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UV-B RADIATION EFFECTS ON PHOTOSYNTHESIS, GROWTH AND CANNABINOID PRODUCTION OF TWO Cannabis sativa CHEMOTYPES
JOHN LYDON, BENJAMIN COFFMAN
Department of Botany, University of Maryland, 1987
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 kJm-* biologically effective UV-B radiation. Infrared gas analysis was used to measure the physiologicalresponse of mature leaves, whereas gas-liquid chromatography was used to determine the concentrationof 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 A’-tetrahydrocannabinol (A”-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 A’-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 A’ THC). Thus the contribution of cannabinoids as selective UV-B filters in C. sutivu is equivocal.
In conclusion, the A9-THC content in leaf and floral tissues of greenhouse grown drug-type C. saliva increased linearly with UV-B dose. Other cannabinoids in drug- and fiber-type plants were unaffected by UV-B radiation. Both drug and fiber chemotypes were physiologically and morphologically tolerant to UV-B radiation. Thus, this tolerance was observed regardless of the concentration or composition of UV-B absorbing cannabinoids. Therefore, it appears that other factors may contribute to the UV-B insensitivity of vegetative C. sativa. Nevertheless, the increased production of A’-THC in floral tissues (where this compound is most concentrated) of drug-type plants upon UV-B irradiation may result in more reproductive success for drug-type than fiber-type C. sativa in high UVB environments as suggested by Pate (1983).