In woad (Isatis tinctoria), field observations indicated, that after p
eriods of dry sunny weather, indigo yields increased significantly, su
ggesting that light intensity and quality affected indigo precursor pr
oduction. Therefore, woad was grown under different light intensities
and in the presence or absence of supplementary UV light. In general,
plants supplied with more light produced more indigo than those given
lesser light. When plants under greater light regimes were transferred
to lesser light conditions, then indigo production declined. The oppo
site was also true, indicating that greater indigo production can be o
btained from plants harvested after periods of increased sunlight.