Last week, we introduced the concept of photobiology, which regards how plants convert sunlight into chemical energy (photosynthesis) and how light interacts with plants, leading to growth, structure, flowers, and overall yield (photomorphogenesis). Now, we’ll get to the meat of it, namely, various colors of the rainbow, and how they impact plants.
Red: Red light impacts photomorphogenesis through phytochromes Pr (red) and Pfr (far red), leaf nutrient content, and stem growth.  Researchers have shown that a high Pfr/Pr ratio is optimal for flower stimulation in plants that require longer “days” via longer exposure to light. This occurs when red light (approx. 650–670 nm) is employed early in the photoperiod, giving way to far-red light (approx. 705-740 nm) at the end.
Interestingly, some cannabis cultivars have shown indifference to Pfr/Pr dependence for flower generation. Researchers reported that some cannabis plants thrive with 18 hours of light and 6 hours of darkness, or a low Pfr/Pr ratio.  So, it’s clear that red light affects the flowering quality, quantity, and duration. Terpene biosynthesis is also regulated by red-light phytochrome Pr. 
Blue: Blue light interacts with cryptochrome and phototropin receptors which oversee physiology and development, including germination, elongation, water movement throughout the plant, and exchange of carbon dioxide.  Blue light governs how plants respond to environmental stressors like bacteria and fungi.  One study found that wavelengths below 445 nm enhanced stem growth, plant height, and anthocyanin formation which causes those wonderful purple hues in cannabis. 
Cannabis plants grown under blue light with a 12-hour on, 12-hour off photoperiod showed enhanced cannabinoid concentrations.  These researchers also found a correlation between ultraviolet (UV)-A and blue wavelengths, that, when used together, augmented cannabigerol concentrations in flowers.
Ultraviolet: Speaking of UV radiation, while wavelengths below 400 nm are not heavily involved in photosynthetic processes, they are involved in photomorphogenesis. UV-B, while detrimental to plants in larger doses, can fortify a plant’s resistance to pests, increase flavonoid concentrations, and improve photosynthetic efficiency.  UV-B has also been shown to increase delta-9-tetrahydrocannabinol (THC) levels in leaves and flowers , which makes sense since cannabinoids have been called a natural sunscreen. 
Green: While plants reflect a lot of green light, finding little utility for these wavelengths, plant morphology can be affected. Like UV-B wavelengths, lower levels of green light can help plant growth while heavier doses can inhibit plants from developing.  And while green light can penetrate deeper into leaves compared to red or blue light , it can cause conflicting responses to important processes induced by blue light . Additionally, green light can reduce THC levels. 
So, the composition of your light source has everything to do with the vitality, yield, and phytochemistry of your crops. Identifying which wavelengths are or aren’t needed can ensure you’re not illuminating your plants with unwarranted radiation.
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Image Credit: PL Light Systems