Why Are Plants Green? To Reduce the Noise in Photosynthesis

https://www.quantamagazine.org/why-are-plants-green-to-reduce-the-noise-in-photosynthesis-20200730/

by theafh • 6 years ago

247 100 6 years ago

russfink• 6 years ago

Why wouldn't fluctuations of sunlight that cause instability in the green spectrum also cause instabilities in the red and blue spectra?

uj8efdkjfdshf• 6 years ago

I think the point is that since the power available at these wavelengths vary sharply with frequency in these regions of the solar spectrum, the plant can easily compensate for brightness fluctuations in these portions of the spectrum with minor tweaks to the target wavelength of the relevant photosystems

londons_explore• 6 years ago

I'd this is the case, it's a shame the article didn't mention it.

It makes sense the changing environmental conditions for a molecule could affect the wavelengths of light it absorbs (for example, pH, temperature, etc), but I'm not sure this has been demonstrated for plant pigments?

Sniffnoy2• 6 years ago

I'm confused though -- how can the plant possibly vary this?

floatrock• 6 years ago

It might be highly efficient to specialize in collecting just the peak energy in green light, but that would be detrimental for plants because, when the sunlight flickered, the noise from the input signal would fluctuate too wildly for the complex to regulate the energy flow.
Instead, for a safe, steady energy output, the pigments of the photosystem had to be very finely tuned in a certain way. The pigments needed to absorb light at similar wavelengths to reduce the internal noise. But they also needed to absorb light at different rates to buffer against the external noise caused by swings in light intensity. The best light for the pigments to absorb, then, was in the steepest parts of the intensity curve for the solar spectrum — the red and blue parts of the spectrum.

I admit there's a bit of a jump there (gotta read the actual article, not the journalist retelling I suppose), but I assume the gist of the math is something like this:

Lets say direct green light delivers a maximum 100 "units of photo-energy" -- gonna play loose with the physics to demonstrate the math.

When a cloud passes over it, lets say the intensity drops to only 75%. Lets also say for now we always convert the energy at 100% efficiency.

So with green light, your 100 units of energy drops by 25 units with each passing cloud.

Now lets say blue light delivers only 80 units of energy. When that same cloud passes over, 80 * 75% = 60 units of energy, or a drop of 20 units.

So, if your process is sensitive to changes in absolute energy, you would rather have a swing of 20 units for every passing cloud than a swing of 25 units. Yeah, you might get less absolute energy (60 units rather than 75), but if the cost of energy swings in your process was very high, the tradeoff might be worth it.

You could also play with the efficiency-of-conversion (ie have higher conversion efficiency at the lower-swing points) for some fun second-order effects.

This is just a toy example of what the underlying dynamics could be. Gotta read the actual paper to understand the actual model they developed. https://arxiv.org/pdf/1912.12281.pdf

wcoenen3• 6 years ago

Now lets say blue light delivers only 80 units of energy. When that same cloud passes over, 80 * 75% = 60 units of energy, or a drop of 20 units. So, if your process is sensitive to changes in absolute energy, you would rather have a swing of 20 units for every passing cloud than a swing of 25 units.

That's not exactly it. The idea is that the blue light absorption can be tuned (presumably by shifting the absorbed wavelengths by a few nanometer) to compensate for external changes. So if the overall light intensity drops 1%, the system responds by shifting the absorbed spectrum to get 1% more power.

This only works in parts of the solar spectrum where power varies sufficiently versus wavelength, hence the preference for blue and red.

FriedPickles• 6 years ago

They do. The paper's point is that plants want to choose two wavelengths with different amounts of average power. This gives the plant a window to tune within to get the desired amount of output power. If the total illumination drops, the plant compensates by tuning more towards the high power wavelength, and vice versa.

But the paper makes the point that the tuning mechanism isn't perfect due to internal noise. If the power level difference between the two input wavelengths is too great, then the random fluctuation of the tuning mechanism will itself create a lot of noise in the output. So basically there's an optimal value for the difference of average power between the two wavelengths.

