Combination of red and UV-A light enhances hemp (Cannabis sativa L.) inflorescence yield and cannabinoid content

Light spectrum plays a crucial role in regulating the growth of hemp (Cannabis sativa L.) plants and the biosynthesis of secondary metabolites. Several studies have demonstrated that additional red-light exposure increases biomass accumulation, while supplementary UV-A light stimulates cannabinoid synthesis. Nevertheless, the potential of stage-specific supplementation of red and UV-A light remains underexplored in its capacity to optimize cannabinoid yield in indoor hemp cultivation. In the present study, the effect of red light in combination with UV-A light on hemp biomass and cannabinoid accumulation was investigated using a high-CBD strain. There were four treatments: (1) white light throughout the growth period (control; V_WR_W); (2) red light supplementation during the vegetative stage (V_WRR_W); (3) UV-A supplementation (V_WR_WUV) during the flowering stage; and (4) combined red and UV-A supplementation (V_WRR_WUV) during the vegetative and flowering stages. Results showed that V_WRR_W promoted the number of effective branches (increased by 18.0%) compared to the control (V_WR_W), resulting in an increase in inflorescence yield by 17.9%. V_WR_WUV increased CBG and CBD content by 52.7% and 12.1%, respectively, relative to the control. The effect of V_WRR_WUV on biomass and cannabinoid accumulation was the strongest among the treatments, with CBG and CBD yields reaching 0.53 g and 4.62 g per plant, representing significant increase of 91.8% (p < 0.01) and 44.1% (p < 0.01), respectively, compared to the control. However, there were no significant differences in CBD yield among the V_WRR_W, V_WR_WUV and V_WRR_WUV treatments, indicating that the combined supplementation of red and UV-A light did not have an additive effect on CBD accumulation. These findings highlight the potential of stage-specific spectral strategy to optimize both plant growth and phytochemical quantity.