In horticulture, red and blue wavelengths were thought to be the only part of the spectrum that drove photosynthesis because chlorophylls are receptive to those colors. New research now reveals that previously dismissed green light can also drive plant growth. Fluence Bioengineering’s chief innovation scientist Dung Duong explains the new data.
The current state of the LED horticulture lighting industry would lead some to believe the world only exists in shades of red and blue. Thanks to narrow readings of research done by Dr. K.J. McRee and Dr. Katsumi Inada, a general consensus in the industry is visible ranges of red and blue light are the only wavelengths of light needed to induce photosynthesis.
However, a better analysis of the experiments conducted by McCree and Inada has shown limitations on what should be concluded from their research. It is important to recognize the discrete findings from McCree and Inada do reinforce historical experiments which reached similar conclusions but also clearly define the boundaries in terms of technology and intent regarding how each of these experiments were performed. While McCree’s research and Inada’s research are groundbreaking for understanding the science behind photosynthesis, there have been assumptions derived by the horticultural lighting industry which are incorrect.
Why Green Light Became “Invisible” in Horticulture Lighting
Before we can fully discuss the misconceptions around visible light, we need to discuss a couple of myths on why red and blue wavelengths are generally thought to be the only wavelengths needed to drive photosynthesis. Chlorophylls are widely recognized as being the primary photoreceptors which allow plants to draw energy from light. Chlorophylls A and B absorb the most amount of energy from the blue and red regions of the electromagnetic spectrum (Figure 1). But chlorophylls are not the only photoreceptors that drive photosynthesis. Carotenoids are another example of photoreceptors which efficiently promote photosynthesis and will absorb energy from the violet to green regions of the electromagnetic spectrum; chlorophylls are simply the most effective.
As already established, chlorophylls are highly receptive to red and blue light (Figures 2a and 2e). Red and blue light will penetrate the top few cellular layers of a single leaf but will generally be absorbed by chlorophylls located in those top few layers. Green light behaves differently when it penetrates the top layers of the leaf (Figure 2c) as it is not as effective at being absorbed by chlorophylls. This characteristic is beneficial as it allows green light to penetrate deeper into each layer and partially transmit through the bottom layer. When this is applied to an entire plant, this characteristic allows green light to efficiently penetrate through to the entire lower plant canopy driving greater photosynthesis throughout the entire plant.
There is an important distinction to be made in research regarding conclusions that are drawn from measurements of a single leaf compared to those of an entire plant. McCree and Inada both identified biological effects associated with various wavelengths of light on single leaves of a wide variety of plants. This is represented in what is known as the action spectrum (Figure 3). The biological effect of green photons historically may have been discounted because the green photons absorbed by carotenoids in single leaves were thought to have a negligible biological effect. However, when this seemingly negligible reaction is measured across an entire plant, the efficiency of green light on every leaf in a plant becomes much more apparent, as seen in Fig.3.
Efficiency: What is Good for the Fixture May Not Be Good for the Plant
To understand this distinction of efficiency, one needs to further dissect how the photoreceptors in a plant interact with green light, particularly at the most basic level: the photon. A basic precept of plant photobiology is how green photons are mostly reflected when hitting chlorophylls at the surface of the leaf, which gives plants their greenish color. However, not all green photons are reflected. Some will pass-through air interfaces in the chloroplasts and will even transmit through chlorophylls.
Because green photons are not absorbed completely by photoreceptors at the surface of the leaf (Figure 2), green light is able to penetrate deeper through a leaf to drive photosynthesis in chloroplasts located towards the bottom surface of the leaf and beyond. This reaction with green light occurs more effectively at high photosynthetic photon flux density (PPFD) than red light emitted at a comparable PPFD. In fact, as PPFD increases, light energy that is absorbed in the upper chloroplasts will reach a saturation point and will be dissipated as heat, while penetrating green light increases photosynthesis by exciting chloroplasts located deep in the mesophyll. And since green light penetrates much more effectively to the lower canopy, green light will help drive photosynthesis across the whole plant as it is absorbed by leaves in the lower canopy not exposed to red or blue light.
One last thing to keep in mind is red light is more energy efficient for light fixtures to emit at high PPFD, but as is evidenced by the heat dissipation of red light, it is not used as efficiently by the plant. Green light, on the other hand, is not as efficient to emit but is more effectively used by the whole plant.
Seeing Green, Again
All this is not to say red or blue light do not have their own uses. Certain photomorphogenic effects can be achieved when narrow-band lighting is employed as a supplemental light. However, for general growing applications, broad-spectrum light, which includes red, blue, and green light should be used to ensure a plant can achieve its full potential. While red and blue light align with peak absorption for chlorophylls A and B and can result in adequate yields, all the other antenna photoreceptors that impact secondary metabolite production are neglected and result in poor quality for the entire plant. Indeed, high-intensity discharge lights (Figures 4a and 4b) have all included green light in the past, and these bulbs typically all possess strong green wavelength emissions.
This is not to mention visual acuity green light provides by complementing red and blue to create a full and broad-spectrum, perceived as white light by the human eye. This enables growers to clearly observe their plants, which is sometimes overlooked as another key contributing factor to the success of plant growth and development.
The best way we have found to put this myth to rest would be to perform side-by-side trials of plants grown under narrow-band red and blue lights at high PPFD compared with plants grown under a comparable PPFD of broad-spectrum light. The resulting plants and yields will most definitely speak for themselves, as it has for the many commercial growers that have employed lighting solutions with a broad-spectrum for their growing applications.