Rheum palmatum (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Rheum palmatum L.

Protologue: Syst. nat. ed. 10: 1010 (1759).

Family: Polygonaceae

Chromosome number: 2n= 22

• Chinese rhubarb, medicinal rhubarb, Turkish rhubarb (En).
• Rhubarbe de Chine (Fr)
• Indonesia: kalembak (Sundanese), kelembak (Javanese), talembak (Madurese)
• Malaysia: kelembak
• Vietnam: dại hoàng chân vịt.

R. palmatum originates from the highlands of most of northern and western China and Tibet, and is nowadays widely cultivated in China and Russia, and to a lesser extent in Europe and the United States, for medicinal and ornamental purposes. In Java, attempts were made to cultivate R. officinale rather than R. palmatum in the 19th Century, but it developed roots of very inferior medicinal value. It is still found here and there in the highland area.

The short, thick rhizomes, and sometimes the smaller roots of R. palmatum but also of the closely related R. officinale Baillon are used widely in China for laxative, tonic, astringent, anti-inflammatory, anti-hypertensive and antitumour purposes, mostly in decoction or as an infusion. In weak doses, they are astringent and a tonic, and when more concentrated, they are laxative and purgative. The rhizomes are employed in a pure form or as part of a mixture against constipation, dysentery, swollen gums, sore throat, sores, furuncles, burns, jaundice, strangury and after childbirth. The dried rhizomes are widely found in Chinese herbalist shops in South-East Asia.

In the European Pharmacopoeia, medicinal rhubarb was already known as a laxative in the Middle Ages, when it was introduced from Russia. It was also well known to the Arabs. The rhizome is taken orally for occasional constipation, but not recommended for children. It should not be taken in cases of intestinal obstruction, stenosis, atony, inflammatory colon diseases, appendicitis and haemorrhoids. Long-term use should be avoided, as it may result in aggravation of constipation, in albuminuria, liver damage and haematuria. There is insufficient information on damaging or undesirable effects during pregnancy, and the excretion of metabolites in breast milk. A laxative effect in breast-fed babies is not known. A harmless side-effect is the yellow or red-brown discolouration of the urine by metabolites. In case of an overdose, the major symptoms are gripes and severe diarrhoea, and fluids and electrolytes should be replaced. Potassium shortage may be responsible for symptoms such as decrease in muscle activity and cardiac arrhythmia. In Europe and the United States, rhizome extracts are often constituents of slimming cures, spring time tonics and blood purifying teas. The leaf blades of Rumex are toxic, because of the high levels of oxalic acid, which can interact with blood calcium. Precipitation of calcium oxalate in the renal tubules can lead to renal failure. There is some prejudice against the use of Rheum because it is said to cause rheumatism. This however, is due to its astringent action, stirring up possible poisons in the blood before eliminating them.

In Java, the rhizomes of R. palmatum (or R. officinale) are used in a cosmetic applied for freckles. In Central Java, rhizome shavings are used in a mixture with tobacco and Sumatran benzoin (from Styrax benzoin Dryand.) in cigarettes, known as "rokok kelembak manyan".

The fleshy petioles of R. × cultorum Thorsrud & Reisaeter or garden rhubarb (parental species: R. rhabarbarum L., R. rhaponthicum L. and/or R. palmatum), widely cultivated in cooler regions in the tropics, are mainly used as a vegetable; the rhizomes are of inferior medicinal value. The petioles of R. officinale are sometimes used as a vegetable.

R. palmatum is also cultivated as an ornamental, especially those forms with striking red inflorescences.

The rhizomes of R. palmatum (and R. officinale) are regularly imported into South-East Asia from China. China and Russia are the main producers of R. palmatum roots, Japan and Korea are minor producers.

