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Why aren't plants black? In theory, a black plant would absorb light from all wavelengths. However green plants (i.e. plants that reflect rather than absorb green) seem to have enjoyed an evolutionary advantage. According to Darwin they must have been the most fit, but what made them the most fit?
Answer 1:

Excellent question! The short answer is that absorbing a narrow band of energies is more efficient than a wide band. Let’s start by recalling that sunlight is a continuous spectrum of energies and the energy of the light (related to the wavelength) determines its color. Not all light energy has equal uses – how a plant (or any matter) absorbs light is related to the light’s wavelength.

Ultraviolet light (above 400nm wavelength) has enough energy to rip some electrons off molecules, breaking carbon double and single bonds. Thankfully, most higher energy light from the sun (X-rays and gamma rays) doesn’t make it down to us on the surface – they are deflected or absorbed by our outer atmosphere and magnetosphere first. Visible light (400 to 800nm) also excites electrons but not enough to break bonds. Lower energy infrared and microwave light cause molecules to vibrate with thermal energy, but don’t excite electrons much.

When plants absorb light, they use it to excite electrons that they can shuttle around to build/break down compounds, storing/releasing chemical energy. Strongly absorbing UV light breaks down bonds uncontrollably and faster than they can be rebuilt. (Ever notice how some houseplants left in direct sunlight will turn bright white? UV damage broke apart the colored pigments, a process called “photobleaching”.) Absorbing infrared light doesn't yield useful energy since the vibrations are much harder to direct into a chemical process. (Excess heat also causes damage by drying plants out. Without water to shuttle chemicals around, cells die, leading to brown dry patches on your houseplants.) Thus, plants absorb mostly in the visible spectrum.

But why absorb only a narrow band of energies in that spectrum? To understand that, we have to look at what can happen once an electron is excited in a molecule:

1. The electron can relax by emitting a photon (light) of similar energy, a process called fluorescence. This is why Highlighter dye looks so brightly colored – the color isn’t just reflection but also conversion of higher energy light into the color you see. If this conversion is slow, like in “glow-in-the-dark” dyes, it is called phosphorescence.

2. The electron can transfer its energy to another molecule next to it. This can keep going until it reaches a compound that can undergo a desired chemical reaction.

3. The electron can relax by releasing its energy as molecular vibration (heat).

These three processes compete every time an electron is excited. If the dye absorbs in a very narrow range of wavelengths, the first process is favored. If it absorbs in a very wide band, there are more ways to release energies during electron transfers so the third process eventually dominates. So, if a plant wants to use an electron for a chemical reaction, it’s most efficient to have an energy absorption range somewhere between these extremes so that electron transfer is favored. Most plants do this by having two sets of pigments that absorb narrowly and are physically separated from each other. What wavelengths they absorb depend on the species, but generally one absorbs higher energies (violet-blue) and the other lower energies (yellow-red).

My research group designs and tests pigments that directly convert solar energy into electricity, using similar design principles as photosynthetic pigments. Our dyes face many of the same problems (photobleaching and inefficient charge transfer) that we solve by creating dyes that absorb narrow energy bands or have charge-transfer cascades to shuttle electrons quickly out to circuitry.


Answer 2:

That's a good question, and ultimately I don't think we know. Chlorophyll, the main pigment that plants use to collect light, is green, but plants do use other pigments to supplement their chlorophyll (such as beta carotene, which is yellow). Other kinds of algae use different pigments yet: phycobilins, which are red or blue, xanthophylls, which are yellow, and fucoxanthins, which are brown. It may have to do with the ability of light to knock electrons loose in molecules, which is how the energy gets transferred into something chemical that the plants can actually use - too much energy and you destroy the receptor instead of collecting it.


Answer 3:

When light hits a pigment molecule in a plant, some of the light is absorbed and some is reflected. For a plant, the pigment chlorophyll absorbs blue and red light and reflects green light as you mentioned. The important point is that a particular pigment doesn’t absorb all light, it just absorbs a small range of wavelengths. Chlorophyll is the main pigment used by plants to use light to make sugars through photosynthesis. Likely, it wouldn’t be possible to have a pigment that absorbs all light and would therefore be black.

There are other pigments in plants that absorb different wavelengths of light, but chlorophyll is by far the most abundant and important. Since evolution has chosen chlorophyll as the best way to make sugar out of light, a plant doesn’t need to have the many different pigments that would make it black. Also, a point about evolution. Evolution doesn’t necessarily arrive at the best possible solution for an organism to survive. It only picks the best solution out of the various options available. For a black plant that is genetically superior to out-compete all other plants, it would have to exist in the first place. It may have been unlikely for a plant to develop such an amazing combination of different pigments.

Therefore, the best choice was a plant that made primarily chlorophyll. And since chlorophyll was good enough for plants to thrive, there was no need for plants to develop a combination of pigments that was more efficient at converting light to sugar.


Answer 4:

That’s an excellent question! If you were designing a plant you might make it black so that it would absorb all wavelengths.

One drawback might be heat. Too much heat could damage the cells or increase evaporation.

A lot of people think that natural selection somehow finds the “best” design. This is not true. The different genes are the result of random mutation. If there is no mutation for black leaves, black leaves can’t become more common. If a mutation for black leaves did happen, and it did work better for the plants, we would expect future generations of that population of plants to have more and more individuals with black leaves.

Can you think of any other consequences of having black leaves? Consider a plant’s relationships with other species.

You might be interested in studying plant ecology.


Answer 5:

There are some plants that are in fact black. Here is an open-access article all about black-pigmented leaves and photosynthesis:

black plants

Very cool question! Plant leaves are like engines, they can overheat and malfunction if they get too hot. Black leaves likely overheat more than green leaves. Also, absorbing different wavelengths depends on the pigment molecules in the plant and how much energy can be harvested from that wavelength. For example, blue light has a lot of energy (think UV and sunburns) while red light has lower energy (infrared/heat waves).

With respect to your evolutionary comments, not everything you see if "most fit." Why do we have a tail bone still? Why do we have an appendix? These are leftovers from past evolutionary adaptations that are not necessarily being "eliminated" from the gene pool. Evolution is a combination of random mutations, random selection at times (think of a volcano killing some plants but not others), and natural selection (why cheetahs run so fast). Black-leaved plants exist today and may have existed in the past but could have been eliminated for any number of reasons. Maybe Black leaves are more likely to get eaten by insects? We may never know :)

Cheers,


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