UV-B RADIATION EFFECTS ON PHOTOSYNTHESIS, GROWTH and CANNABINOID PRODUCTION OF TWO Cannabis Sativa CHEMOTYPES

The effects of UV‐B radiation on photosynthesis, growth and cannabinoid production of two greenhouse‐grown C. sativa chemotypes (drug and fiber) were assessed. Terminal meristems of vegetative and reproductive tissues were irradiated for 40 days at a daily dose of 0, 6.7 or 13.4 kJ m‐2 biologically effective UV‐B radiation. Infrared gas analysis was used to measure the physiological response of mature leaves, whereas gas‐liquid chromatography was used to determine the concentration of cannabinoids in leaf and floral tissue. There were no significant physiological or morphological differences among UV‐B treatments in either drug‐ or fiber‐type plants. The concentration of Δ9‐tetrahydrocannabinol (Δ9‐THC), but not of other cannabinoids, in both leaf and floral tissues increased with UV‐B dose in drug‐type plants. None of the cannabinoids in fiber‐type plants were affected by UV‐B radiation. The increased levels of Δ9‐THC in leaves after irradiation may account for the physiological and morphological tolerance to UV‐B radiation in the drug‐type plants. However, fiber plants showed no comparable change in the level of cannabidiol (a cannabinoid with UV‐B absorptive characteristics similar to Δ9 THC). Thus the contribution of cannabinoids as selective UV‐B filters in C. sativa is equivocal.