Indigo precursors in Fall 2007 Birmingham leaf
Indigo precursors in Spring 2008 Birmingham first year plant.
Indigo precursors in Fall 2008 Birmingham first year plant.
Notice there is less indigo formed in the leaf the Fall of 2008 compared to the Fall 2007 leaf. This could be due to the relative coolness of the summer. Warm weather increases indigo precursor content in the leaves.
Indigo precursors in woad flower 2008 second year plant.
The red isomer of indigo, indirubin, formed on this flower due to bruising when the slide cover was placed over it. The indigo and indirubin formed in the heat of the microscope light. Heat favours the development of indirubin during the precipitation phase of indigo extraction, in a neutral environment.
Unlike other natural dye plants which contain their chromophores within, the woad plant, does not contain indigo or indigotin. The leaves of woad plants contain precursors to indigo. In woad the most important precursors are isatan A, isatan B and indican. The precursors isatan A and isatan B are unstable and require specialized extraction methods to maximize their contribution to the indigo yield. During the extraction process the precursors bond together to form indoxyl. Two indoxyl molecules join together to form indigotin. Depending on the conditions in the extraction vat, the precursors can also bond with indoxyl to form, indirubin, an isomer of indigotin, (indigo red), isoindigo (blue), and isorubin, (indigo brown). (Cardon, pg 337-339)
Indigo precursors are present in woad plants at all stages of growth. There is a higher concentration of indigo precursors in Spring and Summer leaves. Once leaves have been exposed to daytime frost, the indigo precursor content decreases. It increases again the second year as the plant begins to send up its scape. However, the precursor content remains low in second year plants.