Recently, Wageningen University conducted a study on how red and white light affect the growth of medical cannabis, particularly in optimizing the accumulation of plant-specific metabolites under different light intensities. While horticulturists have already used various light spectra and photosynthetic photon flux density (PPFD) to promote plant growth, research on how these factors specifically influence medical cannabis has been limited. This study helps fill that gap.
The research team conducted two rounds of experiments in a climate-controlled indoor environment, cultivating a strain called King Harmony from Perfect Plants. The experiment included two different light intensities-600 and 1200 µmol m-2 s-1-and applied four different light spectra: two low white spectra and two high white spectra. The low white spectra either contained a single 660 nm red light peak or dual red light peaks at 640 and 660 nm. The high white spectra varied in width, with some focusing on narrow bands at 450 nm and 660 nm, and others covering a broad wavelength range from 400 to 750 nm.
The results were intriguing, especially regarding the dual red light peaks. The study found that white light with dual red light peaks at 640 nm and 660 nm significantly increased flower weight and improved light-use efficiency. In contrast, white light with a single 660 nm red light peak was not as effective. This is likely because the maximum absorption peaks of chlorophyll a and b are located near these red light peaks, and these chlorophyll molecules play a crucial role in photosynthesis. Specifically, chlorophyll b binds closely with light-harvesting complexes, while chlorophyll binds with both the core of the photosystem and the light-harvesting complexes.
The role of white light also cannot be overlooked. The researchers noted that increasing the proportion of white light did not directly increase flower weight, but it did alter the balance of blue, green, and red light, which may contribute to enhancing certain therapeutic effects. The experiment showed that when PPFD levels exceeded 1200 µmol m-2 s-1, leaf photosynthesis increased significantly but also triggered stress responses, such as the overstimulation of the photosystem and the production of reactive oxygen species (ROS). At this point, increasing the proportion of green light within the white spectrum helped alleviate this stress by improving light distribution within the leaves, similar to a "detour" effect.
On the other hand, reducing the proportion of white light resulted in taller plants with a more open structure, potentially improving overall light distribution, which could increase both yield and metabolite production. However, the study did not find that the spectrum and PPFD had a significant impact on the total cannabinoid concentration, which contrasts with some earlier research. Differences in experimental conditions, such as PPFD levels and light cycles, may explain this discrepancy.
Additionally, the study found that using low white light spectra under high PPFD conditions sometimes led to discoloration of the flower clusters, likely caused by photoinhibition and ROS production. Despite the discoloration, the affected flower clusters contained higher concentrations of cannabinoids, especially CBD. This suggests that cannabinoids may accumulate as antioxidants to counteract oxidative stress induced by light.
This study provides new insights into medical cannabis cultivation, especially into how to optimize light spectra and intensity to increase yield and metabolite accumulation.
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