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I know plants are green due to chlorophyll.

Surely it would be more beneficial for plants to be red than green as by being green they reflect green light and do not absorb it even though green light has more energy than red light.

Is there no alternative to chlorophyll? Or is it something else?

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    $\begingroup$ I find it even more puzzling why plants don't absorb all of the visible spectrum altogether (resulting in the leaves being black). $\endgroup$ – Adam Zalcman Jan 3 '12 at 19:48
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    $\begingroup$ It is even more strange becouse green algae evolved in water. And red light is absorbed by water. Rhodophyta (red algae) are red due to phycoerythrin but it seems that red color is advantageous for them only in sea depths. $\endgroup$ – Marta Cz-C Jan 3 '12 at 20:39
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    $\begingroup$ @AdamZalcman: There's probably no organic substance with this absorption spectrum, and adding further pigments to cover the entire VIS spectrum might not pay off. $\endgroup$ – Tobias Kienzler Jan 4 '12 at 11:27
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    $\begingroup$ Surely it would be more beneficial: not if the red pigment is less efficient in converting light to energy... $\endgroup$ – nico Jan 4 '12 at 13:04
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    $\begingroup$ If anybody is interested in a short video on the topic (~6.5 min), watch Physics Girl's video: Why aren't plants black? disclaimer: This is not my video, but I think it is very well researched and informative. $\endgroup$ – a tiger Feb 27 '17 at 12:32
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Surely it would be even more beneficial for plants to be black instead of red or green, from an energy absorption point of view. And Solar cells are indeed pretty dark.

But, as Rory indicated, higher energy photons will only produce heat. This is because the chemical reactions powered by photosynthesis require only a certain amount of energy, and any excessive amount delivered by higher-energy photons cannot be simply used for another reaction1 but will yield heat. I don't know how much trouble that actually causes, but there is another point:

As explained, what determines the efficiency of solar energy conversion is not the energy per photon, but the amount of photons available. So you should take a look at the sunlight spectrum:

Solar Radiation Spectrum

The Irradiance is an energy density, however we are interested in photon density, so you have to divide this curve by the energy per photon, which means multiply it by λ/(hc) (that is higher wavelengths need more photons to achieve the same Irradiance). If you compare that curve integrated over the high energy photons (say, λ < 580 nm) to the integration over the the low energy ones, you'll notice that despite the atmospheric losses (the red curve is what is left of the sunlight at sea level) there are a lot more "red" photons than "green" ones, so making leaves red would waste a lot of potentially converted energy2.

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.3

1) Theoretically that is possible, but it's a highly non-linear process and thus too unlikely to be of real use (in plant medium at least)
2) Since water absorbs red light stronger than green and blue light deep sea plants are indeed better off being red, as Marta Cz-C mentioned.
3 And other alternatives, like the semiconductors used in Solar cells, are rather unlikely to be encountered in plants...

Additional reading, proposed by Dave Jarvis:

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    $\begingroup$ I would also add the fact that blue light reaches earth surface greately scattered due to Rayleigh scattering, so the absolute amount of energy carried blue light is comparable (or even less) than that of other parts of the visible spectrum. $\endgroup$ – Alexander Galkin Jan 5 '12 at 14:10
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    $\begingroup$ If leaves were black, they would get too hot. $\endgroup$ – David LeBauer Apr 8 '12 at 2:44
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    $\begingroup$ Also don't forget - evolution only produces something which is good enough - not optimal. If green turned out to be good enough, then there would be impetus to develop black leaves. $\endgroup$ – William Mioch Oct 11 '12 at 23:29
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    $\begingroup$ I'd note as well that there are other photosynthesizers that utilise the energies that plants do not - so-called purple bacteria - and these may predate green photosynthesizers. It's plausible that the "green gap" originally evolved as a means to exploit light not exploited by competing organisms. $\endgroup$ – Jack Aidley Jan 29 '13 at 17:05
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    $\begingroup$ @TobiasKienzler yes, they don't absorb infrared because there isn't enough energy in it to start photosynthesis, and moreover you don't want to overheat; hence reflection $\endgroup$ – sanmai Apr 19 '16 at 1:35
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I believe it is because of a trade off between absorbing a wide range of photons and not absorbing too much heat. Certainly this is a reason why leaves are not black - the enzymes in photosynthesis as it stands would be denatured by the excess heat that would be gained.

This may go some of the way towards explaining why green is reflected rather than red as you suggested - reflecting away a higher energy colour reduces the amount of thermal energy gained by the leaves.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

  • $\begingroup$ I think the solar spectrum is also quite relevant. $\endgroup$ – Tobias Kienzler Jan 4 '12 at 11:29
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    $\begingroup$ It is more to do with the specific photon energy level (see Tobias) than the thermal heat supplied per se. Heat is probably not a concern for temperate/arctic plants which are also green. $\endgroup$ – Poshpaws Jan 4 '12 at 14:25
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    $\begingroup$ Heat is the least of a plant's problems. That is why we can breed black tulips without self-cooking petals. ;) $\endgroup$ – S. Albano Oct 11 '12 at 6:02
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There is quite a fun article here which discusses the colours of hypothetical plants on planets around other stars.

