Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content

Cannabis (Cannabis sativa) flourishes under high light intensities (LI); making it an expensive commodity to grow in controlled environments, despite its high market value. It is commonly believed that cannabis secondary metabolite levels may be enhanced both by increasing LI and exposure to ultraviolet radiation (UV). However, the sparse scientific evidence is insufficient to guide cultivators for optimizing their lighting protocols. We explored the effects of LI and UV exposure on yield and secondary metabolite composition of a high Delta(9)-tetrahydrocannabinol (THC) cannabis cultivar 'Meridian'. Plants were grown under short day conditions for 45 days under average canopy photosynthetic photon flux densities (PPFD, 400-700 nm) of 600, 800, and 1,000 mu mol m(-2) s(-1), provided by light emitting diodes (LEDs). Plants exposed to UV had PPFD of 600 mu mol m(-2) s(-1) plus either (1) UVA; 50 mu mol m(-2) s(-1) of UVA (315-400 nm) from 385 nm peak LEDs from 06:30 to 18:30 HR for 45 days or (2) UVA + UVB; a photon flux ratio of approximate to 1:1 of UVA and UVB (280-315 nm) from a fluorescent source at a photon flux density of 3.0 mu mol m(-2) s(-1), provided daily from 13:30 to 18:30 HR during the last 20 days of the trial. Alt aboveground biomass metrics were 1.3-1.5 times higher in the highest vs. lowest PPFD treatments, except inflorescence dry weight - the most economically relevant parameter - which was 1.6 times higher. Plants in the highest vs. lowest PPFD treatment also allocated relatively more biomass to inflorescence tissues with a 7% higher harvest index. There were no UV treatment effects on aboveground biomass metrics. There were also no intensity or UV treatment effects on inflorescence cannabinoid concentrations. Sugar leaves (i.e., small leaves associated with inflorescences) of plants in the UVA + UVB treatment had approximate to 30% higher THC concentrations; however, UV did not have any effect on the total THC in thesefoliar tissues. Overall, high PPFD levels can substantially increase cannabis yield, but we found no commercially relevant benefits of adding UV to indoor cannabis production.

Automated summary

Increasing light intensity can increase cannabis yield, but adding UV radiation does not provide any commercial benefits in indoor cannabis production.