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Green Light

495nm - 570nm

California Lightworks, URSA, Excite, Fluence, SANlight Spectrum Bar

The middle region of the visible spectrum stretches from green at about 520nm and runs in a yellow color up to about 600nm. It was assumed that the green colors in this area are slightly or not at all absorbed by the majority of plants. Simply it was stated by scientists, that the leaves are green because they reflect the green light. This is the reason why chlorophyll has a green appearance, which in turn gives the leaves the green color. It was completely ignored that carotenoids and other auxiliary pigments absorb light in the middle of the spectrum. Recent studies have showed that the plants absorb the majority of green light.

What Effect Does Green Light have on Plants?

Green Light Wavelength

Choosing the best lighting for growing cannabis can be intimidating, especially when there are so many factors to consider, like spectrum, lumens, PPFD, CRI, CCT, and more! Don’t know what those terms mean? We’ve already covered the basics of horticultural lighting, so read that first if you haven’t already. We are going to dive deeper into one of those factors, and that’s spectrum. The spectrum of light used by a plant is called photosynthetically active radiation (PAR). PAR was first defined in the 60s as the wavelengths from 400 to 700 nm. Today we now understand PAR to encompass wavelengths outside of this range, to include UV and far-red light. This article is part of a series covering all the colors of light, including UV, violet, blue, green, yellow, orange, red, and far-red light! This focus of this article will be on green light. Green light is radiation with wavelengths between 520 and 560 nm and it affects photosynthesis, plant height, and flowering.

Green Light and Vegetative Growth

Green light plays a role in photosynthesis and vegetative growth because it falls within the range of photosynthetically active radiation. However, its effect on plant growth and development is not as well understood as red or blue light. Plants reflect green light, and this is why they appear green to our eyes. This may lead us to think that green light is not used by plants, but it’s simply not true! Only around 5-10% of green light is reflected from a plant and the rest (90-95%) is absorbed or transmitted to lower leaves.

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.

Leaf Reflectance Green 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.

Seed Germination Green Light

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.

When used alongside red and blue light, green is important for both the vegetative, germination and flowering stages of plant growth. Green light penetrates the plant canopy, allowing light to reach the lower branches of the plant. Green light can also be used to manipulate stomatal opening and closing and plant height. In some species, green light can even regulate seed germination and flowering, although this hasn’t been explored in cannabis. The addition of other colors of light (full-spectrum light) has been shown to have further benefits for plant growth.

Green Leaf Green Light Excite LED Grow Lights

When most people think about LEDs being used for horticulture lighting, the first thing that comes to mind is a purple glow being emitted from a fixture consisting of red and blue diodes. This purple glow may be the industry standard for horticulture LEDs, but have you ever wondered why red and blue diodes have historically been the semiconductors of choice by lighting manufacturers? Many manufacturers reference the absorption spectrum of chlorophyll a and b (which peaks in the blue and red regions of the electromagnetic spectrum) as the main reason for providing a purple spectrum.

Absorption Spectrum (Fluence BML)
Figure 2: The Absorption Spectrum of Photosynthesis

At first glance, this seems legitimate since chlorophyll drives photosynthesis after all, but have you considered how the absorption spectrum of chlorophyll is measured? Additionally, have you considered if the absorption spectrum of chlorophyll directly correlates to photosynthesis and plant growth, and what happens if you only target a sole pigment and neglect other pigments responsible for plant growth and development? This article will discuss the differences between absorption spectrum and action spectrum, and (spoiler alert) dispel the myth that “plants don’t utilize green light” to promote plant growth and development.

Action Spectrum

The action spectrum of photosynthesis was created from research that was performed in the 1970s by Drs. McCree and Inada and this work was fundamental in defining the range of photosynthetically active radiation (PAR). Prior to this research, very little work had been performed to determine how varying wavelengths of light influenced photosynthesis and plant growth. These researchers utilized filters to create monochromatic wavebands to determine the influence of light spectra on photosynthesis of single leaves using an assimilation chamber. If you see figure 2, you will notice that plants do in fact utilize green light for photosynthesis, quite efficiently in fact (figure 2).

Action Spectrum (Fluence BML)
Figure 3: The action spectrum of photosynthesis as determined by Drs. McCree and Inada.

So why is there such a difference between the absorption spectrum and the action spectrum if chlorophyll is responsible for photosynthesis? The answer is simple: Chlorophylls are not the only photoreceptors that are responsible for photosynthesis. There are other types of antenna photoreceptors (mainly carotenoids) which also promote photosynthesis, and by utilizing narrow band red/blue LEDs in sole-source lighting conditions these pigments are not able to optimize their light harvesting capabilities. Also, it should be noted that green light does in fact promote photosynthesis in chlorophylls, quite efficiently if fact. Recent work has shown that green light is able to penetrate deeper into leaf surfaces to drive photosynthesis in chloroplast located towards the bottom surface of the leaf, in fact, more efficiently than red light at high PPFD. As PPFD increases, light energy that is absorbed in the upper chloroplasts tends to be dissipated as heat, while penetrating green light increases photosynthesis by exciting chloroplasts located deep in the mesophyll (Terashima et. al., 2009). Additionally, green light penetrates through leaf surfaces much better than red or blue light to reach the lower canopy, which is extremely important in dense canopy production techniques which are common in controlled environment agriculture.


Conclusion

So, what does this all mean for the grower? While we are still in the early-stages of understanding how plants use light and we are still working to understand how different photoreceptors function and interact with one another, the bottom line is that plants absolutely do utilize green light. If your horticulture lighting system is delivering a spectrum which neglects photoreceptors that absorb light in the 500-600 nm region (especially in sole-source lighting applications) you will not be optimizing your growing environment.

Excite LED Grow Lights iSpectrum 125 W Vegging, Flowering Spectrum Graph
Figure 4: Excite iSpectrum 125W Spectrum Graph | Left: Grow, Right: Bloom




Figure 5: URSA Optilux 640W Spectrum Graph


Source: URSA | Fluence Bioengineering | Excite | California Lightworks