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I s'pose this is a variant of the age-old question, "Why are leaves green?" It's fairly easy to ask teh internets and find plenty of answers for that one.

I have a different but related question: why aren't leaves black? That is, chlorophyll varieties in leaves' cells mostly absorb red and blue wavelengths. Why isn't there a pigment in photosynthesis that absorbs green?

Instead, the green gets reflected back to the atmosphere. Its energy is otherwise lost to the plant. Nature is awfully well optimized to so many niches; why would this band of light be wasted?

There are some plants that do absorb green (and have dark leaves). These pigments -- such as anthocyanins, betalains, and carotenoids -- don't have any role in photosynthesis, and play other roles (such as protection against extreme temperature, or acting as an anti-oxidant). Am I missing some pigment in my list that is green-absorbing and participates in photosynthesis? If there were such a plant with a pigment like that, why wouldn't these plants dominate? Is there a large cost to producing the green-absorbing pigment that mostly negates the advantage of the extra energy gained?

After a little more searching on SO, I found the top answer to this question. In slightly different words, it says that light arrives in quanta, the chemistry in photosynthesis is driven by a certain threshold of energy, and any excess energy from a photon goes to waste heat (that tends to denature some of the proteins involved). The answer points to another answer with similar reasoning. However, their explanations don't account for the fact that chlorophylls strongly absorb blue in addition to red, where blue photons have nearly twice the energy of red ones.

Moreover, the answer goes on to say:

Of course, this is still no explanation why leaves are not simply black — absorbing all light is surely even more effective, no? I don't know enough about organic chemistry, but my guess would be that there are no organic substances with such a broad absorption spectrum and adding another kind of pigment might not pay off.

Are there nothing other than educated guesses at an answer? And why not use a combination of pigments, instead of a single, broad-spectrum absorbing pigment? For example, some metabolic pathways use parallel processes. Quoth the Wackypedia:

Sometimes more than one enzyme can catalyze the same reaction in parallel; this can allow more complex regulation: with, for example, a low constant activity provided by one enzyme but an inducible high activity from a second enzyme.

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Evolutionary answer: I like to go one step before green plants and consider the humble alga. Algae were historically classified as green, red, and brown, based on the wavelengths that their characteristic pigments absorbed. It is believed that land plants evolved from a common ancestor of algae, so you might wonder why we don't have similar broad categories of green, red, and brown plants.

Our best guess is that the organisms we call "plants" came from a single evolutionary event and share a common ancestor with green algae, rather than a different group. These green plant-like beings were probably the first big land organisms to try photosynthesis and happened to do well enough to out-compete any other plant-like being that might arise in their environment. Being green was "good enough" to for photosynthesis, allowing these organisms to survive and thrive.

Meta-answer: "Why" questions in biology often have probabilistic (and intuitively unsatisfying) answers. It's very hard to collect data that can "prove" why certain things evolved the way they did. Plus, evolution is not an engineering or design process. It leaves dead ends and doesn't always find the right solution. Founder effects can be very strong. But organisms which last long enough for us to study tend to be "good enough" at what they do. Here, the most parsimonious explanation for why leaves aren't black is that land plants came from green algae and didn't need to produce more of other pigments to take over the world.

Biochemical side-note: Plants can suffer from photooxidative stress when exposed to intense light due to the generation of reactive oxygen species, which disrupt metabolism. Therefore, some non-green pigments, like xanthophylls, are actually responsible for quenching chlorophyll and dissipating energy away from light-harvesting complexes. More energy is not always a good thing.

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  • $\begingroup$ Thanks! If I can restate in my own words, it's two things. (1) Absorbing only red and blue light is good enough to dominate; why bother expending extra effort that isn't necessary (or if it ain't broke, don't fix it). And (2) green light is used for other purposes such as quenching chlorophyll (regulation) and fueling anti-oxidants. Neat! TIL carotenoids play both roles. The xanthophylls you mention assist quenching. Carotenes absorb shorter wavelength light, scatter longer wavelengths, and transmit what they do absorb to chlorophyll (to provide extra energy to the photosystem). $\endgroup$ Dec 29, 2018 at 16:06
  • $\begingroup$ Another intriguing hypothesis is that the first successful photosynthesizer was purple (absorbed abundant green and reflected red and blue). The green algae then exploited a niche by doing the reverse (reflecting green and absorbing red and blue). Eventually the second comers out-competed the first movers, and began to dominate. There was never a need to re-adapt once dominant and absorb green. See this answer: biology.stackexchange.com/a/45335 $\endgroup$ Dec 30, 2018 at 1:02
  • $\begingroup$ Circling back to this a loooong time later, here's another piece of the puzzle: it's hypothesized that eukaryotes engulfed cyanobacteria -- becoming chloroplasts -- and this event only happened once in all of evolution. (That is, no convergent evolution at play among all plants and algae.) All descendants of that common ancestor all have the same green appearance. $\endgroup$ Jul 6, 2020 at 19:49
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In addition to the points mentioned in the other answer, there is also this paper highlighting the importance of a steady energy flow compared to simply optimising energy capture. See this article in Quanta Magazine for a summary.

From the Quanta article:

Gabor and his team developed a model for the light-harvesting systems of plants and applied it to the solar spectrum measured below a canopy of leaves. Their work made it clear why what works for nanotube solar cells doesn’t work for plants: 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.

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  • $\begingroup$ Thanks for the addition. We ask that answers not simply link to outside info in case such links later die. Leave the links but either quote or summarize the main points in your post please. Thanks! $\endgroup$ Sep 25, 2023 at 4:33
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Leaves are usually dark green meaning they actually do absorb some green light and convert it to useful energy, they just reflect more green light than any of the other wavelengths that we can see, thus giving them a green color.

Also, not all plants have green leaves, cactuses for example, have more like green stems with no leaves at all. And in fact, many desert plants have a very strong white tinge to them because of all the white light they reflect in an effort to not overheat and kill themselves by absorbing such absurd amounts of light. Many have evolved reflective wax coatings or tiny hairs that provide shade. The conclusion from this seems to be that the limiting factor for many plants growth is not how much radiation they can absorb per area of leaf, and therefore, there isn't an evolutionary pressure to increase their energy absorption by evolving black pigments.

Note: I disagree with the other answer stating that it is the result of all plants being descendant from a common ancestor that used green pigments. While it is true that all plants are descendant from a common ancestor, I haven't seen compelling evidence that black plants have never evolved or never had the opportunity to evolve. Far more likely in my mind is that they are perfectly possible and don't evolve simply because there is no advantage to doing so. Which I think is incredible; and very surprising. But, I think that's the answer with the most evidence.

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  • $\begingroup$ Hmm. I'm not buying the excess heat thing. Cacti are the exception, not the rule, when it comes to leaf shape. Most leaves are thin: the have an extraordinary amount of surface area relative to volume. Heat transfer to the surrounding fluid seems more than adequate. I say "fluid" because some plants live in air, but others in water. Which brings me to: they evolved in water, and water provides an enormous heat sink. How could heat be a concern for those first-evolvers? $\endgroup$ Oct 17, 2023 at 15:40
  • $\begingroup$ yeah, you're right i think. I don't have much evidence, so this is mostly conjecture. To conjecture, I'd say that forestecol provides a compelling overview of what selection pressures the "first-evolvers" were dealing with. However, to change my answer from above, the most intense limiting factor with the "heat" is probably at the molecule level and substructure level, so it doesn't matter how much water the leaf is, because the clustered pigments and proteins will heat up faster than they can transfer their energy to the surrounding water. $\endgroup$ Oct 18, 2023 at 12:11

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