Indican readily hydrolyzes to release β- D- glucose and indoxyl. The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan. Several plants contain indigo, which, when exposed to an oxidising source such as atmospheric oxygen, reacts to produce indigo dye however, the relatively low concentrations of indigo in these plants make them difficult to work with, with the color more easily tainted by other dye substances also present in these plants, typically leading to a greenish tinge. In Europe, Isatis tinctoria, commonly known as woad, was used for dyeing fabrics blue, containing the same dyeing compounds as indigo, also referred to as indigo. In Central and South America, the species grown is Indigofera suffruticosa, also known as anil, and in India, an important species was Indigofera arrecta, Natal indigo. Until the introduction of Indigofera species from the south, Polygonum tinctorum (dyer's knotweed) was the most important blue dyestuff in East Asia however, the crop produced less dyestuff than the average crop of indigo, and was quickly surpassed in favour of the more economical Indigofera tinctoria plant. A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia. The primary commercial indigo species in Asia was true indigo ( Indigofera tinctoria, also known as I.
A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics, notably the Indian Subcontinent.
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