Keywords

22.1 Introduction

Rheum spp.(Polygonaceae) as one of the oldest and frequently used herbal medicines is mainly distributed in the Asian countries such as China, India, Nepal, Korea, Bhutan, Pakistan , Turkey, Iran, Russia and Tibet [1,2,3]. In the Islamic traditional medicine (ITM ), various medicinal effects have been attributed to the Rheum genus such as healing liver, kidney, womb and bladder diseases, hiccups, diarrhea, constipation, insect bites and relapsing fevers [4, 5]. Several phytochemical studies have demonstrated that the main structures present in different species of this genus are anthraquinones, anthrones and different phenolic compounds such as stilbenes, flavonoids and tannins. These compounds have demonstrated a wide range of pharmacological activities including purgative [6], anti-cancer [7], anti-diabetes [8], anti-oxidative [9], hepatoprotective [10] and nephroprotective [11] in in vitro, in vivo and clinical studies. In the current chapter we present a general report on botany, phytochemistry, ethnobotany and pharmacological activities of Rheum genus.

22.2 Botany

22.2.1 Botanical Description

Rheum L. (Polygonaceae) is a species-rich genus, comprises a total of 44 accepted species (The Plant List, 2013). Rheum species, commonly called rhubarb , includes perennial, stout herbs with procumbent to erect basal leaves and heights range from the procumbent (R. palaestinum) to 2 m tall (R. palmatum and R. webbianum). The mountainous and desert regions of the Qinghai–Tibetan Plateau area (the highest and largest plateau in the world) and adjacent areas of central Asia are putatively centers of both origin and diversification of Rheum , owing to its extremely diversified morphology and high endemism at both species and section level [12]. It is suggested that the rich geological and ecological diversity of these regions, coupled with the habitat isolation due to oscillating climatic conditions during and after the uplifts of the plateau, might have caused the fast radiation and speciation of Rheum [13]. Furthermore, it seems that polyploidy have played an important role in driving diversification and speciation of this genus. More than 50% of taxonomically circumscribed species in Rheum taxa were involved in the polyploidy during their diversification histories [14]. The taxonomy of this genus remains complex, due to the convergent evolution and random fixation of unique morphological characters, which might explain the substantial inconsistencies among gross morphology, pollen exine pattern and trnL-F phylogeny within it [13]. The main habitats preferred by most Rheum taxa are cold and dry alpine meadow, steppe desert and dry slopes [15].

Rheum species are perennial plants, possess roots long, stout. Stem erect, hollow, sulcate, glabrous or strigose. Leaves basal and cauline, simple, sinuatedentate or palmate, the basal ones sparse, dense, or in a rosette, larger than the alternate cauline leaves, the latter sometimes lacking, ocrea usually large, membranous, margin entire, inflorescence simple or branched, usually paniculate, or spike-like or spherical, pedicel articulate, flowers bisexual or polygamo-monoecious, perianth persistent, tepals 6, stamens mostly 9 (6 + 3), rarely 7 or 8, styles 3, short, horizontal, stigmas inflated, recurved, achenes trigonous, winged [16].

According to the website ‘TPL’ (http://www.theplantlist.org.), there are 121 scientific plant names of species rank for the genus Rheum , of these 44 are accepted species names. Table 22.1 summarizes all synonyms of Rheum taxa.

Table 22.1 Scientific names and synonym(s) of reported Rheum species worldwide [according to The Plant List (2013)]

22.2.2 Authentication and Detection of Adulteration

Because of some morphological similarities of the plants and their misidentification by the vendors and consumers, the crude medicinal plants are often substituted or adulterated in commerce which may lead to poor clinical efficacy and adverse effects. In Iran, a number of Rheum taxa are traded in traditional medicine markets and shops such as R. palmatum, R. ribes and R. turkestanicum. Taxonomic assessment revealed that some of them should be considered as adulterated and substituted samples. For instance, R. ribes are admixed with R. turkestanicum and are sold in the market which degrades its quality and efficacy [17, 18]. In various parts of India, R. emodi are adulterated with R. webbianum and R. Spiciforme [19]. In Chinese markets, many adulterants include R. franzenbachii, R. undulatum, R. rhaponticum, Rumex crispus, and R. dentatus are commonly admixed with official Da-huang (the dried rhizomes and roots of R. palmatum, R. tanguticum, and R. officinale), because of similar morphological traits. The results of several previous studies have shown significant differences in the chemical composition of rhubarbs, and it is recommended that clinical practice should be performed for each species individually [20]. Moreover, due to the increasing demand both domestically and internationally and the short supply of official rhubarb , some Rheum taxa, like R. hotaoense has also been used as commercial substitutes in certain regions [21]. Therefore, it is essential to provide an authentic tool for realizing the distinction between different Rheum species and their adulterants. Authentication of these crude medicinal plants is very necessary since without correct identification, the efficacy and safety of products cannot be guaranteed.

