Prota 3: Dyes and tannins/Colorants et tanins
Bull. Mens. Soc. Linn. Paris 1: 399 (1883).
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
Indigo (En). Indigotier (Fr). Anileira (Po). Mnili (Sw).
Origin and geographic distribution
Indigofera longiracemosa is indigenous to coastal areas of Kenya, Tanzania, Comoros, Madagascar and southern India. Occasionally it has been cultivated in Madagascar and introduced elsewhere, e.g. in Indonesia.
In its distribution area, Indigofera longiracemosa is a minor source of indigo, used to dye textiles blue. Historically it is perhaps most important in Madagascar, where the dye of Indigofera longiracemosa is considered superior to the dyes of Indigofera tinctoria L. or Indigofera suffruticosa Mill. The dye was extracted both at domestic and industrial scale. Traditional indigo-dyeing techniques in Madagascar mainly used local Indigofera leaves both fresh and in a composted state. Indigo dyeing was still important there until recent years, especially for the magnificent raffia textiles called ‘lay masaka’, which are still produced e.g. in Kandreho village in the former Sakalava kingdom near Maevatanana. These textiles have ornate designs made with an ‘ikat’ technique in which parts of the warp yarns that are to remain undyed are tightly entwined with a thick thread. The yarns are then dyed and set on the loom, whereby the undyed parts of the yarn form a light-coloured pattern on a dark ground. Other natural fibres such as silk, cotton and wool, absorb indigo blues more easily using a dye bath and the colour is more pronounced.
Indigofera longiracemosa is also used as a green manure. In Madagascar a decoction of the leaves is used as a diuretic. In India the root has been used in tribal medicine as an antidote for snake poisons.
Indigofera plants contain the glucoside indican. After soaking the plants in water or pounding the leaves, enzymic hydrolysis transforms indican into indoxyl and glucose. Indoxyl is then oxidized and polymerized to indigotin (indigo-blue). Indigotin is insoluble in water, so to dye textiles it must be reduced to a soluble ‘leuco’ or colourless form (indigo-white) by a chemical or fermentation process under alkaline conditions. The cloth to be dyed is soaked in this solution. Subsequent re-oxidation of the indigo-white results in the precipitation of the blue indigo colour on the textile. Natural indigo also contains varying proportions of the chemically related dye compounds: indirubin (red), isoindirubin (red) and isoindigo (brown).
Erect annual or short-lived perennial herb or subshrub up to 2 m tall; stem red-brown, glabrescent; branches slender, bearing few small hairs. Leaves alternate, imparipinnate with 5–13 pairs of leaflets; stipules subulate, c. 2 mm long; petiole c. 1.5 cm long; leaflets elliptical, up to 2.5 cm × 1.5 cm, sparsely hairy, distinctly blue-green. Inflorescence a many-flowered, almost sessile, usually long and lax raceme up to 10(–14) cm long, often exceeding the leaves. Flowers bisexual, papilionaceous, c. 4 mm long; pedicel c. 1 mm long, strongly reflexed after flowering; calyx tubular, up to 1 mm long, 5 lobes longer than the tube, brown hairy; corolla pink, densely brown-hairy outside; stamens 10, c. 3 mm long, upper one free, the other 9 united; ovary superior, 1-celled, style long. Fruit a straight cylindrical pod up to 2 cm × 2.5 mm, not constricted between the seeds, pointed, sparsely hairy, brown, 4–6-seeded. Seeds flattened rectangular, 2.5 mm × 1.5 mm, pitted.
Indigofera is a very large genus, comprising about 700 species pantropically, with more than 300 in tropical Africa.
Indigofera stenosepala Baker is another minor source of indigo for dyeing, indigenous to Madagascar, where it is widespread and common. An infusion is used to stimulate the appetite and to treat stomach ulcers and other digestive disorders.
Indigofera longiracemosa is found in lowland, usually coastal regions up to 100 m altitude, in areas with an annual rainfall of 1000–1500 mm. In Madagascar it is found in dune vegetation or grassland on sand, often around villages, up to 200 m altitude. In Indonesia it is grown at about 1650 m altitude and is attacked by pests at lower altitudes.
Indigofera longiracemosa is occasionally cultivated but no cultivation methods are known. Cultivation is probably similar to that of Indigofera arrecta Hochst. ex A.Rich. In Madagascar domestic extraction of indigo was done in several ways. Commonly, harvested plants are stacked in a big container (a barrel with a tapped hole at the bottom). Water is added to cover the plants and they are weighed down with a stone. The plants are left under water for about 12 hours, or as long as air bubbles come to the surface. Then the extraction water is drained off through the hole at the bottom into another container. It is mixed with a solution of slaked lime (in volume proportion 3:1) and well beaten to bring in oxygen and allow the formation of indigotin. Agitation and the addition of small amounts of lime water continue until blue indigo particles are formed. Now stirring continues but without addition of lime water. Then the liquid is left undisturbed for some time to allow the indigo to precipitate on the bottom. After pouring off the waste liquid, the indigo paste is collected into a mould with small holes on all sides that allow the evacuation of remaining water. When the paste is firm enough, it is taken out of the mould and the loaf is left to dry in the shade (sun would taint it pale blue). When dry the loaves are cut and packed, being ready for use or trade. Out of 10 kg fresh leaves, this domestic scale method produces 26 g of indigo with an average indigotin content of about 31%.
In an alternative method for preparing the dye bath or vat in Madagascar, fresh Indigofera leaves are pounded and the paste is left to ferment by specific bacteria. A concentrated infusion of fresh leaves is added repeatedly to keep the paste moist. When the indigotin has fully developed in this compost, the paste is used to set a vat with boiling water and ashes from burnt banana-plant stumps. The ash-lye serves to maintain the alkaline pH necessary for the reduction of the indigotin to soluble indigo-white by a biological process. After a few days, when the reduction is complete, dyeing can take place by repeated alternate dipping and airing of the skins or textiles. Depending on the number of dippings, the shades of blue obtained vary from pale greyish blue to dark navy blue. Different proportions of fresh leaves and compost, and different stages of compost fermentation may produce various shades of greenish blues, while the proportion of indirubin, an isomer of indigotin produced in the process, may give pinkish-purplish shades.
Genetic resources and breeding
Indigofera longiracemosa is not in danger of genetic erosion.
Indigofera longiracemosa will remain a minor but locally important source for indigo dyeing. If current programmes to revive the use of natural dyes in traditional textiles of Madagascar lead to commercial success, interest in natural indigo will also increase, as well as cultivation of Indigofera species.
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Correct citation of this article:
Jansen, P.C.M., 2005. Indigofera longiracemosa Boivin ex Baill. In: Jansen, P.C.M. & Cardon, D. (Editors). PROTA 3: Dyes and tannins/Colorants et tanins. [CD-Rom]. PROTA, Wageningen, Netherlands.