The biggest response I ever got on a social media post was April 1, 2017, when I “introduced the Gavita Laser Lights” on Facebook. Obviously, this was an April Fools post, but it was incredible to see how long this thing lasted, and I still get (serious) questions and inquiries about it.
Mixing fiction with credible data can make a compelling (false) argument, and sometimes reality is stranger than fiction. So, here is the challenge: I will write a short article every edition, and it is up to you to decide whether this is fact or fiction. Call it an exercise in critical thinking and fact-checking.
Let’s first get this out in the open: the spectrum of a wide range of lamps has minimal effect on photosynthesis. That is, if the plants are healthy. If you measure the photosynthesis of plants under a variety of lamps, from incandescent to LED, you will not see a lot of difference. Don’t believe me? Look at research that Bruce Bugbee did back in 1994.¹ A quote:
“PHOTOSYNTHETIC RATE IS SURPRISINGLY LITTLE AFFECTED BY LIGHT QUALITY FROM STANDARD LAMPS.”
The most efficient spectrum in that research came from a low-pressure sodium lamp, which you would never use to grow a plant, of course. The most important aspect of a lamp, purely for photosynthesis, is efficiency in the PAR spectrum and preferably in the orange/red area, and not so much the wide spectrum.
Now, we all know that you can’t grow a decent plant under LPS or incandescent, the first has a too narrow spectrum and the latter way too much infrared. It just illustrates that the spectrum is not so much the main driver for efficient photosynthesis. There are many other plant processes that require various wavelengths (colours) of light.
But how about the McCree curve? It clearly shows that green is about 25% less efficient than other colours of light? Yes and no. Yes, the experiment is valid, and it was repeatedly verified, but no, this is not how you should interpret the results. Without going too much into details, McCree tested leaf disks under quite a low intensity of light. We see that plants as a system, under high-intensity light, have a different response. There is even evidence to suggest that green light is more efficient in high intensity “white” light than red light.² For more about McCree and spectrum, read “Light Matters 5 – Supplementing the Sun”.
If it is not photosynthesis that makes the difference with different spectra, then what is it? For the most part, it is morphogenesis. When you compare light sources and spectra, you see very significant differences on the plant’s growth rate, shape, leaf density, leaf size, the stomata density on the leaf, leaf thickness, internodal distance, and much, much more. In a study done by Sander Hoogewoning³ comparing the effects of CFL, HPS and artificial sunlight, the results were remarkable:
The artificial sunlight treated plant had more than twice the leaf area than the CFL and HPS grown plants.
Another effect that is hard to monitor, but should not be underestimated, is rooting. Under some spectra, plants root better, and a better root system in flowering gives a much better result.
You can imagine that with so many factors playing an important role, and not in the least the genetics you are using, it is very complicated, if not impossible, to give a very specific lighting advice. Maybe for that Indica, you want a bit of extra stretch to keep the plant more open, or maybe you want to suppress the stretching of that Sativa. To make things even more complicated: light intensity plays a role as well.
It is also not automatically so that a wider spectrum gives you a better result, that depends on the ratios of colours in the spectrum. One thing is clear, though: a better result under a better spectrum is not so much the effect of the photosynthetic efficiency of the light, but the morphogenesis of the plant.
The last “Fake or not” was more like an article instead of a claim. The question at hand: does the spectrum of a grow light influence photosynthesis? The answer was no, it doesn’t, but it does influence morphogenesis, which in turn can influence photosynthesis.
When you compare light sources and spectra, you see significant differences in the plant’s growth rate, the leaf’s density, shape, thickness, size and positioning, as well as root development, the density of the stomata on the leaf, internodal distance, and so much more. This is because of the way a plant responds to different wavelengths of light.
Though net photosynthesis does not change very much with different light sources, a different plant shape can increase the light interception and total photosynthesis, resulting in a bigger plant with a better yield. However, adding light for an increased yield is not always the best strategy. For cultivars requiring less light per day, the biggest improvement could be seen instead with a better spectrum.
So, yes, you can “steer” a plant with climate control and a specific spectrum of light. The problem? No plant is the same. Even within varieties, many cultivars will have different optimal spectrum and light requirements. There are, however, a few broad “rules” that are true for many plants.
Spectrum Hardly Influences Photosynthesis: Fake or Not?
