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.


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.

  • $\begingroup$ 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). $\endgroup$ – Armadillo Jim Dec 29 '18 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 $\endgroup$ – Armadillo Jim Dec 30 '18 at 1:02

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