THC isn't produced until the Trichromes react with light, specifically in the UV Spectrum.
Amber-colored trichromes are laden with THC - white trichromes have more precursor elements.
All gas-based lights emit some UV (HPS, MH, Fluorescents) but natural light is best for trichrome and THC production.
Supplement with UVB lighting (sterilization bulbs you'd find at a barber shop, or whatnot - those regular party blacklights are not the right wavelength of ultraviolet.)