The Green Light Myth: Why Green Light is Essential in Your Plant Spectrum

Highlights:

UbiGro maintains green light fraction of sunlight
Other luminescent materials greatly reduce green light
Green light drives photosynthesis deep in leaf tissue and in dense canopies

Plants use a portion of the solar spectrum for photosynthesis. This portion is the visible light region known as photosynthetically active radiation (PAR), which spans from 400-700 nm. However, not all colors of light within PAR are equally effective at driving crop yields. An emerging strategy to modify the sun’s spectrum and enhance crop growth and yields for greenhouse growers is the passive down-conversion of natural sunlight from short to longer wavelengths (from blue to red). This passive down-conversion process can be achieved using luminescent or glowing materials made of organic and inorganic dyes, phosphors, or quantum dots (QDs).

UbiGro is an agricultural film that utilizes nanotechnology called quantum dots to tailor the sunlight spectrum in a greenhouse by absorbing UV (<400 nm) and blue light (400-500 nm) and converting it to red and orange light (600-700 nm) via a process called photoluminescence. The peak emission color of the QDs is easily tuned by simply changing the size of the nanoparticles in the manufacturing process. This way, a specifically tailored light spectrum can be engineered for a particular crop by selecting a specific size of QD to shift a portion of the sunlight spectrum to the desired color.

While the benefits of shifting some UV/blue light into red/orange light have been fairly well-established, this article focuses on the role of green light. We discuss green light’s role in horticulture and its importance when considering what parts of the sunlight spectrum to absorb and what parts to retain when engineering a tailored spectrum for plants.

QD Film vs Dye Film

First, let us examine how the UbiGro QD greenhouse film compares to another passive luminescent greenhouse film, which uses a light-altering dye that converts green light to red light. As shown in the figure below, the UbiGro QD film absorbs only UV and blue light, resulting in a relative boost in red light (>10%), peaking at about 600 nm. Importantly, it does not absorb green light; the relative green light fraction is increased by about +6%. The QD film allows 87% total PAR transmission. This results in a bright, orange-colored greenhouse layer.

In contrast, the luminescent dye absorbs mostly in the green portion of the spectrum, 500-600nm, shifting light into the red, peaking at about 640-660 nm. The relative green loss is -64%. The dye film allows 62% total PAR transmission. This results in a darker, magenta-colored (blue+red) greenhouse layer. Compared to the QD film, the dye film allows less PAR transmission and removes green light rather than UV/blue light to achieve the red boost.

So, is green light important? Would we prefer to leave green in the spectrum or convert it to red? The effects of green light on crop production have been studied, and while horticulture scientists continue to learn about the effects of the spectrum of crop growth and development, some general trends have emerged related to green light.

The Green Light Myth

A common misconception that has historically been popular amongst plant biologists is that green light is ineffective in driving photosynthesis in plants.¹

The widespread interpretation was that green light is not used efficiently in photosynthesis because chlorophyll has a minimum absorption in the green band. The combination of lower absorption and higher reflection for green light than other colors gives plants their green color. However, lower photosynthetic efficiency for green light has not been shown experimentally. Although green light drives photosynthesis less efficiently than does red light per unit of leaf area, on the whole-plant and whole-canopy levels, green light may increase growth due to changes in vertical light distribution, leaf light acclimation, and canopy architecture.²

The contradiction is revealed when analyzing the absorption of isolated chlorophyll in a solution compared to that of a leaf, where leaf chlorophylls are concentrated in chloroplasts, resulting in uneven distribution of chlorophylls within the leaf cells. Because plant tissue reflects and scatters light, the effective light path length is elongated, increasing light absorption, especially green light.³ Leaf absorptance of green light was measured to be 77-88% in a number of species, compared to ~95% for blue and ~91% for red.⁴ So plants do indeed absorb green light, just at a lower rate than other colors.

Growth Rates

Studies have concluded that a high green light fraction can improve growth rates. A study on lettuce using LEDs showed that supplementing a red and blue LED with green light increased growth rates by 47% while holding PPF constant.⁵ A green light fraction of 24% was found to be optimal, while too little or too much green light fraction reduced growth rates.