Now, why the plant doesn't absorb the peak green wavelength and the closest wavelength whose power is exactly the optimal difference less was unclear to me. I think the idea was that the plant also wants to minimize the wavelength difference, and a given power delta can be achieved with less difference in wavelength in the steeper sloping sections of spectrum power graph.

rkwasny• 6 years ago

Related, one of my favourite videos: Richard Feynmann explains fire:

https://www.youtube.com/watch?v=N1pIYI5JQLE

SeriousM• 6 years ago

Finally a clickbait with explanation!

42droids3• 6 years ago

Wanted to say the same.

42droids3• 6 years ago

Ppl who downvote comments like these are responsible for killing any joy and fun on the internet.

kanobo• 6 years ago

Quanta magazine typically does a great job backing up their titles with real science-based content.

raxxorrax2• 6 years ago

Some trees and plants that are still rebellious about this:

https://www.ornamental-trees.co.uk/images/cercis-canadensis-...

Of course those also have chlorophyll, but the color is overridden by other color particles.

Sadly we have no blue trees. We have blue flowers though and it would probably be possible to create one. That would be so awesome.

MarkSweep• 6 years ago

A blue spruce is a fairly blue tree:

https://en.m.wikipedia.org/wiki/Blue_spruce

bregma2• 6 years ago

Blue spruce is actually pretty darn green, but its needs have a waxy coating that alters our perception of the colour of the underlying tissue. New needles, and needles you scrape with your fingernails, reveal their true green colour.

A waxy coating is a strong selector for survival in an environment where preventing evaporation is a must, such as locations in which the ambient temperature drops below the freezing point of water for much of the year.

sgt101• 6 years ago

Interestingly, when organisms retain this kind of diversity it is because of the evolutionary advantage that it confers. In this case that would be the ability of plants in general to evolve towards other colors. For this to be an advantage the driver for that would have to be present in our long term environment. Possibly some sort of moderator appears in the atmosphere? Or does the spectra of the sun change due to some process?

learnstats2• 6 years ago

I think trying to identify evolutionary motivation is generally risky and subject to all kinds of human biases. Any hypothesis is unfalsifiable (so, it's speculation, not science).

Having said that, "Green = provides a convenient way for animals to identify a great source of energy" is likely to be one driver here.

raxxorrax2• 6 years ago

Perhaps the sun isn't the reason. I believe some plants hide from certain kind of predators by changing their colors or maybe they could draw certain insects or birds for additional pollination. Perhaps that was a more significant advantage.

Shivetya• 6 years ago

For color variety, depending on yard location, I have a variation of Loropetalum, one Lilac, and a Japanese Maple. Had a pair of plum trees but the ornamental variety you can buy tends to not last as long.

For low ground stuff Nandina can offer greens, reds, and yellows, in good variety.

mutatio• 6 years ago

Isn't that a cultivar though, artificially maintained in horticulture. Does it occur in significant numbers in wild populations?

tipoftheiceberg7• 6 years ago

The only blue flowers we have are dyed.

We do not know of any “true blue” plant pigments. There are violets and many colors that might look close..

eggsnbacon12• 6 years ago

I have seen many blue flowers personally but since this is a trustworthy internet forum I believe you

irrational2• 6 years ago

What? I have a bunch of hydrangeas in my yard right now that range from light to dark blue. They are definitely not violet or any other close blue color. They are as blue as blue can be. And they haven’t been artificially tampered with. We planted them 15 years ago and have done nothing for them since. Not even fertilizer.

atombender• 6 years ago

The sky doesn't contain any blue pigment, either. And a blue-eyed person has no blue in their irises.

You're referring to pigment; there is no blue plant pigment, true, but that seems a little pedantic, even if it's interesting. Plenty of flowers reflect blue light, which is what people mean when they say an object is "blue".

stpedgwdgfhgdd• 6 years ago

The same applies to software development; local optimization does not weigh up against stability across the entire supply chain.

dvh• 6 years ago

I thought plants are green because sun is green. Edit: not sure about down votes, sun has peak in green spectrum. Our eyes are most sensitive to green, plants are green to utilize it as well.

laszlokorte• 6 years ago

If they would utilize the green light they would not reflect it - so should exactly not be seen as green by your eyes.

cortic3• 6 years ago

I was really looking forward to reading this but its a bit of a disappointment.

