Flavonoids in translucent bracts of the Himalayan Rheum nobile (Polygonaceae) as ultraviolet shields

Abstract

UV-absorbing substances were isolated from the translucent bracts of Rheum nobile, which grows in the alpine zone of the eastern Himalayas. Nine kinds of the UV-absorbing substances were found by high performance liquid chromatography (HPLC) and paper chromatography (PC) surveys. All of the five major compounds are flavonoids, and were identified as quercetin 3- O-glucoside, quercetin 3- O-galactoside, quercetin 3- O-rutinoside, quercetin 3- O-arabinoside and quercetin 3- O-[6"-(3-hydroxy-3-methylglutaroyl)-glucoside] by UV, 1H and 13C NMR, mass spectra, and acid hydrolysis of the original glycosides, and direct PC and HPLC comparisons with authentic specimens. The four minor compounds were characterised as quercetin itself, quercetin 7- O-glycoside, kaempferol glycoside and feruloyl ester. Of those compounds, quercetin 3- O-[6"-(3-hydroxy-3-methylglutaroyl)-glucoside] was found in nature for the first time. The translucent bracts of R. nobile accumulate a substantial quantity of flavonoids (3.3-5 mg per g dry material for the major compounds). Moreover, it was clarified by quantitative HPLC survey that much more of the UV-absorbing substances is present in the bracts than in rosulate leaves. Although the flavonoid compounds have been presumed to be the important UV shields in higher plants, there has been little characterisation of these compounds. In this paper, the UV-absorbing substances of the Himalayan R. nobile were characterised as flavonol glycosides based on quercetin.

The Bracts of Saussurea velutina (Asteraceae) Protect Inflorescences from Fluctuating Weather at High Elevations of the Hengduan Mountains, Southwestern China

“…Based on measurements on a single inflorescence on R. nobile in Nepal, Omori and Ohba (1999) reported that bracts on the inflorescence elevated the temperature of floral tissues and had insulative effects. In addition, the large bracts of R. nobile and R. alexandrae have positive effects on the control of sunlight penetration and protect the inflorescence from ultraviolet radiation (Terashima et al, 1993;Omori and Ohba, 1996;Omori et al, 2000;Tsukaya, 2002;Iwashina et al, 2004).…”

An unusual acylated malvidin 3-glucoside from flowers of Impatiens textori Miq. (Balsaminaceae)

“…To the best our knowledge, this is the first report of the presence of an anthocyanin acylated with 3-hydroxy-3-methylglutaric acid in plants, although there are some reports on the distributions of flavones and flavonols acylated with this acid, such as in Citrus (Kumamoto et al, 1985;Horie et al, 1986), Rheum (Iwashina et al, 2004), et al, 1986), Viscum (Harborne and Baxter, 1999) and Frullania (Kraut et al, 1993). The occurrence of a new type of acylated anthocyanin provides further --6 interesting information for researchers in this field of chemistry.…”

Partly transparent young legume pods: Do they mimic caterpillars for defense and simultaneously enable better photosynthesis?

“…A well-known case of translucent leaves in order to improve photosynthesis, to allow warming of the tissues and even to improve reproduction is that of the glasshouse plants of several Rheum species of the Himalayas. [19][20][21][22] While improving their survival and sexual reproduction by being translucent, these glasshouse plants decrease the danger of UV-B radiation damage by expressing high levels of flavonoids that absorb it. 21,22 Improving photosynthesis also seems to operate when the fast-growing young and partly transparent pods discussed here can perform photosynthesis in both external and internal layers.…”