So why are plants green? This is an easy one. Your eyes don’t lie: Most plants are in fact green. They absorb mostly the red and blue light, reflecting the green light. The scientific proof which predominantly influences our ideas about plant response to different wavelengths of light was produced by Keith McCree in 1970 (published in 1972), in his famous paper “The action spectrum, absorptance and quantum yield of photosynthesis in crop plants” – leading to the definition of PAR light and the, now famous, McCree curve (fig 1).
The McCree curve clearly shows a dip in the spectral response around the green light, which also shows that red light is most efficient. Most horticultural lighting manufacturers nowadays acknowledge this research, and with the introduction of LEDs it is now much easier to create a spectrum which is much more like the McCree curve, providing optimal lighting for the plant at maximum efficiency. You can see this in greenhouses and indoor facilities alike all around the world nowadays: This purple kind of glow is a result of mixing only red and blue LEDs, as they are the most efficient for plant lighting (fig 2).
It is reflected (no pun intended) in the spectrum of most LED plant light products on the market today: they mostly only have predominant red and blue light. You can also see that effect back in the green LED products which are used as a safelight during the dark cycle of a generative short-day plant, without disturbing the flowering cycle. (fig 3)
So there you have it, both supported by science and products evolving from this science. Plants are green because green light is very inefficient for photosynthesis, and it can be used during the dark cycle as a safelight, not interfering with the flowering cycle of short day plants.
A relatively easy challenge for loyal readers of Garden Culture magazine! In a previous article, page 72 of the US-13 Straight out of Compost edition, I describe that the McCree curve is not a guideline for the optimal spectrum of your light, as well as outlining the effect of green light.
FAKE. Green, in fact, is an efficient color. Although you can use pure green light during the night to inspect your crop, it has nothing to do with its efficiency. Let me explain.
McCree did his experiments under a very low intensity (monochrome) light, lighting a leaf disk with different colors to measure the photosynthetic response. He did not research high-intensity light or “white light” (like the sun, with a spectrum of all colors combined), nor did he measure or research the efficiency of light on plants as a complete system. Though McCree’s experiments are repeatable and completely correct, it is the interpretation where many go wrong. To say “plants are green, so green light is reflected – ergo: plants don’t use green light” is a huge mistake. To say “look at McCree, we should only use red and blue light” is also a huge mistake. Here is a link to more recent research that explains why green is an important color, and could even be more efficient than red and blue light!
Ichiro Terashima, Takashi Fujita, Takeshi Inoue, Wah Soon Chow, Riichi Oguchi; Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green, Plant and Cell Physiology, Volume 50, Issue 4, 1 April 2009, Pages 684–697, https://doi.org/10.1093/pcp/pcp034
So why are so many LED fixtures based on red and blue? This has to do with the efficiency of the LEDs. Red and blue are the most efficient colors, where white LEDs are primarily blue LEDs with a phosphorous coating. Over the last years, there has been a great increase in the efficiency of blue and white LEDs, but white LEDs are still considerably less efficient than blue LEDs because of the phosphor losses. In greenhouses, using red and blue only is not an issue: the most dominant color in sunlight, the primary light source for a greenhouse crop, is green! A lot of the spectrum that the LEDs lack is already provided by the sun. Not so much in an indoor facility where your artificial light is your only source.
Though true, it has nothing to do with the efficiency of the green light. Whether a short day plant flowers or not is regulated by a photoreceptor called Phytochrome (P). Phytochrome can have two states: The active state Pfr, which is sensitive to far red light (730 nm), and the inactive form Pr, which is sensitive to red light (660 nm). A long period of darkness will also convert Pfr into Pr, and that is exactly how the plant senses long nights (or short days if you wish). A high amount of Pr in the plant signals that the season is ending and it should start to reproduce. Interrupting the darkness with normal light will immediately transform Phytochrome into its active Pfr state and the dark cycle will start from the beginning, as there is red light in normal light, leaving not enough time to convert the Pfr to Pr. Phytochrome is selectively sensitive to different colors, mostly red (660 nm) and far red (730 nm). If you look at the spectral absorbance spectra for Pr and Pfr you will see that it is not very sensitive to green light. Now you should not overdo it of course, but using a low-intensity monochrome green light will not interfere with the transformation of Pfr to Pr. And that is the reason why true green light will not interfere with your flowering cycle. This is just a very basic explanation of how this process works. There are great resources available on Google Scholar.
Fake or Not?