Tissue Penetration

Under high light intensity, blue and red photons are so strongly absorbed in upper cell layers that the bottom of leaf cells are deficient in those colors; but green light, less strongly absorbed by chlorophylls, penetrates deeper into the leaf to drive photosynthesis.⁶ Green light has been shown to drive leaf photosynthetic rates more efficiently than red or blue light, in a high light background, indicating that whole plant photosynthetic rate could benefit from green light penetration to lower leaf layers.⁷ The result is that green photons may be as efficient as red and blue photons when provided with a high light intensity.⁸In a study where chlorophyll fluorescence was studied in leaf cross sections, it was found that green light penetrated much deeper than red or blue light. 11

Canopy Penetration

While red and blue light drives CO₂ fixation in the upper canopy leaves, it is the green light that penetrates deeper into the canopy and therefore drives CO₂ fixation in lower canopy leaves.⁹ For high-density canopies, converting blue light to green can provide greater light penetration into the canopy by increasing the fraction of highly penetrating wavelengths.¹⁰

Green light has also been shown to be particularly beneficial once the upper canopy as a whole is saturated. ¹² Furthermore, the fact that green light provides a more uniform light distribution within the canopy means that the presence of green photons in a spectrum leads to higher overall canopy photosynthetic efficiency. Shade avoidance responses are induced by green photons, working against the decrease in plant size triggered by the presence of blue photons. This can be seen by analyzing the canopy shade spectrum, which exhibits an increase in G:B ratio because blue light is quickly absorbed and green light penetrates the canopy. ¹³ Green light fraction has also been shown to increase shade avoidance response in lettuce. ¹⁴

Artificial Lighting Trends

Despite a history of using only red and blue LEDs, lighting manufacturers such as HelioSpectra (Sweden) and LED Habitats (Santa Fe, NM) have begun incorporating green LEDs into their fixtures for a fuller spectrum. HelioSpectra performed a study on tomatoes that confirmed the impact of green light on the graded size of tomatoes. “On average, 90% of ripened fruits from plants grown under light treatment with a high green percentage were of A class or categorized as large size, and only 70% from plants grown under 5% green treatment reached full size and were categorized A large. Fruits of plants grown under 40% green light were also 18% heavier on average than fruits of plants grown under the 5% green light treatment ¹⁵.”

Conclusion

In the realm of plant growth, the role of green light, often underestimated due to its lower efficiency in chlorophyll absorption compared to blue or red light, emerges as crucial when considering its unique properties. The higher reflectance and lower absorption of green light not only facilitate photosynthesis deep within plant tissue but also penetrate dense canopies more effectively, highlighting the benefits of green lights in plant growth. This discovery underscores the importance of green light as a vital component of the optimal light spectrum for plant development, aligning with research on the plant growth light spectrum. This insight has led LED manufacturers to incorporate green light into their latest, most advanced fixtures, recognizing its essential contribution to plant health and productivity. Omitting green light in favor of an exclusive focus on red light could undermine the comprehensive benefits of a full-spectrum approach to plant lighting. UbiGro, aligning with this understanding, preserves a significant fraction of green light, minimizing absorption in the green spectrum. Instead of eliminating green light, UbiGro strategically reduces certain wavelengths, like UV and blue light, enhancing the red/orange light fraction. This adjustment promotes increased growth rates and crop yields, leveraging the full spectrum’s potential to support plant health and productivity, which includes the benefits of green lights in photosynthesis and plant growth.

Source

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Kim, Hyeon-Hye; Gions, Gregory D.; Wheeler, Raymond M.; Sager, John C. ‘Green-light supplementation for enhanced lettuce growth under red- and blue-light-emitting diodes.’ HortScience. 2004.
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https://www.heliospectra.com/growers-center/green-light-for-tomatoes

The Green Light Myth: Why Green Light is Essential in Your Plant Spectrum

Dr. Damon Hebert

Dr. Damon Hebert serves as Director of Agriculture Research for UbiQD, Inc., with a background in solar materials and controlled environment cannabis cultivation. He is an advocate for the use of advanced materials to further the industry’s push towards sustainable farming practices. He can be reached at [email protected].