Stars are classified by their spectral type which is dictated by their surface temperatures. The Sun's is relatively hot, and it's spectral energy distribution peaks in the green region of the spectrum. However the majority of stars in the Galaxy are K and M type stars which emit mainly in the red and infrared.

This is relevant to this discussion since any photosynthesis on these worlds would have to adapt to these wavelengths of light in order to proceed. On planets around cool stars plant life (or its equivalent) might well be black!

OK, this is not entirely pie in the sky astrobiologist rubbish. It is actually quite relevant to the search for biosignatures and life on other planets. In order to model the reflectance spectrum of planets we observe (i.e. the light reflected from the primary star) we need to try and take into account any potential vegetation.

For example, if we take a reflectance spectrum of the Earth, we see a characteristic peak in the red "the red edge" which is due to surface plant life.

NASA also has a short page on this here.

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    $\begingroup$ nice links, though I doubt black plants are likely to happen anywhere since that would most likely need too many different pigments $\endgroup$ – Tobias Kienzler Jan 11 '12 at 8:03
  • $\begingroup$ OK, maybe not completely black but very dark. There are various terrestrial plants which have very dark leaves (e.g. Oxalis triangularis). Admittedly, many are cultivars but this does demonstrate that there are some dark pigments available (I suppose anthocyanins?) $\endgroup$ – Poshpaws Jan 12 '12 at 11:08
  • $\begingroup$ good point. Maybe in most cases absorbing green was just the first thing that happened (by evolution) and it was sufficient, but I'm just speculating... $\endgroup$ – Tobias Kienzler Jan 16 '12 at 9:19
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There are two factors at play here. First is the balance between how much energy a plant can collect and how much it can use. It is not a problem of too much heat, but too many electrons. If it were a question of heat, a number of flowers selected for their black pigmentation would have their petals cooked off. ;)

If a plant does not have enough water, is too cold, is too hot, collects too much light, or has some other condition that prevents the electron transport chain from functioning properly, the electrons pile up in a process called photoinhibition.

These electrons are then transferred to molecules that they should not be transferred to, creating free radicals, wreaking havok within the plant's cells. Fortunately, plants produce other compounds that prevent some of the damage by absorbing and passing around the electrons like hot potatos. These antioxidants are also beneficial to us when we eat them.

This explains why plants collect the amount of light energy they do, but does not explain why they are green, and not grey or dark red. Surely there are other pigments that would be able to generate electrons for the electron transport chain.

The answer to that is the same as why ATP is used as the main energy transport molecule in organisms rather than GTP or something else.

Chlorophyll a and b were just the first things that came about that fulfilled the requirement. Certainly some other pigment could have collected the energy, but that region of parameter space never needed to be explored.

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  • $\begingroup$ I think you are right, that the bottleneck is not in collecting the photons, but in processing them without creating too much singlet oxygen. One of important factors might be that chlorophyll plays also a big structural role: It is not only a collector of light, but also allows separation of charge in the special pair of the reaction center and constitutes many pigment-protein complexes structurally. $\endgroup$ – Irigi Oct 14 '14 at 20:10
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I know this question was asked and answered a number of years ago (with many great answers), but I couldn't help but notice that no one had approached this from an evolutionary perspective (like the answer to this question)...

Short Answer

Pigments appear as whatever color is not absorbed (i.e, they appear as whichever wavelength(s) of light they reflect).

Blue light was the most available wavelength of light for early plants growing underwater, which likely led to the initial development/evolution of chlorophyll-mediated photosytems still seen in modern plants. Blue light is the most available, most high-energy light that continues to reach plants, and therefore plants have no reason not to continue taking advantage of this abundant high energy light for photosynthesis.

Different pigments absorb different wavelengths of light, so plants would ideally incorporate pigments that can absorb the most available light. This is the case as both chlorophyll a and b absorb primarily blue light. Absorption of red light likely evolved once plants moved on land due to its increased abundance (as compared to under water) and its higher efficiency in photosynthesis.


