Prota 3: Dyes and tannins/Colorants et tanins
Tent. fl. abyss. 1: 184 (1847).
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
2n = 16
Natal indigo, Bengal indigo, Java indigo (En). Indigotier chessé, indigotier (Fr). Indigueiro, anileira (Po). Mnili (Sw).
Origin and geographic distribution
Natal indigo originates from Africa. It occurs almost throughout tropical Africa, and also in northern and eastern South Africa, Swaziland and southern Arabia. Its range has probably been extended by its cultivation for indigo and subsequent naturalization. It is widely planted in India and south-eastern Asia.
The use of indigo extracted from Indigofera species has a long history. Indigo plants and the dyestuff were already mentioned in the oldest Sanskrit records. The blue dye used for textiles of Egyptian mummies may, in some cases, come from an Indigofera species but woad (Isatis tinctoria L.) is another indigo-blue producing plant known to ancient Egyptians. The most ancient archaeological textiles discovered in West Africa, the ‘Tellem textiles’, were found in funeral caves in the Bandiagara cliff in the Dogon area of Mali. Some of them date back to the 11th or 12th centuries and already include stripes, checks and tie-and-dye patterns done with indigo from Indigofera or Philenoptera species. An early description of the local extraction process of indigo from an Indigofera species in Guinea was left by the Portuguese explorer André Alvares de Almada in 1566. In Africa Natal indigo has been the most important species for the production of indigo since the beginning of the 20th century.
Leafy twigs of Indigofera arrecta and some closely related Indigofera species are the main sources of the indigo dye used since very ancient times for dyeing textiles blue. The leaves and twigs do not contain indigo but colourless precursors that must be extracted and processed to produce the indigo dye. Because of its fascinating deep blue colour, its great colour fastness to light and the wide range of colours obtained by combining it with other natural dyes, indigo has been called ‘the king of dyes’. No other dye plants have had such a prominent place in as many civilizations as Indigofera species. In western Africa indigo is by far the most important dye of plant origin. It plays an important role in traditional cultures of many people, whether only wearers of indigo-dyed textiles like the Tuareg in the Sahara and Sahel region (Niger, Mali), or renowned indigo dyers, among which the Soninke (Sarakole) and Wolof in Senegal, the Marka women of the Djenné region in Mali, the Dyula and Baule in Côte d’Ivoire, the Yoruba and Hausa in Nigeria and the Bamoum and Bamileke in Cameroon. This part of the world is a major centre of textile decoration techniques based on the concept of ‘ resist-dyeing’ and linked with indigo dyeing: intricate patterns are made on the cloth that will resist being dyed. This is done by tying, sewing or plaiting parts of the cloth or by covering them with starch pastes or wax. The piece of cloth is then plunged into the indigo vat to dye the untreated parts. After the resist threads or pastes have been removed, white patterns on a blue ground appear. Light blue patterns on a blue-black ground are formed if a last indigo bath is given after undoing the resist. In Madagascar too, indigo dyeing is very important, for instance in raffia ikats, beautiful textiles in which the ‘resist’ patterns are formed by tying parts of the warp yarns with thick threads before dyeing them with indigo and putting them onto the weaving loom, which again creates white designs on a blue ground. However, the use of natural indigo is rapidly declining and nowadays, synthetic indigo is almost exclusively used, not only in industrialized processes but also at craft level.
Indigofera arrecta is grown as a cover crop and for green manure, especially in tea, coffee and rubber plantations. It gives shade and protection, suppresses weeds and improves the soil. The residue remaining after indigo extraction is also applied as manure. In Malawi the young leaves are eaten as a vegetable. The plants are grazed by all stock.
Numerous applications in traditional medicine have been reported: leaves and roots are used externally to treat itching and in an infusion or decoction as an antispasmodic, sedative, stomachic, febrifuge, vermifuge, abortive, diuretic and purgative, e.g. to treat gum infections, snakebites, gonorrhoea, epilepsy and jaundice; the fruits and seeds are used to treat ophthalmia. In Ghana an aqueous extract of leaves from immature shoots is administered orally to patients with diabetes mellitus. A medicine for the management of peptic ulcer and methods of its preparation and use have been patented. In several regions in Africa it is believed that the indigo in clothes prevents skin complaints. In East Africa the twigs are used for cleaning teeth.