First, green does not have the most energy of the visual spectrum, Blue, or more specific Violet does; https://socratic.org/questions/5348556b02bf347bedff8fed

Second the noise difference from 10% of the green would be negligible compared to the energy we are talking about. Also how does a plant regulate this 'noise'? The only logical explanation would be expand into the green in dark and out of it in light, but they have shown no mechanism for a plant to do that.

Sorry to say that after that fairly long winded article i have come to the conclusion we still don't know exactly why plants aren't all black.

Edit; Thinking about this more, maybe Chlorophyll a and Chlorophyll b take inverted wavelengths of light to produce a rectification effect..? Its interesting, but even if that were true, it would not explain the gap at green.

2nd Edit; I stand corrected, considerably more green light make it through the atmosphere thank you for the information spacemark.

alanbernstein3• 6 years ago

The "most energy" statement isn't about the frequency of light, it's about the power output of the sun, which peaks around green.

jiehong• 6 years ago

That power output really is the area under the curve, in which case most of the sun's energy is in the infrared, actually.

spacemark• 6 years ago

First, green does not have the most energy of the visual spectrum, Blue, or more specific Violet does

The article is correct. Blue photons have more energy, you're right. But more green photons are emitted by the sun, and even more reach the ground through the atmosphere than blue, so the total energy from green photons is significantly greater. Google blackbody spectrum.

https://en.wikipedia.org/wiki/File:Wiens_law.svg

https://lh3.googleusercontent.com/proxy/eDhfILMl70XZ-WUchpgo...

omaranto3• 6 years ago

You do understand that this is a popular science article and that is reporting on this paper, right?

https://science.sciencemag.org/content/368/6498/1490

Did you conclude "we still don't know exactly why plants aren't all black" after reading the paper?

cortic3• 6 years ago

Thanks, looks like the full paper is for paid members only, but i will look into this further.

acdanger• 6 years ago

One theory is that yellow-green wavelengths don't pass through water as readily, hence ancestral sea-dwelling plants adapted to absorb red and blue wavelengths and that adaptation remains "good enough" for contemporary land plants.

Highly recommend the book "How the Earth Turned Green"

https://press.uchicago.edu/ucp/books/book/chicago/H/bo164656...

nuncanada• 6 years ago

Not "good enough"! There are so many known instances where Evolution gets trapped in local maxima and can't escape it... Another local maxima that photosynthesis trapped itself is its efficiency being jumbled by higher O2 concentrations.

jonplackett• 6 years ago

I always just assumed there wasn't much green light, so plants optimised for the rest of the spectrum:

The sky is blue, the sun is orangey.

Where's all this green light I'm missing?

alanbernstein3• 6 years ago

Look at a graph of the spectral power output of the sun, you'll see that it peaks around green.

sp332• 6 years ago

Your green receptors overlap with red, and also a little with blue. There is no single wavelength of light that only stimulates your green photoreceptors without also setting off red or blue ones. https://en.wikipedia.org/wiki/Color#/media/File:Cones_SMJ2_E...

colanderman• 6 years ago

The sun is actually green (that is, is approximated by a black body emitter with a peak in the green region of the visual spectrum, see [1]). Its light appears white because that is how we are conditioned evolutionarily. ("True" white -- i.e. equal spectral energy -- appears blue-grey to us.)

The sun "looks" orange because when you can look at it (sunrise/sunset), its light is heavily filtered by the atmosphere.

[1] https://qph.fs.quoracdn.net/main-qimg-f9bc312e4d114e9e32a627...

projektfu• 6 years ago

Orange is red and green. The intensity we perceive is also not the absolute intensity of that part of the spectrum. Neither is the luminosity equivalent to the power (as in energy over time).

hawktheslayer• 6 years ago

I might just use this trick of nature to inform how I manage my staff at work. We spend so much time trying to maximize efficiently, but perhaps at the cost of stability.

deeweebee• 6 years ago

This is the nerdiest sentence I’ve seen today. Dang!

throwaway_USD• 6 years ago

My understanding of the evolution of Earth flora is that prior to plants being green, the dominant plant life was red (think of red algae blooms) and that the current dominant green plant life likely evolved to use different photons along the EM spectrum where there was less competition.

