The economic value of Cannabis sativa is influenced by inflorescence quality. Exposure to ultraviolet light (UV) has been shown to increase concentrations of Δ9-tetrahydrocannabinolic acid in drug-type plants. High doses of UV-B are physiologically damaging and this damage is theorized to be repaired photo-enzymatically, through exposure to white light or UV-A light. I assessed chlorophyll content, stomatal conductance, sexual lability, vegetative biomass, floral biomass, Δ9-tetrahydrocannabinol and cannabidiol content in C. sativa using light emitting diodes (LED) receiving 4h of UV-B alone or with 6h UV-A daily for five weeks. Significant (or interactive) effects of UV-A were not detected for any trait measured. However, plants exposed to UV-A+B showed a negative correlation between stomatal size and density, not seen under UV-B, suggesting UV-A impacts stomatal physiology. Results suggest a limited impact of UV-A exposure when white light is sufficient. Alternatively, further environmental variables may have obscured any effects from UV-A exposure.
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Jillian Muir-Guarnaccia (2024) studied this question.
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