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Why are leaves green?

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Re: Why are leaves green?

Postby Geokinkladze » Mon Aug 30, 2010 10:56 pm

Great to see this question is still being debated after 5 years on here. I particularly liked Physicist0017's work on the subject and look forward to reading more.

As I understand it from reading the points here and some other work elsewhere the following is true:

1) We percieve plants as green because they reflect light that has mostly the green wavelength.

2) Green is the most effective colour for plants, if not they would have evolved a different colour by now.

3) Due to the properties of other wavelengths the question "why are plants green" is synonymous with "why are plants not black".

It's been suggested that absorbing all colours, and therefore becoming black, would cause plants to suffer heat issues whereas being green allows them to capture a steady stream of light from sunrise to sunset.
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Postby JackBean » Tue Aug 31, 2010 6:17 am

2) no, they do NOT use green light! So it can NOT be the most effective colour for plants.

3) plants are sometimes experiencing overheating even when absorbing blue and red (FR is the "heat light" anyway) and yet they can handle it, so absorbing the whole spectra would not cause much more troubles. But they had to have pigments for all these wavelengths.
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Re:

Postby Geokinkladze » Tue Aug 31, 2010 11:37 pm

JackBean wrote:2) no, they do NOT use green light! So it can NOT be the most effective colour for plants.


Please note I said most effective, I didn't say that it absorbs most light. You seem to equate the most effective colour being that which absorbs most light, which of course would lead to the logical conclusion that black plants are most effective. So my question to you is, if black was a more effective colour than green then why are plants not black?

JackBean wrote:3) plants are sometimes experiencing overheating even when absorbing blue and red (FR is the "heat light" anyway) and yet they can handle it, so absorbing the whole spectra would not cause much more troubles. But they had to have pigments for all these wavelengths.


The way I see it, if plants are experiencing overheating then by definition they "can't handle it". If they can then surely they aren't overheating. For example put me in a sauna and at a certain temperature and for a certain length of time then there's a range of temperatures and time periods I can handle. I will sweat, maybe feel uncomfortable, but the moment I collapse and need to be dragged out, that for me is when I am overheating and I can't handle it.

My personal gut feeling is that being black would cost the plant more in trying to cope with heat than it would gain from the extra energy. I also think data like that provided by Physicist0017 will show that plants still absorb a lot of the energy.
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Postby JackBean » Wed Sep 01, 2010 5:54 am

2) OK, now I understood you mean the color of plants, not the absorbed color.

3) for you, overheating means being dead, for me, overheating and oversunning means not ideal conditions, which do not have to lead to death if handled properly.
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Re: Why are leaves green?

Postby Geokinkladze » Wed Sep 01, 2010 7:58 pm

JackBean wrote:for you, overheating means being dead, for me, overheating and oversunning means not ideal conditions, which do not have to lead to death if handled properly.


Actually my original statement which was "would cause plants to suffer heat issues" is probably more similar to your understanding of overheating than the one you incorrectly painted me as having. For me suffering heat issues means the plant has to make sacrifices that it wouldn't ordinarily need to do, either that or face an inability to procreate (apologies if "procreate" is the wrong term for plants). Either way it's ability to procreate successfully is harmed in some way, if not entirely negated. Having suffered "heat issues" the plant would lose out to more efficient designs which did not have to make such sacrifices. Ultimately I believe green is the most effective colour for plants because it's the colour that is most dominant.

I'm hoping that if I'm wrong someone could explain to me why so I can understand plant biology better. I dont think that it's enough to say "there's enough light in the blue and red spectrum to suffice". I'd love to be proved wrong though.

Also if green is the most effective colour but for some other reason than heat than I'd love to know the reason.

Personally I have a son who is very inquisitive and I hate giving him half answers.
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Postby d3vilprada » Thu Sep 16, 2010 7:01 pm

A chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflect green wavelength. The internal structure of healthy leaves act as diffuse reflector of near-infrared wavelengths. Measuring and monitoring the infrared reflectance is one way that scientists determine how healthy particular vegetation may be.
Leaves appear greenest to us in summer and become red or yellow with decrease in chlorophyll content in autumn.

Majority of the radiation incident upon water is not reflected but either is absorbed or transmitted. Longer visible wavelengths and near-infrared radiations are absorbed more by water than the visible wavelengths. Thus water looks blue or blue-green due to stronger reflectance at these shorter wavelengths and darker if viewed at red or near-infrared wavelengths. The factors that affect the variability in reflectance of a water body are depth of water, materials within water and surface roughness of water.

The majority of radiation on a surface is either reflected or absorbed and little is transmitted. The characteristics of soil that determine its reflectance properties are its moisture content, texture, structure iron-oxide content. The soil curve shows less peak and valley variations. The presence of moisture in soil decreases its reflectance.
When chlorophyll is abundant in the leaf, it dominates both reflection (in the green) and absorption (in blue and red). This accounts for the two absorption bands on either side of the green reflectance band and explains why a leaf appears green.
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Postby DavidMaine » Fri Sep 17, 2010 7:09 pm

Some blue leaves would be nice!
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Re:

Postby Geokinkladze » Sun Sep 26, 2010 6:19 pm

d3vilprada wrote:This accounts for the two absorption bands on either side of the green reflectance band and explains why a leaf appears green.


I think it's pretty clear why plants appear green, the question that still seems unanswered is why plants evolved to be green and not (for example) black?
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Postby Geokinkladze » Sat Jan 29, 2011 4:18 am

Oh well sorry to do this but it's a five year old thread and there hasn't been anything added for five months so...

*bump*

You never know
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Re: Why are leaves green?

Postby Quinten » Thu Feb 10, 2011 11:15 am

Plants might have evolved to be green because during evolution there was another species that absorbed all the green light, which in the spectrum of sunlight contains most energy and is therefor technically most efficient for photosynthesis in our solar system.

A likely canditate is the Halobacterium which shows up as purple because it absorbs most of the green light. what remains is red and blue (mixed into purple).

Andrew Goldsworthy has proposed this theory and thinks that Halobacterium floated on top of the water during the evolution of plants, and algea evolved under water. Because the Halobacterium would have absorbed all the green light, the only light remaining is blue and red. Algea have evolved to absorb this remaining light, red and blue very efficiencly using chlorophyll. Because red and blue are absorbed and green is not absorbed but reflected or passes through, the leaves show up to be green.

Why not black?
You will notice that leaves at the bottom of the tree are darker than leaves at the top, this is because at the bottom there is less risk of overheating because there is more shade. Therefore there is a benefit to reflect some light (remain lighter) because it prevents overheating of the leaves (and death of the plant).
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Postby DavidMaine » Thu Feb 10, 2011 1:24 pm

Quinten - Thank you for rejuvenating the discussion.
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Postby adihutama » Fri Feb 11, 2011 12:38 am

I think I agree more on the physics reason that the biggest energy containment is not in the green spectra, because there is a relation between wavelength and energy content. Theres a formula for it. I kinda forget.

Therefore, I think it is naturally for plant not to absorbed the not-so-big energy, and when they reflect it, they appear green.

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