Dioscorea (PROSEA Medicinal plants)

Plant Resources of South-East Asia
Introduction

Protologue: Sp. pl. 2: 1032 (1753); Gen. pl. ed. 5: 456 (1754).

Family: Dioscoreaceae

Chromosome number: x= 10 (Old World); D. deltoidea: 2n= 20, 40; D. japonica: 2n= 40, 80; D. prazeri, D. zingiberensis: 2n= 20

Major species

Dioscorea deltoidea Wallich ex Kunth,
D. prazeri Prain & Burkill,
D. zingiberensis C.H. Wright.

Vernacular names

Yam (En).
Igname (Fr)
Indonesia: uwi (Javanese), huwi (Sundanese)
Malaysia: ubi
Papua New Guinea: yam
Philippines: yam, ubi.

Origin and geographic distribution

Dioscorea probably originated in the Far East, but spread in early times to the Old and New World, and no overlap occurs now between the species of both worlds. About 600 species are known, 850 according to some authors, distributed through the tropics and warm temperate zones of the world. About 60 species are cultivated or gathered for their edible tubers, and of the 60 Dioscorea species occurring in South-East Asia, about 8 have medicinal use or are poisonous.

Uses

The rhizomes or tubers of many Dioscorea spp. are used in traditional medicine, and some wild species, such as D. deltoidea, D. prazeri, D. tokoro Makino and D. zingiberensis from the Old World, and D. floribunda M. Martens & Galeotti from Central America and Venezuela, are commercially exploited because they contain diosgenin, which can be used as a precursor in the production of oral contraceptives, sex hormones and corticosteroids. In most of their distribution area, D. deltoidea, D. prazeri and D. zingiberensis have no local medicinal use, but are collected or cultivated for cash income.

In India, the tubers of D. deltoidea, D. prazeri and D. tokoro are used as soap, in particular for washing the hair to kill lice, and also for washing silk and wool. The flesh of the rhizome of D. prazeri is very poisonous and thus also used as fish poison. The tuber of D. zingiberensis is used by the Chinese as a cooling medicine.

In Indonesia and China, the grated tuber of D. hispida Dennst. is applied for beginning leprosy, skin diseases and corns, calluses and whitlow of feet. It is also applied on syphilitic sores, together with the tuber of Smilax china L. In Thailand, slices of the tubers of D. hispida are also applied topically to relieve abdominal spasms and colic, and to remove pus from wounds. In the Philippines and China, the tuber is used for arthritis and rheumatism, and for cleaning maggot-infested wounds of animals. The tuber is considered very poisonous, and used for stupefying fish or poisoning arrowheads. A piece of tuber the size of an apple is sufficient to kill a man in 6 hours, the first effects being a feeling of discomfort in the throat, which intensifies to a burning sensation, followed by giddiness, the vomiting of blood, a sensation of suffocation, drowsiness and exhaustion.

In Indonesia and Peninsular Malaysia, the white rhizomes of D. pyrifolia Kunth and the pink to red rhizomes of D. laurifolia Wallich ex Hook.f. are collected and eaten, but also used for poulticing sores, swellings and insect bites. In Indonesia, the stems of D. pyrifolia are used for tying purposes. D. piscatorum Prain & Burkill from the hills of Peninsular Malaysia, Sumatra and Borneo, has rhizomes with dark red flesh, which are very poisonous and are not eaten by wild pigs or other animals. Man is said to eat them roasted, as heat destroys the poison, but if boiled or inadequately prepared they remain bitter. In Peninsular Malaysia, the rhizomes are used to stupefy fish. In the Philippines, the tuber of D. filiformis Blume is eaten with the peel, or a decoction is drunk, to cure malaria.

In Korea, China, Japan and Indo-China, the tuber of D. japonica Thunb. is taken as a tonic and used for the treatment of indigestion, intestinal troubles like dysentery and diarrhoea, and night disturbances of the urogenital system. The mucilage from the freshly crushed tuber is applied to treat intestinal pain caused by stoppage or inflammation; the dried tuber is used as an emollient for boils and abscesses.

In Japan, China and Vietnam, the tuber of D. tokoro is used as a fish poison, as a mild diuretic in treating arthritis and rheumatism, and as a strong resolvent of blood clots.

Many Dioscorea spp. are primarily cultivated for their edible tubers, but some of them have some secondary use as medicinals. Often the tubers or rhizomes contain the poisonous alkaloid dioscorine, and may therefore only be eaten after having been soaked in running water, boiled or baked. The toxic substances from the tubers can cause palpitations, nausea, vomiting, throat irritations, sweating, blurred vision and unconsciousness.

