Dear Dr. Seager,
I remember a thread in this newsgroup some years ago induced by an
astronomer's question: "why are plants green?"
Thanks to GOOGLE's search abilities I could locate this 1997
discussion where Joe Berry, Winslow Briggs and Frantisek Vacha
presented some ideas about the evolution of photosynthetic pigments
which need no further comment. I add these documents at the end of
my posting.
Sincerely,
W. Ruehle
Inst f. General Botany
Joh. Gutenberg University
D-55099 Mainz
Germany
My comments to some special questions in Dr. Seager's posting:
seager at ias.edu schrieb:
>> Hello,
>> I am an astrophysicist with a few plant related questions.
> I hope someone will be able to answer these and to provide
> references, or to point me in the right direction.
> These questions are about the red edge reflectance
> signature of chlorophyll producing plants--the order of magnitude
> increase in reflectance just redward of about 700nm.
Light scattering by intercellulars leads to an increased
chlorophyll absorbance. So the reflected red light (<700nm) is
diminished but reflectance of >700nm remains nearly 100%.
> I have read that the high reflectance redward of 700m is from light
>scattering
> in the air gaps between plant cells, a function that has evolved as a
> cooling mechanism to prevent degradation of chlorophyll.
I do not know your source but possibly you misunderstood something.
The high reflection of plant tissue >700nm results from its low
absorbance and does not differ in its mechanism from inorganic
compounds like silver or MgO. Certainly we are protected against IR
radiation beyond the crown of a tree but the reflecting leafs above
us are not "cooled" by this mechanism but rather by the evaporation
of water. However some leaves have evolved a hairy surface and look
white as a protection against too much light and transpiration. These
hairs reflect all wavelength about equally and with such a vegetation
one would not expect a pronounced red edge.
> This red-edge signature has become
> something of interest to astrophysicists as an indicator
> of life--a civilization 100s of light years away from us with a large
> space telescope would be able to detect the red-edge signature
> on the spatially unresolved Earth.
Could one really resolve a red edge signal in a distance of 100s of
light years from the reflectance of the "blue planet" which originates
mainly from oceans with very diluted chlorophyll contents, relatively
small patches of terrestrial vegetation producing red edge signals,
and fluctuating atmospheric signals?
> 1) Would evolution of a light-harvesting organism
> always lead to a reflectance signature (at a different wavelength
> regime than the harvested one)?
> Or could it also be likely that another method of energy
> dissipation could evolve?
Every compound that has a 1.singulet absorption band produces an edge
between the wavelength of its absorption and light of lower energies
which are no longer absorbed because the 1.singulett state could not
be reached. The magnitude of the red-edge effect is a function of
absorbance. But only a compound generated by living organisms is
likely to cover a planet and will evolve in an optical window of this
planet's atmosphere. So far your idea is promising.
>> 2) Do photosynthetic plants absorb at optical wavelengths
> because of the required energy for molecular electronic
> transitions?
see discussion below
> I'm hoping that there are specific examples from light-harvesting
> organism evolution or existing photosynthetic organisms
> (e.g., bacteria that absorbs light in the infrared) that will
> shed light on these questions, if not an answer to them.
>> Please email any responses directly to me at
>seager at dtm.ciw.edu>> Sincerely,
> Dr. Sara Seager
> Faculty, Carnegie Institution of Washington
> Washington, D.C.
>
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Thread from Sept 1997:
=46rom: Joe Berry <joeberry at biosphere.Stanford.EDU>
Subject: A Question
Date: 1997/09/28
Message-ID: <9709282059.AA01736 at biosphere.Stanford.EDU>#1/1
Distribution: world
Sender: daemon at net.bio.net
Organization: BIOSCI International Newsgroups for Molecular Biology
Newsgroups: bionet.photosynthesis
Dear Photosynthesis Researchers,
I received an interesting question that might be a useful topic for
discussion on the photosynthesis net. The question comes from an
astronomer via Maxine Singer, President of the Carnegie Institution.
Allan Sandage at the Observatories sent me the following
question. Can you help with an answer? He doesn't 'do' email, so
email me the answer and I will send it on to him.
"Why are plants green?? (I suppose this means and not yellow or
blue or red) What evolutionary advantage does green have re
photosynthesis?"
