The growing need for steroidal drugs and the high cost of obtaining them from animal sources led to a widespread search for plant sources of steroidal sapogenins, which ultimately led to the most promising one. It is the largest genus of the family constituted by 600 species of predominantly twining herbs. Among the twining species, some species twine clockwise while others anti-clockwise (Miege, 1958). All the species are dioceous and rhizomatous. According to Coursey (1967), this genus is named in honour of the Greek physician Pedenios Dioscorides, the author of the classical Materia Medica Libri Quinque. Some of the species like D. alata and D. esculenta have been under cultivation for a long time for their edible tubers. There are about 15 species of this genus containing diosgenin. Some of them are the following (Chopra et al, 1980).
D. floribunda Mart. & Gal.
D. composita Hemsl; syn. D. macrostachya Benth.
D. deltoidea Wall. ex Griseb; syn. D. nepalensis Sweet ex Bernardi.
D. aculeata Linn. syn. D. esculenta
D. alata Linn. syn. D. atropurpurea Roxb.
D. Globosa Roxb; D. purpurea Roxb; D. rubella Roxb.
D. bulbifera Linn. syn. D. crispata Roxb.
D. pulchella Roxb.; D. sativa Thunb. Non Linn.
D. versicolor Buch. Ham. Ex Wall.
D. daemona Roxb. syn. D. hispida Dennst.
D. oppositifolia Linn.
D. pentaphylla Linn. syn. D. jacquemontii Hook. f.
D. triphylla Linn.
D. prazeri Prain & Burkil syn. D. clarkei Prain & Burkill
D. deltoidea Wall. var. sikkimensis Prain
D. sikkimensis Prain & Burkill
Among the above said species, D. floribunda, D. composita and D. deltoidea are widely grown for diosgenin production.
1. D. floribunda Mart. & Gal D. floribunda Mart. & Gal. is an introduction from central America and had wide adaptation as it is successfully grown in Karnataka, Assam, Meghalaya, Andaman and Goa. The vines are glabrous and left twining. The alternate leaves are borne on slender stems and have broadly ovate or triangular ovate, shallowly cordate, coriaceous lamina with 9 nerves. The petioles are 5-7cm long, thick and firm. Variegation in leaves occurs in varying degrees. The male flowers are solitary and rarely in pairs. Female flowers have divericate stigma which is bifid at apex. The capsule is obovate and seed is winged all round. The tubers are thick with yellow coloured flesh, branched and growing upto a depth of 30cm (Chadha et al, 1995).
2. D. composita Hemsl.
D. composita Hemsl. according to Knuth (1965) has the valid botanical name as D. macrostachya Benth. However, D. composita is widely used in published literature. It is a Central American introduction into Goa, Jammu, Bangalore, Anaimalai Hills of Tami Nadu and Darjeeling in W. Bengal. The vines are right twinning and nearly glabrous. The alternate leaves have long petioles, membraneous or coriaceous lamina measuring upto 20x18cm, abruptly acute or cuspidate-acuminate, shallowly or deeply cordate, 7-9 nerved. The fasciculate-glomerate inflorescence is single or branched with 2 or 3 sessile male flowers having fertile stamens. Male fascicle is 15-30cm long. The female flowers have bifid stigma. Tubers are large, white and deep-rooted (upto 45cm) (Chadha et al, 1995).
3. D. deltoidea Wall. ex. Griseb.
D. deltoidea Wall. ex. Griseb. is distributed throughout the Himalayas at altitudes of 1000-3000m extending over the states of Jammu-Kashmir, H. P, U. P, Sikkim and further into parts of W. Bengal. The glabrous and left twining stem bears alternate petiolate leaves. The petioles are 5-12 cm long. The lamina is 5-15cm long and 4-12cm wide widely cordate. The flowers are borne on axillary spikes, male spikes 8-40cm long and stamens 6. Female spikes are 15cm long, 3. 5cm broad and 4-6 seeded. Seeds are winged all round. Rhizomes are lodged in soil, superficial, horizontal, tuberous, digitate and chestnut brown in colour (Chadha et al, 1995). D. deltoidea tuber grows parallel to ground covered by small scale leaves and is described as rhizome. The tubers are morphologically cauline in structure with a ring of vascular bundles in young tubers which appear scattered in mature tubers (Purnima and Srivastava, 1988). Visible buds are present unlike in D. floribunda and D. composita where the buds are confined to the crown position (Selvaraj et al, 1972).
Importance of Diosgenin: Diosgenin is the most important sapogenin used as a starting material for synthesis of a number of steroidal drugs. For commercial purposes, its -isomer, yamogenin is also taken as diosgenin while analysing the sample for processing. Various steroidal drugs derived from diosgenin by artificial synthesis include corticosteroids, sex hormones, anabolic steroids and oral contraceptives. Corticosteroids are the most important group of steroidal drugs synthesized from diosgenin. First group of corticosteroids regulates carbohydrate and protein metabolism. The second group consists of aldosterone, which controls balance of potassium, sodium and water in the human body. The glucocorticoids in the form of cortisone and hydrocortisone are used orally, intramuscularly or topically for treatment of rheumatoid arthritis, rheumatic fever, other collegen diseases, ulcerative colitis, certain cases of asthma and a number of allergic diseases affecting skin, eye and the ear. These are also used for treatment of gout and a variety of inflammations of skin, eye and ear and as replacement therapy in Addison’s diseases. The minerato corticoides, desoxycorticosterone or desoxycortone are used in restoring kidney functions in cases of cortical deficiency and Addison’s disease.
