Green light is radiation with wavelengths between 520 and 560 nm and it affects photosynthesis, plant height, and flowering. Plants reflect green light and this is why they appear green to our eyes. As a result, some growers think that plants don’t use green wavelengths, but they actually do! In fact, only around 5 – 10% of green light is reflected from leaves and the rest (90 – 95 %) is absorbed or transmitted to lower leaves [1]. Green wavelengths get used in photosynthesis. Chlorophyll pigments absorb small amounts of green wavelengths. Light that doesn’t get absorbed is transmitted to leaves that are shaded out from direct light. This means that leaves at the bottom of the canopy get more green light than leaves at the top. A high proportion of green wavelengths compared to other colors tells lower leaves that they are being shaded out, so they are able to react accordingly. Lower leaves may react by opening or closing their stomata or growing longer stems that help the leaves reach brighter light [1, 2, 3]. When it comes to growing cannabis, many cultivators are interested in the quality of light used for the flowering stage. In many plants, flowering is regulated by two main photoreceptors: cryptochrome and phytochrome. Both photoreceptors primarily respond to blue light but can also respond to green, although to a lesser extent. Green can accelerate the start of flowering in several species (although cannabis has yet to be tested) [1, 4, 5]. However, once flowering has begun, it’s important to provide plants with a “full spectrum” light that has high amounts of blue and red light, and moderate amounts of green, in order for photosynthesis to be optimized. Green light mediates seed germination in some species. Seeds use green wavelengths to decide whether the environment is good for germination. Shade environments are enriched in green relative to red and blue light, so a plant can tell if it is shady or sunny. A seed that senses a shaded environment may stay dormant to avoid poor growing conditions [1]. Some examples of plant species where researchers have documented this response are: ryegrass (a grass that grows in tufts) and Chondrilla (a plant related to dandelion) [1, 6]. Although green wavelengths generally tell plants NOT to germinate, there are some exceptions! Surprisingly, green wavelengths can stimulate seed germination in some species like Aeschynomene, Tephrosia, Solidago, Cyrtopodium, and Atriplex [1, 6, 7]. Of course, light is not the only factor affecting seed germination – it’s a combination of many factors, such as soil moisture, soil type, temperature, photoperiod, and light quality. When combined with red and blue light, green can really enhance plant growth [1, 8]. However, too much green light (more than 50% of the total light) can actually reduce plant growth [8]. Based on the most current research, the ideal ratio of green, red, and blue light is thought to be around 1:2:1 for green:blue:red [9]. When choosing a horticultural light, choose one that has high amounts of blue and red light and moderate amounts of green and other colors of light. Not many studies can be found about the effect of green light on cannabis growth or metabolism. However, if one reads carefully, there are clues and data available even from the very early papers. Mahlberg and Hemphill (1983) used colored filters in their study to alter the sunlight spectrum and study green light among others. They concluded that the green filter, which makes the environment green by cutting other wavelengths out, reduced the THC concentration significantly compared to the daylight control treatment. It has been demonstrated that green color can reduce secondary metabolite activity with other species as well. For example, the addition of green to a light spectrum decreases anthocyanin concentration in lettuce (Zhang and Folta 2012). If green light only reverses the biosynthesis of some secondary metabolites, then why put green light into a growth spectrum at all? Well, there are a couple of good reasons. One is that green penetrates leaf layers effectively. Conversely red and blue light is almost completely absorbed by the first leaf layer. Green travels through the first, second, and even third layers effectively (Figure 2). Lower leaf layers can utilize green light in photosynthesis and therefore produce yields as well. Even though a green light-specific photoreceptor has not yet been found, it is known that green light has effects independent from the cryptochrome but then again, also cryptochrome-dependent ones, just like blue light. It is known that green light in low light intensity conditions can enhance far red stimulating secondary metabolite production in microgreens and then again, counteracts the production of these compounds in high-intensity light conditions (Kim et al. 2004). In many cases, green light promoted physiological changes in plants that are opposite to the actions of blue light. In the study by Kim et al. blue light-induced anthocyanin accumulation was inhibited by green light. In another study it has been found that blue light promotes stomatal opening whereas green light promotes stomatal closure (Frechilla et al. 2000). Blue light inhibits the early stem elongation in the seedling stage whereas green light promotes it (Folta 2004). Also, blue light results in flowering induction, and green light inhibits it (Banerjee et al., 2007). As you can see, green light works very closely with blue light, and therefore not only the amount of these two wavelengths separately is important but also the ratio (Blue: Green) between these two in the designed spectrum. Furthermore, green light has been found to affect the elongation of petioles and upward leaf reorientation with the model plant Arabidopsis thaliana both of which are a sign of shade avoidance symptoms (Zhang et al. 2011) and also gene expression in the same plant (Dhingra et al. 2006). As mentioned before, green light produces shade avoidance symptoms which are quite intuitive if you consider the natural conditions where the plants grow. Not all the green light is reflected from the highest canopy leaves in nature but a lot of it (50-90%) has been estimated to penetrate the upper leaves at the plant level ((Terashima et al., 2009; Nishio, 2000). For the plant growing in the understory of the forest green light is a signal for the plant of being in the shade of a bigger plant. Then again, the plants growing under unobstructed sunlight can take advantage of the green photons that can more easily penetrate the upper leaves than the red and blue photons. From the photosynthetic pigments in higher plants, chlorophyll is crucial for plant growth. Dissolved chlorophyll and absorb maximally in the red (λ600–700 nm) and blue (λ400–500 nm) regions of the spectrum and not as easily in the green (λ500–600 nm) regions. Up to 80% of all green light is thought to be transmitted through the chloroplast (Terashima et al., 2009) and this allows more green photons to pass deeper into the leaf mesophyll layer than red and blue photons. When the green light is scattered in the vertical leaf profile its journey is lengthened and therefore photons have a higher chance of hitting and being absorbed by chloroplasts on their passage through the leaf to the lower leaves of the plant. Photons of PPFD (photosynthetic photon flux density) are captured by chlorophyll causing an excitation of an electron to enter a higher energy state in which the energy is immediately passed on to the neighboring chlorophyll molecule by resonance transfer or released to the electron transport chain (PSII and PSI). Despite the low extinction coefficient of chlorophyll in the green 500–600 nm region it needs to be noted that the absorbance can be significant if the pigment (chlorophyll) concentration in the leaf is high enough. The research available clearly shows that plants use green wavelengths to promote higher biomass and yield (photosynthetic activity), and that it is a crucial signal for long-term developmental and short-term dynamic acclimation (Blue:Green ratio) to the environment. It should not be dismissed but studied more because it brings more opportunities to control plant gene expression and physiology in plant production. REFERENCES Banerjee R., Schleicher E., Meier S. Viana R. M., Pokorny R., Ahmad M., Bittl R., Batschauer. 2007. The signaling state of Arabidopsis cryptochrome 2 contains flavin semiquinone. The Journal of Biological Chemistry 282, 14916–14922. Dhingra, A., Bies, D. H., Lehner, K. R., and Folta, K. M. 2006. Green light adjusts the plastic transcriptome during early photomorphogenic development. Plant Physiol. 142, 1256-1266. Folta, K. M. 2004. Green light stimulates early stem elongation, antagonizing light-mediated growth inhibition. Plant Physiol. 135, 1407-1416. 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