According to the definition of the European Pharmacopoeia, "Rhei Radix" consists of the whole or cut rhizomes of R. palmatum or R. officinale, of hybrids of these two species or a mixture. The smell is pleasant, and the taste is sweetish, astringent and bitter at the same time. It contains about 2.2% of 1,8-hydroxyanthracene derivatives, consisting mainly (60-80%) of monoglucosides of rhein, chrysophanol, aloe-emodin, physcion and emodin. Dianthrone glycosides are also present together with free aglycones and a small amount of anthrone glycosides, depending on the time of harvesting and the conditions of drying. The anthracene derivatives may occur in various oxidation stages; anthraquinones can be reduced to anthrones, which may be oxidized to dianthrones. Dianthrones, on the other hand, can be reduced back to anthrones, which may oxidize to anthraquinones relatively easily. The rhizomes also contain about 1% gallotannines.

The 1,8-hydroxyanthracene derivates possess a laxative effect, which may be based on 2 distinct mechanisms, (1) an influence on the motility of the large intestine, resulting in accelerated colonic transit, thus reducing fluid absorption and (2) an influence on secretion processes, resulting in enhanced fluid secretion. Defecation takes place after a delay of 8-12 hours due to the time taken for transport to the colon and metabolic conversion of hydroxyanthracene glycosides by the bacteria of the large intestines to the active forms, the anthrones. Laxative drugs containing anthraquinone derivatives should be used with caution, however, as daily and prolonged use can lead to a dependence syndrome and "cathartic colon".

In vitro studies have demonstrated antimutagenic and anti-oxidant properties of anthranoid constituents. Antimicrobial properties were found as well, against Bacteroides fragilis and Trichomonas vaginalis in mice. The extract has also been reported to decrease blood urea in rats with chronic renal failure. A water extract of the fruits showed strong suppression activity on hepatitis B virus surface antigen, and was also active against herpes simplex virus 1 and influenza viruses.

There are no studies on single dose toxicity, repeated dose toxicity, reproductive toxicity or carcinogenicity of the preparations of "Rhei radix". Some isolated anthraquinones (aloe-emodin, emodin, physcion and chrysophanol) gave positive results in in vitro genotoxicity studies, but all in vivo genotoxicity studies were negative. The anthranoids in their anthrone form inhibit cell growth and thymidine incorporation in human cultured cells, and inhibit both DNA replication and repair synthesis, through an imperfectly understood mechanism. Because of these effects on the cell cycle, anthrones are used locally in the treatment of psoriasis and other skin infections.

Hot water extracts of R. palmatum rhizomes showed molluscicidal activity against the snails Oncomelania hupensis, Biomphalaria glabrata and Bulinus globosus, which are vectors of Schistosoma japonicum, S. mansoni and S. haematobium respectively. Activity was highly correlated with the anthraquinones rhein and the anthrone chrysophanol as a 0.03% solution killed 50% of the snails after 2 days. The extract of the rhizomes has a controlling effect of 50-67% on tomato mosaic virus, if sprayed 24 hours before or immediately after inoculation.

Anthraquinone glycosides with similar uses as those found in Rheum are found in ripe pods of Senna and Cassia, and in Aloe leaves. Psyllium (Plantago spp.), a natural bulk laxative that does not cause dependency or "cathartic colon", is better suited for prolonged use. It is for this reason one of the substitutes of anthraquinone-containing laxative drugs.

The action of R. palmatum differs from other laxatives in that the tannin present may exert an astringent action after purgation.

• A large, erect, perennial herb, up to 2.5 m tall; rhizome and roots thick, branched, almost fleshy, bright yellow.
• Leaves partly in a radical rosette, partly spirally arranged on an erect stem; leaves of rosette borne on a thick, subcylindrical petiole, up to 30 cm long, blade orbicular, 50-90 cm × 50-70 cm, base cordate, deeply palmately lobed, 3-5-veined, lobes ovate-oblong or lanceolate, apex acute, entire, dentate or pinnatifid, upper surface normally rough; ocrea large, membranaceous.
• Inflorescence a loose, hairy panicle with racemiform branches, foliated to the tip.
• Flowers bisexual, fascicled; tepals in 2 whorls of 3, mostly red, sometimes pink or whitish, little or not accrescent after anthesis; stamens 9; ovary trigynous, styles 3, recurved, stigmas thick.
• Fruit a trigonous nutlet, 10 mm × 8 mm, brown, much longer than perianth, 3-winged, scarious.