22.2.3 Threat Categorization and Conservation Prioritization

Several Rheum species, particularly from Kashmir Himalaya are under tremendous risk and have been considered as “threatened” by several agencies such as International Union for Conservation of Nature (IUCN), United Nations Environment Programme (UNEP) and World Wide Fund for Nature (WWF) [22]. Multiple factors such as habitat loss and extensive collection from the wild have caused a significant decline in the natural resources of them. In India, various sub-endemic plant taxa belonging to the genus Rheum , were identified as threatened including R. moorcroftianum “critically endangered”, R. webbianum “Endangered” and R. australe “vulnerable”, due to a critical decrease in their population [23]. R. wittrockii is an endangered and rare species that grows in Kazakhstan. It is a very useful medicinal plant and is being used for cooking in a variety of dishes [24]. Some Rheum species, such as R. alexandrae and R. nobile are monocarpic perennial species, meaning that they only produce flowers once in a lifetime and are only reproduced through their seeds. These plants are “potentially endangered” and overexploitation of their wild resources should be forbidden [25]. Both of these rare noteworthy taxa-endemic to the high eastern Himalayas-are being used in traditional Tibetan medicine [26, 27]. Three other Rheum species including R. tanguticum, R. officinale and R. palmatum are endemic and “endangered” plant species that grow in China. In recent years, due to the overutilization and the loss of habitat of R. tanguticum, it has been named as “endangered” in the China higher plants endangered list [28, 29]. Moreover, R. palmatum and R. officinale wild resources have decreased due to a decrement in R. tanguticum natural resources, thus, both are considered as “threatened” taxa in China [30]. There is a strong need for conservation priorities and management strategies of such valuable Rheum gene pool through establishment of herbal gardens and medicinal plants nurseries for ex situ conservation, coupled with education and awareness programs for large-scale cultivation [23]. If overharvesting and habitat destruction of these valuable species continues, they may vanish from the area within a few years.

22.3 Phytochemical Constituents

According to previous studies, the most important chemical structures from Rheum taxa are anthraquinones, anthrones and phenolic compounds (stilbenes, flavonoids, phenolic glycosides, phenolic acids, cinnamic acid derivatives and tannins). These compounds have been classified in Table 22.2.

Table 22.2 The most important chemical structures isolated from different parts of Rheum spp.

22.4 Ethnobotanical and Ethnomedicinal Knowledge

The traditional uses of Rheum species in ethnomedicine mainly originate from Asia and Europe. 25 species of Rheum have been reported to be beneficial among which R. australe, R. palmatum, R. ribes and R. webbianum have the highest number of citations in the world. Different parts of Rheum taxa including roots, petioles, fruits and seeds have been used as ethnomedicine for a long time. There are some reports on the traditional uses of R. palmatum and R. rhaponticum in European countries, particularly Bulgaria and Spain. In these countries, the roots are the most used part for treating fever, heart problems, stomachache and jaundice [60, 61]. Stems of R. rhabarbarum commonly known as “Rhabarber” have been recommended as depurative in Germany [62]. In Kazakhstan, R. wittrockii was used by Kazakhs against gastro-enteric and skin ailments. Moreover, R. altaicum was advised as an anti-inflammatory and for treating skin problems. Stalks of R. compactum and R. wittrockii were eaten by local people [63].

China represents most of the distribution range of the Rheum taxa in the world [16]. Many species of this genus are used in traditional Chinese medicine and many reports are found highlighting their traditional and ethnomedicinal applications. Among them, R. officinale, R. palmatum and R. tanguticum are the official rhubarbs. In Chinese markets, dried roots and rhizomes of R. tanguticum and R. palmatum are called “north rhubarb ”, while that of R. officinale are called “south rhubarb ” [28, 29]. These taxa are known for their purgative, anti-bacterial, astringent, anti-carcinogenic, and stomachic properties [64]. Further, the roots and rhizomes of R. palmatum-commonly known as Zhang Ye Da Huang or Chinese rhubarb-have been recommended to treat abdominal distension, constipation and stomach pain [65]. Also, the roots of R. officinale, known as Da Huang, have been recognized as a wound healing agent and purgative [66, 67]. Leaf petioles of R. acuminatum, R. austral, R. globulosum, R. inopinatum, R. lhasaense, R. palmatum, R. pumilum, R. rhomboideum and R. tanguticum are used as condiment in China [68].

