3-O-rutinoside and kaempferol 3-O-gentiobioside, and five C-glycosylflavones, orientin, isoorientin, vicenin-2 and apigenin 6-C-hexoside-8-C-pentoside were isolated from the former species. On the other hand, quercetin 3-O-rutinoside alone was found in the latter species (Iwashina et al., 2008a). Common kaempferol and quercetin 3-O-glucosides were also isolated from the flowers of B. semperflorens Link & Otto, together with their aglycones (Kwon et al., 2019). Acylated flavonol glycosides, quercetin 3-O-(p-coumaroylglucoside) and 3-O-(p-coumaroyldiglucoside) were detected in the flowers of B. elatior, together with kaempferol and quercetin, and their 3-O-glucosides and 3-O-rhamnosylglucosides (Karima et al., 2017). As polymethoxylated flavonols, pachypodol (5,4'-dihydroxy-3,7,3'-trimethoxyflavone) and ternatin (5,4'-dihydroxy-3,7,8,3'-tetramethoxyflavone) were isolated from the leaves of B. glabra Aubl. (Ensemeyer and Langhammer, 1982). Other methoxylated flavonols, kaempferol 3-methyl ether 7-O-diglucoside and quercetin 3-methyl ether 7-O-diglucoside, were found in B. manicata (Brongn.) Klatzsch. (Harborne and Hall, 1964). Harborne and Hall (1964) reported the anthocyanins and flavonols from the leaves and flowers of 13 Begonia species. In the flowers and leaves of almost species, flavonols, e.g. quercetin 3-O-xyloside, 3-O-glucoside and 3-O-rutinoside, were isolated, together with anthocyanins such as cyanidin 3-O-sambubioside, 3-O-sophoroside and 3-O-xylosylrutinoside. However, flavonols were not found in B. coccinea Hook., B. × richmondensis and B. fuchsioides Hook. var. rosea (Harborne and Hall, 1964). As another flavonoid class, flavan and proanthocyanidins, (+)-catechin, (−)-epicatechin, and procyanidins B1 and B2, were reported from the leaves of B. glabra and B. fagifolia Fischer (Ensemeyer et al., 1980; Ensemeyer and Langhammer, 1984). However, in spite of Begonia is one of the largest genus of the plant kingdom, the reports of flavonoids are meager. In this experiments, foliar flavonoids of 122 taxa and three hybrids of the genus Begonia were surveyed and chemotaxonomically discussed. Additionally, the flavonoids of another genus Hillebrandia, which consists of only one species, H. sandwicensis, was also isolated.

Section snippets

Plant materials

Hillebrandia sandwicensis, and 121 species, one variety, one strain and three hybrids (total 127 taxa) of the genus Begonia were cultivated in the nursery of the Research Center of Biodiversity Academia Sinica, Taipei, Taiwan. Their origins and collection numbers are listed in Table 1. Voucher specimens are deposited in the herbarium of the Research Center of Biodiversity Academia Sinica (HAST), Taiwan.

General

UV spectra were measured with Shimadzu MPS-2000 multipurpose recording spectrophotometer

Identification of flavonoids

Twenty-one flavonoids were obtained from the leaves of Hillebrandia sandwicensis and Begonia spp. Of these compounds, 10 and 11 were flavonol O-glycosides and C-glycosylflavones, respectively (Fig. 1). Flavonoid 1 was identified as quercetin 3-O-rutinoside (rutin) and found in many species to be the major compound. Flavonoids 2, 3, 4 and 18 were identified as kaempferol 3-O-rutinoside, isorhamnetin 3-O-rutinoside, quercetin 3-O-glucoside and quercetin 3-O-rhamnoside, respectively. Flavonoid 5

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