In China, the tubers of both cultivated and wild D. bulbifera L. are considered bitter, cooling, diuretic and antidotal. They are used for boils, swellings, sore throat and snakebite. In the Philippines, the powder obtained from scraping the bulbils is valued for rubbing on abdominal oedema. In Papua (Indonesia), the plant is used to treat diarrhoea. In Indo-China, the tuber of D. alata L. is eaten during convalescence from phthisis, kidney and spleen disorders. In Colombia, the leaves are applied as poultices on pimples and tumours, and in baths to relieve various skin irritations and bites from centipedes. The starch from the tuber is applied as a paste to haemorrhoids. In Costa Rica, an infusion of the leaves is drunk as a sudorific, febrifuge and digestive stimulant. The tuber of D. hispida is also eaten as a famine food, after boiling properly as careless washing can cause poisoning. Piper lolot DC. is believed to be a counter-poison for D. hispida.

Production and international trade

In Peninsular Malaysia and India, the tubers of D. japonica and D. zingiberensis are imported from China. In China and India, the tubers of D. deltoidea, D. prazeri and D. zingiberensis are collected or cultivated for the pharmaceutical industry, but statistics are scarce. In China in 1995, about 35 000 t of D. zingiberensis tubers were collected for diosgenin extraction.

In Mexico, the export of Dioscorea spp. as a source of steroid precursor, was important until a law was introduced allowing only extracts to be exported. The high cost of the fabrication of the extract caused manufacturers to focus on β-sitosterol, the by-product of vitamin E production.

Properties

In general, the rhizomes, tubers, or sometimes the seeds or the whole plants of about 50 Dioscorea species contain tannins, saponins and alkaloids. However, only about 20 species contain enough diosgenin (at least 2%) to be of economic importance, e.g. D. deltoidea, D. prazeri, D. tokoro, D. zingiberensis. Dioscorea spp. containing high levels of diosgenin and pennogenin are D. composita Hemsl. and D. floribunda, both originating from the New World.

The steroidal sapogenins isolated from medicinal Dioscorea have an additional ring added to the steroid backbone that makes them particularly similar to human sex hormones. Cortisone and hydrocortisone are two other important hormones that can be synthesized using diosgenin as starting material. They are used for the treatment of severe allergenic reaction, for arthritis and Addison’s disease, caused by malfunction of the adrenal glands. In the rhizomes and tubers, diosgenin is present in the form of its glycosides: dioscin, gracillin, or both. Dioscin is composed of 2 molecules of L-rhamnose and 1 molecule of D-glucose while gracillin contains 2 molecules of D-glucose and 1 molecule of L-rhamnose. Diosgenin causes a general paralysis of the central nervous system.

The compound dioscorine is also present in D. bulbifera and D. pentaphylla L., and to a lesser extent in some races of D. alata. It has not been detected in D. esculenta (Lour.) Burkill.

The tubers of D. deltoidea contain, besides diosgenin, also the steroid glycosides deltoside, oligofurostanosides and deltonine, while the leaves contain deltofolin, an acylated spirostanol saponin.

Treatment of tomato and cucumber plants with furastonol glycosides obtained from cell cultures of D. deltoidea decreased their susceptibility to infection by the nematode Meloidogyne incognita, while the fecundity of the nematode decreased 5-fold, females were smaller and the sex ratio shifted towards an increase in males.

The tubers of D. hispida contain the toxic isochinuclidine alkaloid dioscorine and also dioscoricine. The first compound is bitter and behaves like picrotoxin, producing paralysis of the central nervous system.

The rhizomes of D. prazeri also contain prazerol, prazerigenin-D-glycosides, prazerigenin A-3-O-α-L-rhamnopyranosyl-(1->6)-β-D-glucopyranoside and prazerigenin A-3-O-β-D-glucopyranoside. The tubers of D. japonica contain allantoin, arginine and choline, and also the glycans dioscoran A-F, which exhibit significant hypoglycaemic activity in normal and hyperglycaemic mice. A methanol extract of the aerial parts, containing (+)-β-eudesmol and paeonol, showed a suppressive, antimutagenic effect on umu gene expression of the SOS response in Salmonella typhimurium TA1535/pSK1002 against the mutagen furylfuramide.

The rhizomes, female flowers and seeds of D. tokoro also contain other steroid saponins and their genins: dioscin, yononin, tokoronin and kogagenin.

Adulterations and substitutes

Fenugreek seed (Trigonella foenum-graecum L.) and the related T. corniculata (L.) L. contain 0.5-2% diosgenin and small amounts of similar steroids, and could therefore be a useful commercial alternative source of diosgenin. Derris elliptica (Wallich) Benth. is a more stable substitute for Dioscorea piscatorum as a fish poison, and also as a pesticide to kill parasitic worms in rice.