In Nepal, the crushed and boiled roots of R. acuminatum and R. australe are used for indigestion, menstruation problems and blood purification. Besides, the roots paste is applied externally on fractured and broken bones. The petioles of R. acuminatum have been used to treat diarrhea, constipation, cold, cough and headache and also to make pickles [69].

In Iranian traditional medicine, people use the roots of R. turkestanicum to treat diabetes, hypertension and cancer [70]. The roots of R. palmatum are traditionally used for liver diseases, constipation, and backache and as cardiac tonic, appetizer and anti-lithiatic. Moreover, different parts of R. ribes (Persian name: Rivas) are used for various ailments of human and animals.

Fresh young stems of R. tibeticum -known as Sheepod in Pakistan- are being used as vegetable and mild laxative agent [71]. In the Pakistan folk medicine, the powdered rhizomes of R. australe are used topically for treating wounds and orally for curing constipation [72]. Furthermore, different parts of R. spiciforme have been recognized for curing digestive disorders and as blood purifier and tonic for livestock [73].

In India, R. australe has been used to treat abdominal pain and constipation, loss of appetite, asthma, bronchitis, fever, cuts, dysentery, eye disorders, sprains, swellings, ulcers and wounds. R. moorcroftianum has been recommended to treat colds and internal injuries. Rheum webbianum has been advised as an astringent agent and purgative and is used for curing wounds, abdominal disorders and boils [23]. The most frequent traditional uses of Rheum taxa in different countries is to treat gastrointestinal diseases, skin problems, abdominal complaints, kidney ailments, jaundice, diabetes, bronchitis, worms and boils. Apart from their medicinal applications, several species of Rheum are used as natural dyes. The roots of R. acuminatum, R. austral, R. moorcroftianum and R. webbianum are good sources to obtain yellow color which is used in cosmetics, textile industry or as a food colorant [68, 69, 74].

22.5 Nature of Rheum spp. Described in ITM

In Islamic Traditional Medicine (ITM) four genus of Rheum have been described including R. palmatum (Râvand Sini), R. rhaponticum (Râvand Shâmi), R. turkestanicum (Râvand Torki) and R. ribes (Ribâs). The temperament (Mizaj) of R. palmatum, R. rhaponticum and R. turkestanicum are mentioned as warm and dry in second degree, but R. ribes is described as cold and dry in second degree (Table 22.3). In different traditional references, Rivand is mentioned as resolving (moḥallel), cutting (moqaṭeʿ), deobstruent (mofaṭeḥ), purifying (monaqi), attenuating (molaṭef), and also astringent (qabeḍ). Ribas (R. ribes) is attenuating, astringent and gastrointestinal tonic [75].

Table 22.3 Rheum spp. used in ITM

22.6 Medicinal Uses of Rheum spp. in ITM

The different properties of Rheum spp. are categorized according to the organs of the body on which they exert their effects.

22.6.1 Respiratory System

One of the most important therapeutic applications of R. palmatum and R. rhaponticum in ITM is respiratory problems. These rhubarb species are considered as expectorant, corrosive moisture desiccant and anti-cold cough. They also improve asthma, hemoptysis and orthopnea [4, 5, 75]. In contrast, R. ribes is considered as a harmful plant for respiratory system in ITM [5].

22.6.2 Central and Peripheral Nervous System

Rheum palmatum and R. rhaponticum are known as good remedies for brain purgation, headache and memory [4]. It has been reported that R. ribes is deleterious for central and peripheral nervous system [5].

22.6.3 Liver, Kidney and Spleen

Avicenna, in his book (Canon), together with several other scientists such as Ibn Al-Baytâr have recommended rhubarb preparations as anti-weakness and analgesic of liver and kidney [4, 5]. It is liver deobstruent and is able to break kidney and bladder stones. It also removes rigidity and edema from spleen and is useful in treating dropsy [5, 75].

22.6.4 Gastrointestinal System

According to the studied texts, rhubarb is very useful for internal organs, especially stomach and liver. It is believed that all of the species can attenuate stomach problems [4, 5, 75]. A dense extract of R. ribes is used to reduce thirst and to cure nausea and hemorrhoids. The astringent property of R. ribes seeds can be effective in curing diarrhea. The other species of Rheum can be beneficial in treating flatulence, hiccups, diarrhea, constipation and intestinal ulcers [4, 76].

