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Indigofera coerulea Roxb.

Protologue
Fl. ind. ed. 1832, 3: 377 (1832).
Family
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
Chromosome number
2n = 16
Synonyms
Indigofera articulata auct. non Gouan.
Vernacular names
Indigo (En). Indigotier (Fr). Anileira (Po). Mnili (Sw).
Origin and geographic distribution
In tropical Africa Indigofera coerulea occurs in the Sahel zone from Mali east to Somalia, and south to Kenya and Uganda. It is also found from Algeria though Arabia to India and Sri Lanka and it has been introduced in Mauritius. Indigofera coerulea was formerly much cultivated for indigo in drier areas and its wide distribution has possibly been influenced by this fact.
Uses
All aboveground parts of Indigofera coerulea are a source of indigo, which is used to dye textiles blue. In the Ogaden area of Ethiopia dried, ground up leaves and roots are used as a wound dressing. An extract of the leaves is drunk against constipation and applied as a wash against infected eyes.
Properties
Indigofera plants contain the glucoside indican. After soaking the plants in water, enzymic hydrolysis transforms indican into indoxyl and glucose. Indoxyl can be oxidized to indigotin (indigo-blue). Indigotin is insoluble in water, so to dye textile it must be reduced to a soluble form by a chemical or fermentation process under alkaline conditions. Textile fibres dipped in the vat become impregnated with the soluble and colourless ‘leuco’-form of the dye (indigo-white) and subsequent oxidation (‘airing’) results in the precipitation of the blue indigotin on the textile. Natural indigo also contains varying proportions of a chemically related red dye called indirubin.
Botany
Herb up to 1 m tall, with slightly angled stem, densely covered with appressed, silvery hairs. Leaves alternate, pinnately compound with 5–9(–11) leaflets; stipules triangular-subulate, 1–3 mm long; petiole up to 2 cm long, rachis 6 cm; petiolules c. 2 mm long; leaflets oblong-obovate, up to 32 mm Χ 23 mm, densely hairy. Inflorescence an axillary, sessile, many-flowered raceme up to 4 cm long. Flowers bisexual, papilionaceous; pedicel c. 1 mm long, strongly reflexed after flowering; calyx tubular, 5-lobed, 1.5 mm long, densely white or golden-brown hairy; corolla golden-brown hairy outside; stamens 10, upper one free, the other 9 united, 3–3.5 mm long; ovary superior, 1-celled, style long. Fruit an indehiscent, curved, hairy, cylindrical pod c. 15 mm Χ 2–3 mm Χ 1.5–2 mm, with 3–4 oblong-elliptical segments, upper suture about 1.5 mm wide, silvery hairy when young, brown when old, 3–4-seeded. Seeds ellipsoid, smooth.
Indigofera is a very large genus, comprising about 700 species pantropically, with more than 300 in tropical Africa. Based on the hairiness of the leaflets, 2 varieties of Indigofera coerulea have been distinguished: var. coerulea with upper side of the leaflets glabrous and var. occidentalis J.B.Gillett & Ali with upper side of the leaflets hairy. Indigofera coerulea has been much confused with Indigofera articulata Gouan from Egypt and Arabia, which has 3–5 leaflets, globular pod segments and a narrow pod suture.
Ecology
Indigofera coerulea grows in subdesert and open Acacia-Commiphora bushland, at 200–1250 m altitude, in areas with an annual rainfall of 200–250 mm.
Management
Although Indigofera coerulea has been cultivated, details about cultivation methods are not known. It is supposed that cultivation will be more or less similar to that of, for example, Indigofera arrecta Hochst. ex A.Rich.
Genetic resources and breeding
Indigofera coerulea is widespread and not in danger of genetic erosion.
Prospects
The use of natural indigo as a dye has almost disappeared and has been largely replaced by synthetic indigo. The revival of interest in natural dyes might also involve Indigofera coerulea, which now is only of marginal importance as a source of blue dye in dry areas.
Major references
• Burkill, H.M., 1995. The useful plants of West Tropical Africa. 2nd Edition. Volume 3, Families J–L. Royal Botanic Gardens, Kew, Richmond, United Kingdom. 857 pp.
• Cardon, D., 2003. Le monde des teintures naturelles. Belin, Paris, France. 586 pp.
• Gillett, J.B., Polhill, R.M., Verdcourt, B., Schubert, B.G., Milne-Redhead, E., & Brummitt, R.K., 1971. Leguminosae (Parts 3–4), subfamily Papilionoideae (1–2). In: Milne-Redhead, E. & Polhill, R.M. (Editors). Flora of Tropical East Africa. Crown Agents for Oversea Governments and Administrations, London, United Kingdom. 1108 pp.
Other references
• Gillett, J.B., 1958. Indigofera (Microcharis) in tropical Africa. Kew Bulletin, Additional Series 1, H.M.S.O., London, United Kingdom. 166 pp.
• Lemmens, R.H.M.J. & Wessel-Riemens, P.C., 1991. Indigofera L. In: Lemmens, R.H.M.J. & Wulijarni-Soetjipto, N. (Editors). Plant Resources of South-East Asia No 3. Dye and tannin-producing plants. Pudoc, Wageningen, Netherlands. pp. 81–83.
• Neuwinger, H.D., 2000. African traditional medicine: a dictionary of plant use and applications. Medpharm Scientific, Stuttgart, Germany. 589 pp.
Author(s)
• P.C.M. Jansen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands


Editors
• P.C.M. Jansen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
• D. Cardon
CNRS, CIHAM-UMR 5648, 18, quai Claude-Bernard, 69365 Lyon, Cedex 07, France
General editors
• R.H.M.J. Lemmens
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
• L.P.A. Oyen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article:
Jansen, P.C.M., 2005. Indigofera coerulea Roxb. In: Jansen, P.C.M. & Cardon, D. (Editors). PROTA 3: Dyes and tannins/Colorants et tanins. [CD-Rom]. PROTA, Wageningen, Netherlands.