There is a great deal of controversy and misinformation regarding the role (or lack thereof) of Green spectrum in photosynthesis. One common misconception is that because plants are green (due to the photosynthetic pigment Chlorophyll being GREEN)they do not absorb and consequently metabolize green light. In part this is true, for plants do reflect away more green light than other spectrum’s and that is precisely WHY they look green. But they reflect only a portion of the Green light away. Some does penetrate into the leaf and can be utilized, so in this article I am going to examine this complicated and poorly understood process in a quest to understand the best spectrum for cannabis.
Plants absorb light for photosynthesis using photo-active pigment compounds that absorb certain ranges of frequencies and reflect others.The photons (light) that are absorbed effectively create the energy for the plant by taking carbon from carbon dioxide in the air, and water and minerals from the roots and combining them to create highly reactive enzymes that are then used to produce carbohydrates (sugars) in the Chloroplasts (Plant energy factories and home of chlorophyll pigments)
There are several photo-active pigments in plants that can create energy. The most important is Chlorophyll, the pigment that makes plants green. Chlorophyll resides INSIDE the Chloroplast, which is the place where this photosynthetic chemical conversion takes place, and for this reason Chlorophyll is the most efficient of the photosynthetic pigments. All other pigments create an enzyme that must be transported through an amazingly efficient quantum process to the Chloroplast to make the final carbohydrate. Read the book “Quantum Biology” for more details.
There are actually two forms of Chlorophyll referred to as Chlorophyll A and B, and both forms absorb light in both the Red and Blue spectrums and reflect away green and yellow. Chlorophyll A is considered the primary photosynthetic pigment in plants, and it has more jobs than just metabolizing light, where Chlorophyll B functions very similarly to the other pigments.
The next most important photosynthetic pigment is Carotene or the Carotinoid system. Carotinoids are pigments ranging from yellow to red,and they are the pigments responsible for the red and yellow colors leaves take on as they die in the fall. The reason for this change is that Chlorophylls are in a layer at the outer surface of the leaf, and the Carotinoids deeper down in the leaf. So in a healthy leaf we see only the outer layer of green Chlorophyll.
The Chlorophyll requires an on-going supply of nitrogen to survive and nitrogen is one of 5 transportable minerals that can be moved around by the plant after a leaf is fully grown. The other non-transportable minerals are supplied to a leaf when they grow and they cannot be moved. So when a leaf is allowed to die by a plant,the nitrogen is moved away from that leaf for other uses and the Chlorophyll degrades. But the Carotinoids do not and so as the outer layer of green Chlorophyll degrades, the deeper yellow/red carotinoids become visible.
It is this structural hierarchy that also makes Chlorophyll more efficient photo- synthetically, because as long as the green Chlorophyllis present, a large percentage (but NOT all) of Green /yellow spectrum is reflected away before it can penetrate deep enough to be reach the Carotinoid pigments.
This is also why when the top of a plant gets too close to alight, they turn yellow. The plant recognizes that the light in that area is so intense that it doesn’t NEED the Chlorophyll to supply that section of the plant properly, because the carotinoids will handle it just fine. So the plant moves the nitrogen for the Chlorophyll to more deserving areas that have less light and leaves the Carotinoids to do the job.
But here is where the real complexity arises. In a environment where light is abundant,such as with direct sunlight, the underlying Carotinoid system actually increases the leafs total energy capacity.It is literally a secondary energy system to absorb the green light that is not reflected away by the chlorophyll, and it is very helpful in a canopy where a major portion of the leaves are partially or fully shaded. But this extra capacity costs efficiency because over 50% of the green light is reflected away. This reflected light can be still absorbed in deeper canopies through the bottom of the leaves, and so green light can help support highly shaded areas of the plant and this is clearly why the carotinoid system evolved, to use green light that is not utilized by chlorophyll, in more shaded areas.
But in an indoor grow, every PHOTON costs money. So it is not cost efficient to waste energy on green photons in the interest of increasing the overall capacity of the leaf. It is much more efficient to eliminate all shaded areas though pruning and shaping a plant, AND providing the light in a highly diffused (multi-directional)form so that each leaf (and flower site) sees an even, uniform level of light at or below the leaf Chlorophylls capacity for absorption. With shading thus eliminated we can then modify the light so it contains ONLY the spectrums best utilized by the efficient Chlorophyll layer, namely the 4 bands in red and blue regions referred to as the Chlorophyll A and B absorption peaks.
There is a lot of discussion currently circulating regarding the importance, (or lack thereof) of Green spectrum primarily because High Pressure Sodium (HPS) lights are extremely effective Flower lights and yet they produce roughly half their spectrum in the Green Yellow region. But the current crop if top-tier LED lights with roughly equal electrical efficiencies (umols/J) to that of HPS, can save up to 30% or more in power consumption with equal yields per sq foot of canopy IF the above described optimal conditions are met. And remember, FULL SPECTRUM white LED’s must provide roughly an equal portion of green light to produce white light, and so the savings potential in these light is significantly less than LEDs which specifically target the chlorophyll peaks.
It is important to remember that the more shading you have in your canopy, the more effective the green spectrum becomes. So to fully leverage the value of photosynthetically optimized spectrum, it is important to minimize shading. And the more shading you have the more effective green rich light sources like HPS or sunlight become. We also do not know if green light has a role in other plant processes so eliminating green light completely may not be desirable.
Controlling the spectrum has additional benefits in regards to the phenomenon of photomorphogenesis, or the effect of various spectrum on the pheno-expressions of plants, (and this topic is discussed in depth in a different paper,) but it appears that Green light has a similar effect to Red light,namely it promotes stem, flower and fruit, where Blue Light promotes leaf and root production.
So in conclusion, significant savings can be realized by minimizing Green spectrum in the light provided to plants, but results will vary depending on the shape of the canopy and the diffusion level of the light supplied.