Funny enough as I understand visible light and the EMR spectrum there is no "green" (color/wave length/energy) rather the color green is a construct originating not in the light spectrum but in the mind of the observer.

andrewflnr• 6 years ago

I was thinking about the same hypothesis but IIRC it was highly speculative, with the main evidence for it simply being that modern plants evolved to use those other photosynthesizers' castoffs. We may not need that hypothesis anymore if we have a solid reason to avoid green anyway.

Zaak• 6 years ago

There is a range of wavelengths of light that humans perceive as green. The same is true for every color that is part of the rainbow. In contrast, the "pure purples" do not appear in the rainbow, and there is no single wavelength of light that humans perceive as purple (it requires red light plus blue light).

Balgair• 6 years ago

The perception of color is a pretty wild area of science. Colors seem to be culturally dependent. In that, people literally cannot see the difference between blue and green if their language does not have words to distinguish them. Even when big rewards are given for the 'correct' answer. Colors also follow certain patterns, with colors like blue being the last to be named in a culture.

https://www.youtube.com/watch?v=gMqZR3pqMjg

Fun fact, the color blue does not appear at all in the Iliad, Homer describes the ocean as wine, despite the stunning blue colors of grecian seas.

Ericson2314• 6 years ago

Land plants are newer than algae, but green algae is older than red algae (as evidenced by the endosymbiosis order). But maybe green algae got dominated by the others pretty early on.

edit actually that's brown algae that's the derived one. red and green it looks like both come from the original endosymbiosis.

VerDeTerre• 6 years ago

I had heard of the similar Purple Earth Hypothesis[1] wherein organisms with photosynthesis based on retinal arose in the oceans early on. Chlorophyll-based life developed deeper and took advantage of the red and blue light that filtered through.

The hypothesis seems pretty speculative, but maybe it's compatible with this new research, which could explain why green plants came to dominate despite retinal being simpler.

[1] https://en.wikipedia.org/wiki/Purple_Earth_hypothesis

thescriptkiddie• 6 years ago

You're probably thinking of magenta [0]. Green light absolutely does exist, it has a wavelength of around 500-550 nm [1].

[0] http://www.biotele.com/magenta.html

[1] https://en.wikipedia.org/wiki/Light#/media/File:EM_spectrum....

sergeykish3• 6 years ago

I wonder how could one come up with such a "green" theory

* green in rainbow and prism makes it equal to other rainbow colors

* green in RGB requires pure color for wider gamut

* birds receptors are not screwed [1]

[1] https://upload.wikimedia.org/wikipedia/commons/e/e8/BirdVisu...

zadkey• 6 years ago

Why are some plants purple like tradescantia pallida?

Chris2048• 6 years ago

But, why do plants need a regular amount of light energy? Can't a black plant just absorb everything available?

_5659• 6 years ago

The color of plants and the statistics of light is fascinating.

Tangentially related, I was developing a CV app for farmers in an early stage weed startup right before legalization in California so they could monitor analytics on the health of their sprouts as well as identify strains. The camera conditions were all over the place and data preprocessing stage was an absolute nightmare. We tried everything from filters to background removal, augmentation techniques through transformations, noise induction for generalization - nothing really improved the baseline models because the photos honestly sucked. Farmers ignored our guidelines on lighting and framing and format and kept sending in inconsistent garbage.

I had a eureka moment. What if we sent each of the farmers a little piece of square cardboard painted magenta? The idea was that the increase in contrast would allow us to process the leaf contour a little bit better. The fact that the card was square meant some farmers even took the time to take the plant indoors so they could frame it better within the edges. Data quality improved dramatically. It worked.

Unfortunately legalization did not work out as we planned, farmers disappeared and the market was monopolized by corporations, there wasn't any interest in helping develop strains locally.

Why Are Plants Green? To Reduce the Noise in Photosynthesis

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