Why aren't plants black?

[Maximilian]

On several ocations I've stumbled upon explanations like "Photosystems in plant ancestors evolved in a different environment and now it's sort of "stuck" the way it is, it's too hard to change it", "The plant would become too hot!", and "The plants can't handle so much energy". I am a bit skeptical about these explanations, but if you have any data testing any of these hypothesis I'd love to know about it.

 

While reading "Photobiology: The Science of Life and Light" (Amazing book) I stumbled upon what looks like an explanation. The problem is that I can't fully understand it, and I would like your help.

 

You can read it here: http://books.google.com.mx/books?id=bRWd5bGhXM4C&lpg=PA156&vq=Why%20are%20plants%20green%3F&hl=es&pg=PA156#v=onepage&q&f=false

 

 

On the third paragraph it says:

"As for the idea that an ideal pigment should absorb everything, we should remember that the better a substance absorbs, the better it emits, and the transformation of radiant energy into other energy forms is just the balance between absorption and re-radiation"

 

I am trying to understand this and I am unsure about whether I'm on the right path or not. Some of the reactions in photosynthesis are reversible (Example: http://jgp.rupress.org/content/34/6/809.full.pdf), so I guess it means that by increasing the absorption you increase the emission and increases the chances of the reaction going backwards and emitting a photon... If this is correct, that would mean that being able to absorb (and emit) green light would somehow lead to a decrease in net absorption. I am still quite confused, if anyone could enlighten me or point me in the right direction that would be GREAT.

[michel123456]

Interesting question. That's a change from the question why plants are green. I found this pdf where the abstract gives no hope:

The physiological reason that higher plants are green is unknown.

I have no idea why plants have not chosen to be black bodies.

I posted this mainly to show you someone read your question...

[Maximilian]

Interesting question. That's a change from the question why plants are green. I found this pdf where the abstract gives no hope:

 

I have no idea why plants have not chosen to be black bodies.

I posted this mainly to show you someone read your question...

 

Thank you Michel, that's a very interesting paper.

[Moontanman]

Higher plants have evolved from green algae, the pigment chlorophyll reflects green light and absorbs blue and red (simplification) There are algae that use pigments that make the cell look purple and some complex algae are red and some are brown. If i had to just guess I would say it's because the pigment of choice was best at allowing the plant to use light. Plants don't just absorb light the pigment absorbs light of a specific wave length due to factors that control the chemical processes involved. It's not just a matter of absorbing the most light, the light has to initiate a particular chemical reaction. The pigment is what initiates this reaction.

 

Purple sulfur bacteria use light but don't release oxygen, then there are the blue green cyanobacteria, not to mention other green algae that don't release oxygen. Red algae use blue light, they evolved from using light that had been filtered by sea water, brown algae come closest to absorbing all the light they get. Many of the algae if not all still contain chlorophyll but they use other pigments to augment the collection of light... now I feel confused, much of my information comes from years of growing various types of algae under various conditions.

 

But the root of the idea is that first of all not all plants are green and simply absorbing all the light is not the idea, using light to initiate chemical reactions is the idea... and the pigment is what the plant uses to harvest the light and certain pigments absorb certain wave lengths of light. It is quite possible that no suitable pigment exists that would allow plants to absorb all the wave lengths of light emitted by the sun...

 

On a side note, there are some unusual bacteria that live around deep sea vents that use infrared light as well.