Production and international trade
Already in antiquity indigo was traded from India to the Mediterranean region, but this became more important during the Middle Ages. Large-scale cultivation of Indigofera started in the 16th century in India and south-eastern Asia. Later, large plantations were established in Central America and the southern United States. The large-scale export of indigo from Asia to Europe started in about 1600 and had to compete with dye from woad which was cultivated mainly in France, Italy, Germany and Great Britain. By the end of the 17th century, indigo had almost completely replaced woad. Synthetic indigo, which came into commercial production in 1897, proved catastrophic to the production of natural indigo, and by 1914 only 4% of the total world production was of plant origin. Then followed a period in which synthetic indigo lost some of its importance due to competition from new synthetic dyes, but the ever growing popularity of blue jeans enormously contributed to a revival of interest in indigo. At present, the crop is still cultivated for dye production on a small scale in India and in some parts of Africa, southern Arabia (Yemen), Central America and Indonesia. The most important present-day centre of indigo production from Indigofera is probably the northern part of Karnataka state in India. In India, annual production of Indigofera dye gradually decreased from 3000 t from 600,000 ha in 1890, to 50 t from 4000 ha in the 1950s, to an annual export fluctuating between 2 t and 20 t in the 1990s.
Indigofera plants contain the glucoside indican. After soaking the plants in water, enzymic hydrolysis transforms indican into indoxyl and glucose. Indigotin (often also called indigo or indigo-blue) formation results from the grouping of two molecules of indoxyl in the presence of oxygen. Indigotin is insoluble in water, so to dye textiles it must be reduced to a soluble form (colourless ‘leuco’ indigo or indigo-white) by a fermentation process under alkaline conditions or by a chemical reducing agent such as sodium dithionite. Subsequent oxidation by airing of the textile after its removal from the dye bath, results in the regeneration of indigotin and fixation of the blue colour onto the textile. Natural indigo can contain varying proportions of a chemically related red dye called indirubin and of the minor isomeric compounds isoindirubin (red) and isoindigo (brown).
The leaves of Indigofera arrecta contain 4.5% N, 0.02% P2O5, 1.95% K2O and 4.5% CaO. Whole plants of Natal indigo showed good palatability in sheep.
An aqueous extract prevented the development of hyperglycaemia in genetically obese diabetic mice. In tests with rats, an intraperitoneal administration of a hot water extract of dried leaves decreased the plasma glucose levels of fasting normoglycaemic rats, but did not prevent the rise in plasma glucose after an oral glucose load. It was suggested that the extract is insulinotropic and may require functional β-cells to be active. In tests with healthy, non-diabetic, young adult male volunteers in Ghana, an extract of Natal indigo increased the erythrocyte sedimentation rate and decreased lymphocyte concentration in the blood. It did not alter the mean systolic and diastolic pressures, nor did it change fasting blood glucose, whereas serum marker enzymes and metabolites for hepatic and renal functions remained normal. These data suggest that the species may not have overt toxic reactions but could affect the immune status of users. The extract also showed no acute and subchronic toxic effects in tests with mice.
Adulterations and substitutes
Several other, often unrelated, plant species are sources of indigo dyes. Examples include woad from Europe, Polygonum tinctorium Aiton from China, Korea and Japan, Assam indigo (Strobilanthes cusia (Nees) Kuntze) from Indo-China and Thailand and Marsdenia tinctoria R.Br. from tropical Asia. In West Africa, gara (Philenoptera cyanescens (Schumach. & Thonn.) Roberty) and Gambian indigo (Philenoptera laxiflora (Guill. & Perry) Roberty) are important sources of indigo. They also contain indican and are often combined with indigo from Indigofera species in traditional indigo dyeing processes. Other natural sources of indigo are a mutant of the fungus Schizophyllum commune and the purple snail Hexaplex trunculus, which mostly contains precursors of indigo and only minor proportions of the bromine derivatives of indigo of which the famous Tyrian purple dye of antiquity was composed. The most important substitute of natural indigo, however, is the synthetic industrial product.