The rhizome of R. palmatum measures 10-15 cm in diameter after several years. In Russia, R. palmatum flowers in July; in Europe, from May to June. Rheum L. is wind pollinated.

Several subspecies are distinguished in R. palmatum: subsp. palmatum is cultivated as an ornamental in Europe and leaves of R. palmatum subsp. tanguticum (Maxim.) Stapf are more elongated and less deeply lobed than those of subsp. palmatum, and more often cultivated as a medicinal in Europe.

R. officinale is closely related to R. palmatum, and their rhizomes are used indiscriminately. R. officinale is more robust than R. palmatum, the leaves are 30-100 cm across, 3-7-veined, and generally less deeply lobed; the flowering stem is up to 3 m tall, much branched, tepals greenish, nutlets reddish. It is widely cultivated in China where it is now running wild, and is sometimes cultivated in West and Central Europe. R. officinale occurs naturally in the highlands of western China and eastern Tibet, and towards the South touching the distribution area of R. palmatum.

R. palmatum grows in scrub and grassland of highlands, at 2500-4400 m altitude.

R. palmatum is mainly propagated by rhizome cuttings. R. palmatum can also be quickly multiplied through meristem tips, grown on a Murashige and Skoog (MS) medium, containing naphthalene acetic acid and benzylaminopurine (BAP), where shoots are formed. Subsequent transfer of the shoots to a BAP free medium resulted in root development within 3 weeks, and the shoots could then be transplanted to peat pots.

Biosynthesis of anthraquinones in cell cultures of R. palmatum was established on MS medium supplemented with 2,4-D and kinetin. However, it was difficult to obtain high yields of specific compounds, and even where yields approach or exceed those found in whole plants, in many of these cultures only oxyanthraquinones and dianthrones are produced and the pharmacologically more potent dianthrone glycosides (sennosides) are peculiarly absent. A MS medium, at pH 5.5-5.8 and in darkness, stimulated hairy root growth and biosynthesis of free anthraquinones. The addition of auxin, however, inhibited the biosynthesis of these compounds. The highest yield of sennosides A and B (0.0083% and 0.004%, respectively) was obtained from callus from cut portions of a hybrid between R. palmatum and R. coreanum Nakai, on a MS medium containing 20 g maltose/l, 1 ppm indole acetic acid and 1 ppm 4-PU-30.

In tests in Germany, medicinal rhubarb plants produced less active compounds when nutrients were added. The 1,8-hydroxyanthracene derivate concentration was highest in untreated plants.

Rheum is attacked by a brown spot disease, caused by Ascocyta rhei. Rheum leaves are sometimes damaged by leaf-eating insects and mites and in Russia the damage can be considerable.

Rhizomes of cultivated and wild plants are dug up. In China, the rhizomes of R. palmatum are harvested in September or October, sometimes around May, 6-10 years after planting.

In China, rhizome yield of R. palmatum can be as high as 8-12 kg of rhizomes per plant.

The rhizomes of R. palmatum are peeled, and either sliced in small pieces and dried in the sun or oven, or dried as a whole, and cut into pieces after drying. The dried pieces are very hard, with a smooth surface, pale yellow, with a whitish netting pattern. Before use, the pieces are powdered.

A small genebank collection of R. palmatum and R. officinale is kept in Gatersleben, Germany. Wild Rheum plants are possibly threatened by genetic erosion because plants are widely harvested in the mountainous areas of China and Tibet. Breeding programmes for medicinal purposes exist mainly in China and Russia, but other countries, such as New Zealand, are showing some interest as well. Breeding for ornamental purposes is mainly done in Europe and the United States.

"Rhei radix" is a well-known medicine still in use, and present in many Pharmacopoeias worldwide. The potential for cultivation of R. palmatum in some high land regions in South-East Asian countries needs further investigation.

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• G.H. Schmelzer & S.F.A.J. Horsten