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Nothing says "nature" like a lush green forest. But why are plants green in the first place?

Hosted by: Stefan Chin

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Sources:
https://science.sciencemag.org/content/sci/368/6498/1490.full.pdf
https://science.sciencemag.org/content/sci/suppl/2020/06/24/368.6498.1490.DC1/aba6630-Arp-SM.pdf
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[♪ INTRO].

Nothing says “nature” more than a luscious green forest or a grassy green hillside. But… why green?

Why are almost all plants this color? I mean, sure, they use the green pigment chlorophyll for photosynthesis. But why is chlorophyll green?

Now, scientists actually don’t know for sure. But one idea is that it might be a way for plants to protect themselves against fluctuating light from the Sun. Now, you might think that green is just as good as any other color… but it’s actually kind of a surprising choice for a plant.

You see, plants get energy by absorbing sunlight. So you’d think they’d want to soak up as much of it as they can. But if they managed to absorb all of the light that hit them, they’d be black.

Which… they’re definitely not. And the fact that they look green tells us that that’s the color they reflect the most of. Now, what’s weird about that is that green is one of the colors that the Sun emits the most of.

Yet for some reason, plants reject this huge portion of the Sun’s rays. And according to a paper that came out in June 2020, scientists believe they just might have figured out why. The team behind the study, led by a group at UC Riverside, proposed that maybe plants aren’t trying to absorb as much light as possible.

Maybe they’re also trying to absorb light that doesn’t fluctuate too much. Because, imagine you’re a leaf in a jungle canopy:. The shadows of the leaves above are constantly shifting, meaning the amount of sunlight that falls on you can vary a lot from moment to moment.

But a plant is kind of like an electric power grid— it’s vulnerable to sudden changes. Like, a surge in power can damage cells, whereas a sudden drop in power can mean the plant doesn’t have enough juice to keep functioning. So in nature, there’s all this variability in the amount of sunlight reaching a plant, and somehow it needs to regulate the amount of light it takes in so it never gets a harmful surge or drop in power.

And the researchers think that the colors plants absorb can help them do that. See, while plants reject green light, they absorb multiple other colors of light from either end of the spectrum. Bluer wavelengths have more energy than green, while redder ones have less.

And the researchers hypothesized that the plant averages out the energy from the different wavelengths of light it absorbs, making it more resistant to sudden shocks. The key here is that the amount of light from each wavelength can vary independently from moment to moment. So, a surge in solar energy at, say, red wavelengths, should be less destructive if it’s averaged out among all the other wavelengths the plant absorbs.

And according to the team’s hypothesis, plants are fine-tuned by evolution to absorb exactly the wavelengths that will balance each other out most effectively. If plants absorbed all the light from the Sun, they’d be much more vulnerable to its fluctuations. To investigate their idea, the researchers made a simple simulation of a chloroplast—the part of the plant where photosynthesis happens —and designed it to absorb energy from just two colors of light.

They wanted to test how simulated chloroplasts would deal with the same kind of variability natural ones face, so they used simulated sunlight and varied its brightness over time. And they learned a few things. For one, the amount of power that came out the end of the network depended on which two wavelengths the plant was absorbing.

Which makes sense: Sunlight has different intensities at different wavelengths. The more sunlight there is to absorb at those wavelengths, the more power the plant can produce. But it turned out that the difference between the two input wavelengths also mattered a lot.

When the difference between the two wavelengths was too large, the molecular network didn’t do enough to even out the noise. On the other hand, if the difference was too small, the signal got dampened too much, and the output was too low. So the best combination seemed to be sort of a compromise:.

Just the right wavelengths to absorb enough sunlight in the first place, and just enough difference between the wavelengths to handle fluctuations. And that wasn’t all. The scientists could even use the model to predict what color different photosynthesizing lifeforms should be, based only on the sunlight in their environment.

As they expected, the model told them that the ideal color for plants in normal sunlight is green. But it also showed that under a dense canopy of trees, a shade of purple is the optimal color. And sure enough, there are certain photosynthesizing bacteria living in dense forests that are exactly that shade.

So it seems like, once again, evolution has solved a complex physics problem that we didn’t even know it was working on:. It’s narrowed in on the perfect color to make plants grow. So maybe when it comes to photosynthesis, it really is easier being green.

Thank you for watching this episode of SciShow! If you liked this episode and you’re interested in learning even more about the incredible inner workings of plants, you might like our episode on how plants tell time. You can watch that right after this! [♪ OUTRO].