22.6.5 Skin

In ITM , rhubarbs have a good potential for eliminating skin freckles and lentigo as well as healing dermatitis and psoriasis [4, 75]. Rheum ribes is used for erysipelas, rosacea acne and herpes in a preparation with barely (Hordeum vulgare Linn) flour [75].

22.6.6 Reproductive Organs

Rheum taxa. Except R. ribes, reduce libido and remove the womb pain [75].

22.6.7 Joints and Muscles

Rheum spp. except R. ribes are used as anti-sciatic and analgesic. They also improve muscle weakness and nerve rupture [4, 75].

22.6.8 Heart

According to ITM, R. ribes can empower the principle organs of the body meaning heart, brain and liver. It can also improve tachycardia [4].

22.6.9 Mental Properties

Rheum ribes is anti-depressant and exhilarant. Moreover, it attenuates melancholia and misanthropy [76].

22.6.10 Bacterial Infections

Rhubarbs are mentioned as anti-recurrent fevers in ITM [5]. Also, R. ribes is considered as an anti-smallpox and anti-measles herb [4].

22.6.11 Other Properties

Topical use of R. ribes extract is useful for improving eyesight [5, 76].

22.7 Pharmacological Aspects

At present, various pharmacological activities of Rheum species including anti-cancer, anti-viral, anti-fungal, anti-bacterial, anti-oxidative, anti-dermatitis, hypoglycemic, hypolipidemic and kidney and liver protective have been shown in different in vitro, ex vivo and in vivo studies (Tables 22.4, 22.5, 22.6). The effect of different preparations of rhubarb in treating atherosclerosis, acute bleeding of the upper gastrointestinal tract, constipation, dysenteric diarrhea and depression has been demonstrated in some clinical trials as well (Table 22.7).

Table 22.4 In vitro studies of Rheum spp.
Table 22.5 Ex vivo studies of Rheum spp.
Table 22.6 In vivo studies of Rheum spp.
Table 22.7 Clinical studies of Rheum spp.

22.8 Conclusion

Traditional medicine around the world plays an important role in exploring new drugs. Utilizing from accurate instructions of famous scientists, ITM texts are valuable sources for detecting new drugs. In this review, the applications and lucrative properties of Rheum spp. were investigated in ITM books, and adapted with the results reported in pharmacological studies.

Rhubarb is said to possess a wide range of therapeutic applications in the traditional and folklore medicine such as healing gastrointestinal, liver, kidney, womb and bladder diseases, diarrhea and constipation [4, 5], skin problems, diabetes, bronchitis and boils. The medicinal effects of rhubarb may owe to the several chemical compounds present in this plant specifically anthraquinones, anthrones and different phenolic compounds such as stilbenes, flavonoids and tannins. The effect of this herb in treating both constipation and diarrhea is due to anthraquinones and tannins, respectively. At low doses, rhubarb is said to act as anti-diarrheal, while at higher doses it is cathartic [148]. Many effects of rhubarb including anti-cancer, anti-microbial, anti-oxidant, anti-diabetic, anti-dyslipidemic, anti-pigmentation, nephroprotective, hepatoprotective and immunoprotective have been shown in in vitro and in vivo studies. In addition, some clinical trials have indicated the efficacy of this plant in treating atherosclerosis, gastrointestinal bleeding, diarrhea and dysmenorrhea which is in agreement with the traditional and folklore medical applications of rhubarb . The results of previous studies have shown significant differences in the chemical composition of Rheum genus, and it is recommended that clinical practice should be performed for each species individually [20]. Though many pharmacological activities of Rheum spp. have been investigated, further studies especially clinical trials are needed to fill the present gaps in our knowledge of different aspects of this plant. According to the potency of rhubarb in treating different diseases, the demand for this plant in international and domestic markets is growing leading to excessive exploration and a sharp drop in the wild resources of rhubarb as well as damage to the wildlife [149]. Several Rheum species are under immense risk and have been considered as “threatened” and it is of great importance to stop overharvesting and habitat destruction and establish rhubarb gardens in combination with education and awareness programs for large-scale cultivation and conservation of this genus [23].

Taken together, rhubarb is a valuable medicinal plant that has been useful in treating several diseases for centuries. Due to the different therapeutic effects mentioned in the ITM texts and folklore medicine as well as various pharmacological and clinical studies and notable phytochemicals isolated from Rheum spp., this genus is highly recommended to the herbal pharmaceutical industry for manufacturing several oral and topical formulations beside exploring new lead compounds and drugs for treating skin, gastrointestinal, metabolic and reproductive diseases.