Annual to perennial herb or subshrub up to 2(–3) m tall; stem erect, copiously branched, slightly ridged, covered with appressed, whitish or brownish, 2-branched hairs. Leaves arranged spirally, imparipinnate; stipules subulate to bristle-like, 2–9 mm long; petiole up to 1.5 cm long, thickened at base, rachis up to 6 cm long; stipels subulate, up to 1 mm long; petiolules c. 1 mm long; leaflets 7–21, narrowly elliptical-oblong, up to 20 mm × 7 mm, usually glabrous above, appressed hairy below. Inflorescence a usually sessile, many-flowered axillary raceme up to 5 cm long but usually much shorter; bracts lanceolate, c. 1 mm long, caducous. Flowers bisexual, papilionaceous; pedicel c. 1 mm long, strongly reflexed in fruit; calyx c. 1.5 mm long, the tube about as long as the 5 triangular lobes, brownish appressed hairy; corolla c. 5 mm long, pinkish or reddish, standard obovate, narrowed gradually to the base, wings with very short claws, keel laterally spurred; stamens 10, 3–4 mm long, upper one free, the other 9 united into a tube; ovary superior, 1-celled, with long style. Fruit a linear pod 12–17 mm long and c. 2 mm wide, straight, slightly tetragonal, brown when ripe, 4–6-seeded with slight constrictions between the seeds. Seeds shortly oblong, c. 2 mm × 1.5 mm, rhombic in cross-section. Seedling with epigeal germination; cotyledons thick, short-lasting.
Other botanical information
Indigofera is a very large genus comprising approximately 700 species and is distributed throughout the tropics and subtropics of Africa, Asia and the Americas. Africa and the southern Himalayas are richest in species. Over 300 species have been recorded for tropical Africa.
For indigo production several Indigofera species are used but there are 3 closely related major ones: Indigofera arrecta, Indigofera tinctoria L., which probably originates from tropical Asia but is now distributed pantropically, and Indigofera suffruticosa Mill., originating from tropical America and now locally cultivated elsewhere in the tropics, including Africa and Madagascar but not in tropical East Africa. The origin and identity of Indigofera plants cultivated for dye production is often obscure as a result of introduction, selection and the close affinity of species. Indigofera arrecta is sometimes difficult to separate from Indigofera tinctoria. The latter usually differs in its larger and less numerous leaflets and longer fruits containing more seeds. In East Africa, but not in West Africa, Indigofera arrecta generally occurs at higher altitudes (1000–2000 m) than Indigofera tinctoria (below 1000 m). Indigofera suffruticosa differs from Indigofera arrecta by its short, 10–15 mm long, red-brown pods. Intermediate specimens between these 3 species have been found, possibly of hybrid origin.
Growth and development
Seeds germinate in about 4 days. Plants may start to flower 3 months after sowing. Like many other leguminous plants, Indigofera arrecta forms root nodules with nitrogen-fixing capacity with e.g. Rhizobium indigoferae. The total lifespan for dye crops is 2–3 years when grown as a ratoon crop.
Indigofera arrecta occurs in open deciduous forest, upland evergreen bushland, often in forest margins, and secondary regrowth. It occurs at 200–2700 m altitude, in regions with an annual rainfall of 400–1800 mm. The plant is deep-rooting and withstands drought well. When used as a cover crop, Natal indigo can only be grown in gardens or plantations with little or no shade. An established crop can withstand very wet soil for up to 2 months.
The crop normally requires little attention after sowing. Weeding is done when needed. As a cover crop, Natal indigo is slashed at regular intervals. Production of seed is usually poor in plants that have been cut. Seed production therefore requires plants to be grown specifically for this purpose.
Propagation and planting
Propagation is usually by seed. Seed yields of 675–1200 kg/ha have been reported for India. The seeds have a hard seed coat and soaking overnight in water or scarification with sulphuric acid is needed before sowing. Fields are prepared by hoeing or by one or two ploughings after showers followed by light harrowing before and after broadcast sowing. Sowing in a nursery and transplanting into the field may also be practised.
Diseases and pests
Natal indigo is attacked by Ralstonia solanacearum (synonym: Bacillus solanacearum), several fungi and nematodes.
Branches are harvested when the plants are 4–5 months old and have formed a closed stand, usually at the flowering stage. The plants can be cut again 2–4 months later. Up to 3 harvests are possible per year when it is grown as a ratoon crop.
The dye yield from Natal indigo is higher than from any other Indigofera species. Annual yields of 22–100 t green matter per ha have been reported in India; the recorded output of indigo cake is 135–325 kg/ha per year.