Description

Dioecious perennial plants, with rhizomes, or with large and fleshy tuber(s), lignified or renewed annually, varying greatly in number, size and form; roots thick and unbranched or thin, fibrous and branching, sometimes short and spiny; stems climbing and twining, 2-12(-40) m long, tough, often woody at the base and winged with longitudinal ridges.
Leaves usually alternate, simple, base cordate, apex acuminate, pointing downwards, palmately veined, or palmately compound with 3-5 lobes or leaflets pinnately veined; petiole with a pulvinus at both sides. Axillary buds often more than one per axil, arranged vertically, with the youngest lowest, sometimes developing into a bulbil.
Inflorescence axillary, unisexual, racemose or spike-like.
Male flowers usually small, green, opening slightly at anthesis; perianth in 2 whorls of 3 segments; stamens 6, all fertile in South-East Asia. Female flowers usually produced in much smaller numbers; perianth lobes fleshy, opening more widely at anthesis; ovary 3-locular, inferior; stigmas 3.
Fruit a 3-locular capsule, 1-3 cm long, 3-winged or strongly angled, dehiscent.
Seed flattened, margins partly or completely winged.

Growth and development

Pollination of Dioscorea is effected by small, night-flying insects. The pollen is glutinous and often sweetly scented.

Other botanical information

The taxonomy of Dioscorea is confusing, because the group is very closely knit, and a general review is missing. A good review needs to include in order of importance: underground parts, direction of twining, wings, colour and shape of the seeds, position of the capsules, degree of segregation of male and female flowers, and characters such as hairs and glands.

Ecology

Most Dioscorea are confined to the tropics and do not tolerate frost, although some are montane and are tolerant to low temperatures. In order to produce good tubers, Dioscorea spp. need fertile, loamy, and well-drained soils, as they do not tolerate waterlogging.

Propagation and planting

Many wild Dioscorea reproduce by seed, and also vegetatively by rhizomes or tubers and/or bulbils. The seeds are dispersed by wind. Bulbil dormancy is most pronounced immediately after harvest and declines with time in storage. D. deltoidea exhibits no dormancy when planted in evergreen tropical regions, and continues growing and flowering throughout the year. D. deltoidea is best propagated vegetatively, through tubers or bulbils, as the plants grow faster and yield more diosgenin.

Stem cuttings of Dioscorea do not normally develop adventitious roots, but in some species a mass of tuberous tissue, or small bulbil-like organs, develop in the leaf axils of the cuttings and may be used for propagation. This method is not used in economic production, although it is used for D. floribunda grown for diosgenin.

In vitro production of active compounds

Tissue cultures of several Dioscorea spp., but especially D. deltoidea, were tested for their diosgenin content. D. deltoidea contained (0.2-)1.6-3.8(-8)% diosgenin in suspension cultures, and (0.3-)1-2.5% in callus cultures, D. zingiberensis contained 0.4% diosgenin in callus cultures, D. composita contained 0.02-0.06% diosgenin in callus or suspension cultures, and D. floribunda contained 0.1-1.3% diosgenin in callus cultures. D. deltoidea cells grown in continuous culture, showed a time-dependent concentration of furostanol glycosides, and when the concentration approached a maximum value, cell viability declined rapidly.

Husbandry

Dioscorea has specific growth requirements in order to ensure the production of a high level of diosgenin in tubers or rhizomes. Waterlogging is harmful and too high soil temperatures are disadvantageous. D. zingiberensis accumulates most diosgenin when grown in the (semi-)shade, at a rainfall level of 800-900 mm/year, and a mean annual soil temperature of 15-17°C.

In Pakistan, pieces of the tubers of D. deltoidea of 7 cm long and weighing 25-30 g were planted under different growth conditions and time of season, in thin forest and left for 2-10 years. Tuber yield increased up to 6 years after planting, and decreased afterwards. In India, monsoon planting gave more sprouts and a higher yield than autumn planting and the application of N and P fertilizer increased yields. In another experiment, maximum tuber yield was obtained with a low level of nitrogen and high levels of potassium and phosphorus, and no significant difference in percentage of diosgenin yield was obtained. In India, D. deltoidea was successfully grown on ridges, with shade provided by Sesbania aegyptiaca (Poir.) Pers., in sandy loams.

In India, diosgenin content in the tubers of D. deltoidea was found to be positively correlated with leaf width and negatively correlated with petiole length, both under natural conditions of growth and under cultivation.