Long Answer

Early Plants Develop Modern Photo-system

It turns out, just like the variability in transmittance of different wavelengths of light through the atmosphere, certain wavelengths of light are more capable of penetrating deeper depths of water. Blue light typically travels to deeper depths than all other visible wavelengths of light. Therefore, the earliest plants would have evolved to concentrate on absorbing this part of the EM spectrum.

https://disc.sci.gsfc.nasa.gov/education-and-outreach/additional/science-focus/ocean-color/images/spectral_light_absorption.gif

However, you'll notice that green light penetrates relatively deeply as well. The current understanding is that the earliest photosynthetic organisms were aquatic archaea, and (based on modern examples of these ancient organisms) these archaea used bacteriorhopsin to absorb most of the green light.

http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/imgbio/plantblack.gif

Early plants grew below these purple bacteriorhopsin-producing bacteria and had to use whatever light they could get. As a result, the chlorophyll system developed in plants to use the light available to them. In other words, based on the deeper penetrative ability of blue/green light and the loss of the availability of green light to pelagic bacteria above, plants evolved a photosystem to absorb primarily in the blue spectrum because that was the light most available to them.

  • Different pigments absorb different wavelengths of light, so plants would ideally incorporate pigments that can absorb the most available light. This is the case as both chlorophyll a and b absorb primarily blue light.

  • Here's two example graphs (from here and here) showing the absorption spectrum of typical plant pigments:

    Photosynthesis

So Why Are Plants Green?

As you can guess from the above paragraphs, since early under water plants received so little green light, they evolved with a chlorophyll-mediated photo-system that did not have the physical properties to absorb green light. As a result, plants reflect light at these wavelengths and appear green.

But Why Are Plants Not Red?...

Reason to ask this question:

This would seem to be equally plausible given the above information. Since red light penetrates water incredibly poorly and is largely unavailable at lower depths, it would seem that early plants would not develop a means for absorbing it and therefore would also reflect red light.

  • In fact, [relatively] closely related red algae did evolve a red-reflecting pigment. These algae evolved a photo-system that also includes the pigment phycoerythrin to help absorb available blue light. This pigment did not evolve to absorb the low levels of available red light, and so therefore this pigment reflects it and makes these organisms appear red.

    • Interestingly, according to here, cyanobacteria that also contain this pigment can readily change it's influence on the organism's observed color:

      The ratio of phycocyanin and phycoerythrin can be environmentally altered. Cyanobacteria which are raised in green light typically develop more phycoerythrin and become red. The same Cyanobacteria grown in red light become bluish-green. This reciprocal color change has been named 'chromatic adaptation’.

  • Further, (although it's still under debate) according to work by Moreira et al (2000) (and corroborated by numerous other researchers) plants and red algae likely have a shared photosynthetic phylogeny:

    three groups of organisms originated from the primary photosynthetic endosymbiosis between a cyanobacterium and a eukaryotic host: green plants (green algae + land plants), red algae and glaucophytes (for example, Cyanophora).

So what gives?

Answer:

The simple answer of why plants aren't red is because chlorophyll absorbs red light.

This leads us to ask: Did chlorophyll in plants always absorb red light (preventing plants from appearing red) or did this characteristic appear later?

  • If the former was true, then plants don't appear red simply because of the physical characteristics that the chlorophyll pigments evolved to have.

  • As far as I know, we don't have a clear answer to that question.

    • (others please comment if you know of any resources discussing this).
  • However, regardless of when red light absorption evolved, plants nevertheless evolved to absorb red light very efficiently.

    • A number of sources (e.g., Mae et al. 2000, Brins et al. 2000, and here) as well as numerous other answers to this question, suggest that the most efficient photosynthesis occurs under red light. In other words, red light results in the highest "photosynthetic efficiency."

      • This NIH page suggests the reason behind this:

      Chlorophyll a also absorbs light at discrete wavelengths shorter than 680 nm (see Figure 16-37b). Such absorption raises the molecule into one of several higher excited states, which decay within 10−12 seconds (1 picosecond, ps) to the first excited state P*, with loss of the extra energy as heat. Photochemical charge separation occurs only from the first excited state of the reaction-center chlorophyll a, P*. This means that the quantum yield — the amount of photosynthesis per absorbed photon — is the same for all wavelengths of visible light shorter than 680 nm.

Why Did Plants Remain Green?

So why have plants not evolved to use green light after moving/evolving on land? As discussed here, plants are terribly inefficient and can't use all of the light available to them. As a result, there is likely no competitive advantage to evolve a drastically different photosystem (i.e., involving green-absorbing pigments).

So earth's plants continue to absorb blue and red light and reflect the green. Because green light so abundantly reaches the Earth, green light remains the most strongly reflected pigment on plants, and plants continue to appear green.

  • (However, note that other organisms such as birds and insects likely see plants very differently because their eyes can distinguish colors differently and they see more of the strongly reflected UV light that ours cannot).
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The biologist John Berman has offered the opinion that evolution is not an engineering process, and so it is often subject to various limitations that an engineer or other designer is not. Even if black leaves were better, evolution's limitations can prevent species from climbing to the absolute highest peak on the fitness landscape. Berman wrote that achieving pigments that work better than chlorophyll could be very difficult. In fact, all higher plants (embryophytes) are thought to have evolved from a common ancestor that is a sort of green alga – with the idea being that chlorophyll has evolved only once. (reference)

Plants and other photosynthetic organisms are largely filled with pigment-protein complexes that they produce to absorb sunlight. The part of the photosynthesis yield that they invest in this, therefore, has to be in proportion. The pigment in the lowest layer has to receive enough light to recoup its energy costs, which cannot happen if a black upper layer absorbs all the light. A black system can therefore only be optimal if it does not cost anything (reference).

