Chlorophyll and carotenoid pigments capture green light and use it for photosynthesis. Chlorophyll absorbs low amounts of green relative to red and blue light, so it’s best to provide a plant will at least all three types of light. When combined with red and blue light, green light further enhances plant growth. But too much green light (more than 50% of the total light) reduces plant growth. At this time, the ideal ratio of green, red, and blue light (as well as other colors of light) is not yet known for many species, including cannabis. For one tomato variety, the ideal ratio is 1:2:1 for G:B:R. Likely, the ideal spectrum for cannabis vegetative growth will be strain-dependent. When choosing a horticultural light, choose one that that has high amounts of blue and red light and moderate amounts of green and other colors of light.
Green light is easily transmitted through leaves. When sunlight or another source of full-spectrum light reaches a plant, the leaves transmit high amounts of green light and low amounts of red and blue light. This means that the leaves at the bottom of the canopy receive a modified spectrum that is low is blue and red light and enriched in green. Green light is absorbed by photoreceptors. One type of photoreceptor is cryptochrome, and this photoreceptor controls stomatal opening and stem elongation. Depending on the species, green light can either cause stomata to open and close and stems to stretch or stay short. In some species, like mustard and fava bean, green light closes stomata. In other species, like sunflower, green light opens stomata4! At this time, it’s not clear whether green light opens or closes stomates on cannabis leaves. Green light (via the action of cryptochrome) also controls stem elongation. When a plant is shaded, the stems elongate so that the leaves can reach more light. When plants are given high amounts of green light, they think they are being shaded and their stems elongate and the leaves become larger so that the plant reaches more light.
Figure 1: When full-spectrum light hits the leaves of a plant, the photosynthetic pigments absorb much of the red and blue light. Therefore, the light reaching the lower leaves of the canopy is enriched in green and far-red light. Photoreceptors in the lower leaves receive this altered spectrum and signal to the lower leaves that they are being shaded.
Green Light and Seed Germination
Green light mediates seed germination in some species. Seeds use green light to evaluate whether the environment is good for growing. If a seed germinates in a shady spot, it can be detrimental to the plant because it will not get enough light to grow. A seed that senses a shaded environment may avoid these adverse conditions by staying dormant and not germinating. Shade environments are enriched in green relative to red and blue light. Seeds of different species show a range of responses to green light. Green light prevents seed germination in some species like ryegrass (a grass that grows in tufts) and Chondrilla (a plant related to dandelion). Surprisingly, green light can stimulate seed germination in a number of uncommon species like Aeschynomene, Tephrosia, Solidago, Cyrtopodium, and Atriplex. There are several factors that affect seed germination, such as soil moisture, soil type, temperature, photoperiod, and light quality. Light quality prevents germination at inappropriate times that could potentially compromise survival. At this point, the impact of green light on cannabis seed germination is not known.
Green Light and Flowering
When it comes to growing cannabis, many cultivators are most interested in the quality of light used for the flowering stage. In many plants, flowering is mainly regulated by two main photoreceptors: cryptochrome and phytochrome. Both photoreceptors primarily respond to blue light but can respond to green light as well, although to a much smaller degree. Green light is effective at accelerating flowering in a number of species. Although no cannabis-specific experiments have been performed, increased green light may encourage flowering. 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 light, in order for photosynthesis to be optimized.