Handling after harvest
In small-scale cultivation in Africa, harvested branches are often pounded to a soft pulp and made into balls, which are sold on the market after drying. This is the method described by André Alvares de Almada in 1566 and in all early reports. In large-scale cultivation the branches are put in a water containing tank or pit immediately after harvesting. After some hours of fermentation, during which enzymic hydrolysis leads to the formation of indoxyl, the liquid is transferred to another pit or tank and stirred continuously for several hours to stimulate oxidation of the indoxyl to indigotin. Afterwards, the solution is left to rest and the insoluble indigotin settles down to the bottom as a bluish sludge. The water is drained off, and after the indigotin has been washed to get rid of impurities, it is pressed, dried and usually cut into cubes which can be packed and sold.
To dye textiles, indigotin must be reduced to a soluble form under alkaline conditions. In traditional processes (indigo vats), the reduction of indigotin into soluble indigo-white is achieved through a bacterial fermentation. The reducing bacteria are obtained in the bath by adding vegetable matter such as the crushed indigo balls, or, when indigo powder is used, crushed balls of Philenoptera leaves, or, according to some recipes, molasses, coconut-milk, banana or guava leaves. An alkaline pH of around 8.2 is adequate and is maintained by adding a potash lye prepared from the ashes of different calcinated plants specially selected for this purpose in each region (among the Marka women of Mali the most appreciated species are Adansonia digitata L. (baobab), Anogeissus leiocarpa (DC.) Guill. & Perr., Balanites aegyptiaca (L.) Delile, Bauhinia reticulata DC. and Faidherbia albida (Delile) A.Chev.). Less often, the bath is made alkaline by adding freshly slaked lime. In the industrial process, an alkaline solution of sodium dithionite is used for reducing indigo to indigo-white. This polluting chemical process has now been adopted by most craft dyers. Often a gum (e.g. from Acacia senegal (L.) Willd. or Anogeissus leiocarpa (DC.) Guill. & Perr.) is added to make the coloured textile more durable and wind-proof. After the textile has been dipped into the solution of indigo-white, it turns blue when exposed to the air. Several dips are needed to impart a dark blue colour to the textile and especially to cotton fabrics, each dip followed by exposure to the air for some time. In Africa very often dyeing is followed by calendering, a finishing process that consists of beating indigo powder onto the surface of the cloth using heavy mallets to give the textile an iridescent, metallic shine, like on the Tuaregs’ veils. This finish has both aesthetic and medicinal purposes: since the indigo powder is not fixed durably onto the cloth it discharges onto the skin of the people who wear such cloth and acts as a disinfectant and cicatrising drug. Textiles dyed with indigo are very colour-fast to light and washing, less so to rubbing.
Indigofera arrecta is widespread in Africa and is unlikely to be liable to genetic erosion. The share of related species in the gene pool of plants cultivated as Indigofera arrecta is unclear, as is the influence of these species and of cultivated plants on the genetic variation of wild-growing Indigofera arrecta.
Several germplasm collections of Indigofera exist, the largest being maintained at CSIRO, St. Lucia, Queensland, Australia (365 accessions) and CIAT, Cali, Colombia (250 accessions). In Africa, collections are present in Ethiopia (ILRI, Addis Ababa, 60 accessions), Kenya (e.g. National Genebank of Kenya, Kikuyu (40 accessions) and in South Africa (Pretoria).
In spite of its cultural importance, the use of indigo of vegetable origin has almost disappeared, being replaced by synthetic indigo. In West Africa colourful printed textiles, which have a long history in local fashions, are now the most important type of textiles worn by the majority of people and they are mainly imported from Asia or Europe. In recent years, with worldwide increasing concern for sustainability and the demand from consumers for natural products, there has been a revival of interest in natural indigo. Much research still has to be done to optimise indigo production from the most promising plants. Natal indigo is a multipurpose species; it is very useful as a forage and as a cover crop and green manure in agriculture, and its medicinal properties deserve more research.
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Correct citation of this article:
Lemmens, R.H.M.J. & Cardon, D., 2005. Indigofera arrecta Hochst. ex A.Rich. In: Jansen, P.C.M. & Cardon, D. (Editors). PROTA 3: Dyes and tannins/Colorants et tanins. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, flowering and fruiting branch; 2, part of stem with leaf and inflorescence; 3, flower; 4, fruit; 5, seed
Redrawn and adapted by W. Wessel-Brand