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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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I find it even more puzzling why plants don't absorb all of the visible spectrum altogether (resulting in the leaves being black). –  Adam Zalcman Jan 3 '12 at 19:48
    
@CrazyJugglerDrummer - I thought about it but didn't want to get the chlorophyll response without people reading =P –  Joe Clarke Jan 3 '12 at 20:38
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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. –  Marta Cz-C Jan 3 '12 at 20:39
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@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. –  Tobias Kienzler Jan 4 '12 at 11:27
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Surely it would be more beneficial: not if the red pigment is less efficient in converting light to energy... –  nico Jan 4 '12 at 13:04
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up vote 35 down vote accepted

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...

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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. –  Alexander Galkin Jan 5 '12 at 14:10
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@AlexanderGalkin: True, but that should already be included in the red curve (together with atmospheric absorption): for blue you can see a stronger deviation from sunlight (yellow curve) than for red/IR (Unfortunately I don't have the dataset but it would be better visible for the photon density) –  Tobias Kienzler Jan 5 '12 at 15:06
    
If leaves were black, they would get too hot. –  David Apr 8 '12 at 2:44
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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. –  William Mioch Oct 11 '12 at 23:29
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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. –  Jack Aidley Jan 29 '13 at 17:05
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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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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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nice links, though I doubt black plants are likely to happen anywhere since that would most likely need too many different pigments –  Tobias Kienzler Jan 11 '12 at 8:03
    
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?) –  Poshpaws Jan 12 '12 at 11:08
    
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... –  Tobias Kienzler Jan 16 '12 at 9:19
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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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I think the solar spectrum is also quite relevant. –  Tobias Kienzler Jan 4 '12 at 11:29
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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. –  Poshpaws Jan 4 '12 at 14:25
    
Heat is the least of a plant's problems. That is why we can breed black tulips without self-cooking petals. ;) –  S. Albano Oct 11 '12 at 6:02
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