Both male and female sex hormones are also synthesized from disosgenin. The main male sex hormone (androgen) which is produced from disogenin is testosterone. The main female sex hormones produced are oestrogen and progesterone. Recently oestrogen has also been used in cosmetic lotions and creams to improve the tone and colour of skin. One of the main uses of progesterone during recent years has been as antifertility agent for oral contraceptives. These artificial steroids have increased oral activity and fewer side effects, as they can be used in reduced doses. Oral contraceptives are also used for animals like pigs, cows and sheep to control fertility and to give birth at a prescribed period in a group of animals at the same time. These compounds are also used to reduce the interval between the lactation periods to have more milk and meat production. Anti-fertility compounds are also used as a pest-control measure for decreasing the multiplication of pests like rodents, pigeons and sea gulls (Husain et al, 1979).
Although yam tubers contain a variety of chemical substances including carbohydrates, proteins, alkaloids and tannins, the most important constituents of these yams are a group of saponins which yield sapogenins on hydrolysis. The most imp ortant sapogenin found in Dioscorea are diosgenin, yamogenin and pannogenin. Diosgenin is a steroid drug precursor. The diogenin content varies from 2-7% depending on the age of the tubers. Saponins including 5 spirastanol glucoside and 2 furostanol glucoside, 4 new steroid saponins, floribunda saponins C, D, E and F. Strain of A and B are obtained from D. floribunda (Husain et al, 1979). Rhizomes of D. deltoidea are a rich source of diosgenin and its glycoside. Epismilagenin and smilagenone have been isolated from D. deltoidea and D. prazeri (Chakravarti et al, 1960; 1962). An alkaloid dioscorine has been known to occur in D. hispida (Bhide et al,1978). Saponin of D. prazeri produced a fall of blood pressure when given intravenously and saponin of D. deltoidea has no effect on blood pressure (Chakravarti et al,1963). Deltonin, a steroidal glycoside, isolated from rhizomes of D. deltoidea showed contraceptive activity (Biokova et al, 1990).
Agrotechnology: Dioscorea species prefer a tropical climate without extremity in temperature. It is adapted to moderate to heavy rainfall area. Dioscorea plants can be grown in a variety of soils, but light soil is good, as harvesting of tubers is easier in such soils. The ideal soil pH is 5.5-6.5 but tolerates fairly wide variation in soil pH. Dioscorea can be propagated by tuber pieces, single node stem cuttings or seed. Commercial planting is normally established by tuber pieces only. Propagation through seed progeny is variable and it may take longer time to obtain tuber yields. IIHR, Bangalore has released two improved varieties, FB(c) -1, a vigorously growing strain relatively free from diseases and Arka Upkar, a high yielding clone. Three types of tuber pieces can be distinguished for propagation purpose, viz. (1) crown (2) median and (3) tip, of which crowns produce new shoots within 30 days and are therefore preferred. Dipping of tuber pieces for 5 minutes in 0.3% solution of Benlate followed by dusting the cut ends with 0.3% Benlate in talcum powder in mo ist sand beds effectively checks the tuber rot. The treatment is very essential for obtaining uniform stand of the crop. The best time of planting is the end of April so that new sprouts will grow vigorously during the rainy season commencing in June in India. Land is to be prepared thoroughly until a fine tilth is obtained. Deep furrows are made at 60cm distance with the help of a plough. The stored tuber pieces which are ready for planting is to be planted in furrows with 30cm between the plants for one year crop and 45cm between the plants for 2 year crop at about 0.5 cm below soil level. The new sprouts are to be staked immediately. After sprouting is complete, the plants are to be earthed up. Soil from the ridges may be used for earthing up so that the original furrows will become ridges and vice versa. Dioscorea requires high organic matter for good tuber formation. Besides a basal doze of 18-20t of FYM/ha, a complete fertilizer dose of 300kg N, 150kg P2O5 and K2O each are to be applied per hectare. P and K are to be applied in two equal doses one after the establishment of the crop during May-June and the other during vigorous growth period of the crop (August- September). Irrigation may be given at weekly intervals in the initial stage and afterwards at about 10 days interval. Dioscorea vines need support for their optimum growth and hence the vines are to be trailed over pandal system or trellis. Periodic hand weeding is essential for the first few months. Intercropping with legumes has been found to smother weeds and provide extra income. The major pests of Dioscorea are the aphids and red spider mites. Aphids occur more commonly on young seedlings and vines. Young leaves and vine tips eventually die if aphids are not controlled. Red spider mites attack the underside of the leaves at the base near the petiole. Severe infestations result in necrotic areas, which are often attacked by fungi. Both aphids and spider mites can be very easily controlled by Kelthane. No serious disease is reported to infect this crop. The tubers grow to about 25-30 cm depth and hence harvesting is to be done by manual labour. The best season for harvesting is Feb-March, coinciding with the dry period. On an average 50-60t/ha of fresh tubers can be obtained in 2 years duration. Diosgenin content tends to increase with age, 2.5% in first year and 3-3.5% in the second year. Hence, 2 year crop is economical (Kumar et al, 1997).