Red and yellow light is longer wavelength, lower energy light, while the blue light is higher energy. It seems strange that plants would harvest the lower energy red light instead of the higher energy green light, unless you consider that, like all life, plants first evolved in the ocean. Seawater quickly absorbs the high-energy blue and green light, so that only the lower energy, longer wavelength red light can penetrate into the ocean. Since early plants and still most plant-life today, lived in the ocean, optimizing their pigments to absorb the reds and yellows that were present in ocean water was most effective. While the ability to capture the highest energy blue light was retained, the inability to harvest green light appears to be a consequence of the need to be able to absorb the lower energy of red light (reference).

Some more speculations on the subject: (reference)

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There are several parts to my answer.

First, evolution has selected the current system(s) over countless generations through natural selection. Natural selection depends on differences (major or minor) in the efficiency of various solutions (fitness) in the light (ho ho!) of the current environment. Here's where the solar energy spectrum is important as well as local environmental variables such as light absorption by water etc. as pointed out by another responder. After all that, what you have is what you have and that turns out to be (in the case of typical green plants), chlorophylls A and B and the "light" and "dark" reactions.

Second, how does this lead to green plants that appear green? Absorption of light is something that occurs at the atomic and molecular level and usually involves the energy state of particular electrons. The electrons in certain molecules are capable of moving from one energy level to another without leaving the atom or molecule. When energy of a certain level strikes the molecule, that energy is absorbed and one or more electrons move to a higher energy level in the molecule (conservation of energy). Those electrons with higher energy usually return to the "ground state" by emitting or transferring that energy. One way the energy can be emitted is as light in a process called fluorescence. The second law of thermodynamics (which makes it impossible to have perpetual motion machines) leads to the emission of light of lower energy and longer wavelength. (n.b. wavelength (lambda) is inversely proportional to energy; long wavelength red light has less energy per photon than does short wavelength violet (ROYGBIV as seen in your ordinary rainbow)).

Anyway, chlorophylls A and B are complex organic molecules (C, H, O, N with a splash of Mg++) with a ring structure. You will find that a lot of organic molecules that absorb light (and fluoresce as well) have a ring structure in which electrons "resonate" by moving around the ring with ease. It is the resonance of the electrons that determine the absorption spectrum of a given molecule (among other things). Consult Wikipedia article on chlorophyll for the absorption spectrum of the two chlorophylls. You will note that they absorb best at short wavelengths (blue, indigo, violet) as well as at the long wavelengths (red, orange, yellow) but not in the green. Since they don't absorb the green wavelengths, this is what is left over and this is what your eye perceives as the color of the leaf.

Finally, what happens to the energy from the solar spectrum that has been temporarily absorbed by the electrons of chlorophyll? Since its not part of the original question, I'll keep this short (apologies to plant physiologists out there). In the "light dependent reaction", the energetic electrons get transferred through a number of intermediate molecules to eventually "split" water into Oxygen and Hydrogen and generate energy-rich molecules of ATP and NADPH. The ATP and NADPH then are used to power the "light independent reaction" which takes CO2 and combines it with other molecules to create glucose. Note that this is how you get glucose (at least eventually in some form, vegan or not) to eat and oxygen to breath.

Take a look at what happens when you artificially uncouple the chlorophylls from the transfer system that leads to glucose synthesis. http://en.wikipedia.org/wiki/Chlorophyll_fluorescence Notice the color of the fluorescence under UV light!

Alternatives? Look at photosynthetic bacteria.

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Tobias Keinzler does a good job of explaining why black plants would not work, this is an explanation of why plants are green and not some other color.

Color of foliage is based on whatever the color is of bacteria (or archaea) that get incorporated to become chloroplasts. Or more specifically the color of their light absorbing pigments. there is a huge range in nature for color in photosynthetic organisms, plants are green becasue chlorophyll is green, it could have just as easily been red or purple. http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html

There is decent evidence that chloroplast ancestors absorb the margins of the visible spectrum becasue halobacterium absorb the major constituents, becasue the chlorophyll users did not compete with them directly instead absorbing the leftover light. It was only later when they got incorporated into larger cells that they came to dominate and eventually giving rise to plants. Plants are not green becasue green is better, plants are green becasue that is the first efficient photosynthetic pigment to evolve that did not compete with the dominate photosynthesizer.

enter image description here

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