Green leaves and green light: what's really going on

by /u/SuperAngryGuy of /r/HandsOnComplexity

This article challenges the claim that 'plants can't use green light', 'plants are green because they reflect all green light', and other iterations. My counterclaim is the McCree curve used in botany, and every paper on photosynthesis studies by wavelength demonstrates that most plants use green light efficiently (particularly compared to blue light and at higher lighting levels).

The claim and my problem with it

Leaf penetration by light color (source)

There is a lot of confusion about how green plants absorb light, and the notion that 'plants can't use green light' or 'plants are green because they reflect all green light'. This confusion comes from biology books that show you a chart for pigments in a solvent or photosynthetic bacteria/algae, and not how higher green land plants actually respond to light.

This has been used by many low end predatory LED grow light sellers, such as making outrageous claims about the photosynthetic performance of red/blue only LED grow lights, by hitting some 'magical wavelengths'. I have seen this claim collectively cause a lot of financial harm to people, and consumers may not be making good choices by thinking the spectral output of a lower wattage red/blue LED grow light is somehow going to make up for the low lighting levels: it absolutely will not!

The counterclaim (and what's really going on)

Most green light is absorbed and is used for photosynthesis. Every scientific paper on plant lighting by wavelength for photosynthesis backs the claim that plants use green light.

There is the claim that 'plants are green because they reflect all green light' or that 'plants can't use green light'. This can be explained with reflectance, absorption, and transmittance. You are likely going off the pigments dissolved in a solvent chart if you believe this, and that's a relative absorption chart in vitro (e.g cuvette), not the McCree curve that is an absolute chart of how plant leaves respond to light by wavelength for photosynthesis in vivo (living leaf). There is a pretty big difference here. Also, at no point is chlorophyll in a solvent truly at zero percent absorption of green light in higher resolution charts.

Unlike chlorophyll in a solvent, in a green plant leaf we have relatively dense chloroplasts, containing thylakoid membranes stacked as disks (grana), that holds the cholophyll in a 3D structure called a quantasome (basic photosynthesis unit). There is a much higher density level of chlorophyll in a leaf than chlorophyll in a solvent extract. So in vitro, with just relatively loose pigments suspended in a solvents, there is going to be a different measurement and spectral characteristics than in a green leaf in vivo, which is a dense solid lattice that changes optical characteristics (such as broadening the adsorption bands).

In this example of the spectral reflectivity profile of a high nitrogen marijuana leaf, about 90% of the green light is being absorbed.

Terashima et al has entered the chat

Schematic of penetration of monochromatic blue, green, and red light. Green light may reach the bottom layer of cells due to the chlorophyll’s weak absorption in the green (the green window) and scattering of green light within the leaf (the detour effect). (source)

The intra-leaf light absorption profile is in most cases steeper than the photosynthetic capacity profile. In strong white light, therefore, the quantum yield of photosynthesis would be lower in the upper chloroplasts, located near the illuminated surface, than that in the lower chloroplasts. Because green light can penetrate further into the leaf than red or blue light, in strong white light, any additional green light absorbed by the lower chloroplasts would increase leaf photosynthesis to a greater extent than would additional red or blue light. Ichiro Terashima et al, Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light.

What the team found was the green light started outperforming red light at about 300 uMol/m2/sec as measured with a pulse amplitude modulated fluorometer. Red and blue light gets quickly absorbed by the chlorophyll near the leaf surface, but green is able to drive photosynthesis deeper.

When I see people mentioning this is only for higher white light conditions, then I can tell they have not read the paper. We are looking at net photosynthesis rates in the above paper and that is what really counts, not absolute absorption. Also, the absorbed green light can transmit deeper through leaf material more effectively and potentially used for photosynthesis more efficiently.

This is because the top layers of chloroplasts that contains chlorophyll becomes saturated (as per PI curves), while green light can penetrate deeper into leaf tissue (sieve effect) and reflected around until absorbed by a chlorophyll molecule (scattering) or by an accessory pigment. This efficiency can be measured through the amount of chlorophyll fluorescence or a gas exchange chamber.

So what high intensity light source has a lot of green light that plants evolved to use? The sun! At full sunlight PPFD of around 2000 umol/m2/sec would be considered very intense light compared to what the average indoor grower would use. With thin leaves (e.g. apple) I can measure perhaps 150 uMol/m2/sec of sunlight through an upper leaf that will illuminate a lower leaf with nearly all green light (which is a very efficient lighting level for photosynthesis). For more information refer to J. N. Nishio (2000). Why are higher plants green?

Your eyes can deceive you, don't trust them

Spectrometer shot of a chlorotic (yellow) leaf

Our eyes have a combined sensitivity curve where the peak of our sensitivity is also were the peak reflectivity is going to be for a green plant. So, it's true plants do reflect more green light than red or blue, but the way we perceive light is naturally much higher biased for green light with a 555 nm sensitivity peak, which is the same as a green plant's reflectivity peak. This allows use to notice very tiny variations of green which can be use to more precisely diagnose a plant if a gatherer. Coincidence?

It should be noted that the maximum absorption wavelength for chlorophyll in leaves in vivo is 675-680 nm (chlorophyll A), and not 660 nm as often cited (chlorophyll B is about 645 nm). This can be seen in the spectrometer shot as a dip in the 675-680 nm range from small amounts of chlorophyll A left over. The blue absorption are carotenoids which have perhaps a 30-70% efficiency at transferring the absorbed light energy to a photosynthetic reaction center through chlorophyll A. Depending on the plant, there may be 2.5ish-7 or so chlorophyll A molecules for every chlorophyll B molecule but mostly around a 3:1 ratio.

Space Bucket with a high power green LED and a pole bean (source).

The 30-70% efficiency claim (depending on type and the paper) about carotenoids is why I've always thought it is odd that any grow light seller would brag about targeting carotenoids. Carotenoids are there to help the plant with intense lighting and shunting some of the higher energy blue photons absorbed away from chlorophyll.

So... Why not use green LEDs?

Green LEDs are electrically inefficient compared to red and blue LEDs, and this problem is known as the 'green gap' in physics/engineering. The most efficient green LEDs that I know are actually blue LEDs with a green phosphor. That is why white LEDs are used instead that have a strong green light component. I've done pure LED green grows, using green COBs in a Space Bucket, and just to prove the point that it can be done.

View related studies and papers (14)