In India, several field trials were effected at 100, 500, 1000 and 1500 m altitude with 2-year-old plants of D. prazeri and D. composita. Removal of flowers and fruits resulted in a higher diosgenin content in the tubers as was the case with foliar applications of KNO₃, P, Mg+Mn+B, gibberellic acid or indole acetic acid. Application of a base fertilizer with N and K inhibited diosgenin formation. Altitude did not have any effect on diosgenin content between 100 and 1000 m, but it increased at 1500 m altitude.

Diseases and pests

Diosgenin content in tubers of D. composita, D. floribunda and D. prazeri, infected by the tuber rot-causing fungus Fusarium solani, increased during the early phase of infection and then decreased with incubation time. Nematodes, like yam nematode (Scutellonema bradys), Meloidogyne spp. and Pratylenchus spp. cause considerable damage in Dioscorea. D. japonica is attacked by yam mosaic virus.

Harvesting

The tubers of wild and cultivated Dioscorea are harvested manually at the end of the growing season.

Yield

In India, diosgenin content per 100 g of dry tuber or rhizome was 3.1% in D. composita, 1-5(-8)% in D. deltoidea, 4.7% in D. floribunda, 2.6% in D. mexicana Scheidw., 2-3% in D. prazeri, 6-16% in D. zingiberensis and 1% in the rhizome and seeds of D. tokoro. Other Dioscorea spp. generally have a lower yield. In comparison, Costus speciosus Sm. yields about 1.6% diosgenin.

In India, yields of up to 50 g of fresh tubers per D. deltoidea plant are obtained in temperate Kashmir, but in tropical Bangalore, improved clones yield 1.25 kg of fresh tubers per plant, containing 5-8% diosgenin on dry weight basis, while 3% is considered a reasonable level for economic processing. In another experiment, fresh tuber weight was 1.8-2.3 kg/plant after 2 years, yielding 4-6% diosgenin. Fresh D. zingiberensis tubers yield 3% diosgenin on average in China, the lowest levels being in the south and highest in the north.

Handling after harvest

Tubers of D. japonica are dried and then sliced in thin pieces before transporting. The optimal time for fermentation of the tubers of D. deltoidea is 48 hours as this increases the diosgenin content by about 45%. When storing tubers and bulbils of D. japonica, dormancy is prolonged for 500 days at 20°C and by application of gibberellic acid at 100 μM. The extraction procedure for diosgenin normally begins with a mineral acid treatment to hydrolyze the glycosides. After filtering, the insoluble fraction is neutralized, washed and treated with petroleum ether or toluene, which extracts diosgenin.

Genetic resources and breeding

Due to heavy collection in their natural habitat in India for the pharmaceutical industries, the natural resources of D. deltoidea and D. prazeri are being rapidly depleted. Immediate conservation should receive high priority. Plantations of diosgenin-producing Dioscorea could prevent this over-collection. Large germplasm collections of edible Dioscorea exist in West Africa and Central America. A small collection of D. japonica germplasm is kept in Mie-kene (Japan). Breeding programmes of Dioscorea spp. for diosgenin production exist in India, China, Japan, Vietnam and Russia.

Prospects

Research on different Dioscorea spp. to elucidate the composition of tubers and bulbils for their diosgenin and sapogenin content is needed. Although many diosgenin-producing species do not grow naturally in the Malesian region, they could well be cultivated there as a cash crop.

Literature

Burkill, I.H., 1951. Dioscoreaceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana. Series 1, Vol. 4. Noordhoff-Kolff, Djakarta, Indonesia. pp. 293-335.
Flach, M. & Rumawas, F. (Editors), 1996. Plant Resources of South-East Asia No 9. Plants yielding non-seed carbohydrates. Backhuys Publishers, Leiden, the Netherlands. pp. 85-97.
Hang, Y.Y., 1996. Determination of the content of main constituents and pharmacologic experiments on Dioscorea japonica in China. Journal of Plant Resources and Environment 5(2): 5-8. (in Chinese)
Hikino, H., Konno, C., Takahashi, M., Murakami, M., Kato, Y., Karikura, M. & Hayashi, T., 1986. Isolation and hypoglycaemic activity of dioscorans A, B, C, D, E and F: glycans of Dioscorea japonica rhizophores. Planta Medica 3: 168-171.
Pal, A. & Sharma, A.K., 1977. Diosgenin content of Old and New World species of Dioscorea with special reference to the chromosomal races. Indian Journal of Experimental Biology 15(12): 1109-1112.
Zinovieva, S.V., Udalova, Z.V., Vasiljeva, I.S. & Paseschnichenko, V.A., 1997. Action of sterol glycosides on Meloidogyne incognita infecting tomato and cucumber roots. Russian Journal of Nematology 5(2): 77-80.

Selection of species

Authors

R.C.K. Chung