Light is the primary energy source for plants. Not only is it necessary for photosynthesis, but certain wavelengths impact the accumulation of secondary metabolites, including cannabinoids, terpenes and flavonoids.
This blog post unpacks the impact of the light spectrum on cannabis and how different wavelengths influence yield, cannabinoid production and plant morphology.
Light spectrum and intensity
The light spectrum is determined by wavelengths. For instance, ultraviolet light encompasses a part of the spectrum whose wavelength is 10–400 nanometers (nm).
The sun emits light with wavelengths of 300–1,000 nm. However, plants only photosynthesize light with wavelengths of 400–700 nm. This is known as the photosynthetically active radiation (PAR) range. The most influential light wavelengths for plant development and growth are generally considered to be red, 600–700 nm, and blue, 420–450 nm (Figure 1).
Figure 1. Absorption spectra of plant photosynthetic pigment. Image source: See footnote 4.
Plants sense light within the PAR range through three types of photoreceptors: phytochromes, cryptochromes and phototropins. Each respond to different wavelengths and generate a range of physiological responses, including the production of secondary metabolites (Figure 2).
Figure 2. The impact of wavelengths on Cannabis sativa L. secondary-metabolite responses, with corresponding photoreceptors (↑: increase; Δ: varying depended on light treatments; ↓: decrease; ?: unknown). Image source: See footnote 5.
We can measure light intensity by seeing how many photons, or light particles, within the PAR range reach the crop canopy. This is called the photosynthetic photon flux density (PPFD), and it is measured in micromoles per square meter per second (μmol/m²/s).
Several cannabis studies have found that increasing light intensity, or PPFD, can positively effect photosynthesis and plant growth due to greater net photosynthetic and transpiration rates. The exact light intensity requirements vary by cultivar, but research has shown that cannabis yields rise linearly with increasing PPFDs up to 1400 μmol/m²/s.[1]
Light spectrum’s impact on cannabis
Since indoor cannabis needs artificial lighting for photosynthesis, growers must be mindful of how they provide light, as it impacts plant development and physiological responses.
The duration of light is one management factor. Shortening the photoperiod enough causes photosensitive cannabis varieties to “flip” from the vegetative to the flowering phase.
Another factor is the amount of blue and red light provided to the plant, as the ratio between the two can induce changes in the cannabidiome.[2] Specifically, exposure to a broad spectrum at a given blue:red light ratio may enhance growth and cannabinoid concentration, compared to exposure of just one or the other.[3]
One research study[4] looked at whether the blue:red light ratio affects cannabinoid metabolism and whether plant growth and secondary metabolism intensifies under a full spectrum with a similar blue:red ratio. The results found that the light spectrum impacts cannabis in several ways:
- Yield: Cannabis flowering was most prolific in a spectrum restricted to a 1:1 blue:red ratio. However, in two of the three varieties tested, a 1:4 blue:red ratio produced similar results.
- Chemical profile: Accumulation of CBGA, the primary cannabinoid and a precursor for most other cannabinoids, was stimulated by blue-rich light versus far-red-rich HPS light. While other major cannabinoids like CBDA, THCA and CBCA were also affected by light quality, researchers noted that their responses were cultivar-specific and less pronounced compared with CBGA.
- Plant morphology: Blue light was more conducive to keeping plants compact than a red:far-red ratio.
While some recent studies suggest cannabis plants may benefit from the full spectrum (i.e., white light) over monochromatic red and blue lights, the researchers of the above-mentioned study did not find that in their experiment. Instead, providing cannabis plants white light with a blue:red ratio of 1:1 produced the lowest yield.
The researchers concluded that while genetic variability plays a role in every cannabis plant’s response to different wavelengths, the light spectrum does influence plant development and the cannabinoid profile. Therefore, growers can fine-tune wavelengths to affect yields and cannabinoid production.
Figure 3. Relative photon flux of the spectral properties of the experimental treatments. 1:1 and 1:4 represent blue: red light ratios supplied by LED. HPS is high-pressure sodium. Presented values are for 0.5 nm intervals. Image source: See footnote 1.
Artificial light sources
Several types of lights can be used in indoor cannabis cultivation. The options range in intensity and spectrum:
- Metal halide (MH) is a high-intensity discharge (HID) lamp that is relatively uniform in intensity at all wavelengths.
- High-pressure sodium (HPS) is another HID lamp with red and far-red-rich wavelengths.
- Fluorescent fixtures vary in spectrum, with a high peak in the shorter wavelengths and decreased intensity as wavelengths increase.
- Light emitting diodes (LEDs) can be custom-made to cover desired parts of the spectrum.
Ultraviolet (UV) light, which is below the PAR range, may also be used to influence the production of secondary metabolites. Research shows flavonoid concentrations are higher when plants are grown under UV light, as well as blue and far-red light,[5] but more studies are needed to understand the impact of UV light on cannabis secondary metabolite production.[6]
Conclusion
While the genetic differences between various cannabis cultivars mean that one plant may not respond to specific light wavelengths and light intensity in the same way as others, these factors can enhance cannabinoid yields and the accumulation of secondary metabolites. Growers need to take care in selecting and managing artificial light sources to ensure they provide energy within the PAR range and mimic optimal natural conditions.
Emerald Harvest Team
[1] Rodriguez-Morrison, Victoria et al. “Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment.” Frontiers in plant science vol. 12 646020. 11 May. 2021, doi:10.3389/fpls.2021.646020.
[2] The entire spectrum or profile of cannabinoids present in cannabis plants.
[3] Danziger, Nadav, and Nirit Bernstein. 2021. “Light matters: Effect of light spectra on cannabinoid profile and plant development of medical cannabis (Cannabis sativa L.).” Industrial Crops and Products 164. https://doi.org/10.1016/j.indcrop.2021.113351.
[4] Ibid.
[5] Eichhorn Bilodeau, Samuel, Bo-Sen Wu, Anne-Sophie Rufyikiri, Sarah MacPherson, and Mark Lefsrud. 219. “An Update on Plant Photobiology and Implications for Cannabis Production.” Frontiers in Plant Science 10. https://doi.org/10.3389/fpls.2019.00296.
[6] Desaulniers Brousseau, Vincent, Bo-Sen Wu, Sarah MacPherson, Victorio Morello, and Mark Lefsrud. 2021. “Cannabinoids and Terpenes: How Production of Photo-Protectants Can Be Manipulated to Enhance Cannabis sativa L. Phytochemistry.” Frontiers in Plant Science 12. https://doi.org/10.3389/fpls.2021.620021.