THanks, Maxine
I would appreciate hearing the thoughts of other photosynthesis
researchers. I have included my answer and a response from Winslow
Briggs below.
Thanks,
Joe Berry
Carnegie Institution of Washington
Stanford, CA 94305
joeberry at biosphere.stanford.edu
An Answer:
Here is one way to look at it: Chlorophyll's absorption is at
wavelenths <700 and >400 nm. This "window" was probably prescribed by
the chemistry of the primordial oceans. These are thought to have
contained high concentrations of Fe+2 ion (which absorbs strongly at
wavelengths >700 nm) and dissolved organic compounds (which absorb in
the blue and near UV). Thus, chlorophyll is a pigment that "fits"
into a window of available light energy. In this sense, it is ideally
suited for photosynthesis. On the other hand, chlorophyll is green
because it dosen't completely fill the window. This is not an
advantage, and plants have evolved a number of accessory pigments to
fill the hole in the chorophyll absorption spectrum. These pigments
donate absorbed photon energy to chorophyll.
__________________________________________________________________
Subject: Re: (Fwd) Question
Author: "Winslow Briggs" <BRIGGS at andrew.stanford.edu> at Internet
Date: 9/25/97 11:26 AM
Let me add to Joe's comment:
There aren't any conjugated double bond pigments that I know that have
extremely broad absorption bands. Below 400nm, the increasing energy
of the photons raise the spectre of photochemical damage. Beyond 700
nm, the energy levels are sufficiently low that except in exceptional
cases they are insufficient for effectively driving photochemistry. A
compromise: an absorption band safely above the UV, and one
sufficiently down in the red that useful photochemistry is still
possible. My guess is that a single band in either wavelength region
would probably be selected against. The situation in higher plants is
not perfect, as Joe points out, and accessory pigments are made in
some algae to fill in the gaps. Even higher plants use carotenoids,
absorbing in the blue, to enhance energy capture, but these still do
not extend too far into the green window left by chlorophyll.
It seems to me that given the properties of conjugated double bond
systems in absorbing light energy, making a molecule with two major
bands within the biologically constrained wavelength range is not all
that simple, and chlorophyll is an ideal solution.
(Note the waving of hands!).
=46rom: Frantisek Vacha <vacha at GENOM.UMBR.CAS.CZ>
Subject: Re: A Question
Date: 1997/10/03
Message-ID: <102310A2B96 at genom.umbr.cas.cz>#1/1
Distribution: world
Sender: daemon at net.bio.net
Organization: BIOSCI International Newsgroups for Molecular Biology
Newsgroups: bionet.photosynthesis
Another wiev why plants are green
Two reasons:
=46irst. We have to ask why plants use chlorophyll or generally
porphyrins.
According to my opinion nature hadnlt much choices and plants used the
most convenient way to develop a useful pigment system. Well before
chlorophyll-like organisms there have been heterotrophic organisms with
=46e-porphyrins, hems. Hem is suitable for many enzymatic reactions but
its
absorption properties are not good (main peak at about 400 nm and then
some nothing about 550 nm) and having a heavy metal Fe in the centre its
properties as a species for energy transfer, energy conservation (longer
excitation times) or even charge separation are bad. However, nature had
already developed path for synthesis of a potentially good pigment
(chlorophyll). Note that the synthesis pathway of hem and chlorophyll is
the same to the IX-protoporphyrin. Protoporphyrine and even
Mg-protoporphyrin have absorption mainly at about 400 nm and almost
nothing in the red region. The advantage of absorption in the red is
made
by reduction of a 7-8 bond of protochlorophyll. The Mg atom in the
centre
in not needed for such absorption profile as seen on pheophytin but it
is
definitely needed for porphyrins to became pigments suitable for
photosynthesis.
However, there is also bacteriorhodopsin in Halobacterium and in
Holococcus. Is it photosynthesis? Synthesis of bacteriorhodopsin has
different pathways from chlorophyll. Here it is seen that nature had
tried
more paths to evolve photoautotrophic organisms. And everything could
have
been orange!
Second. Why isnlt the question aewhy are plants red-brown?" ? There are
green sulphur bacteria and purple bacteria. Green sulphur bacteria are
actually not very green (depends on the level of carotenoids) and their
red
absorption maximum (Qy transition) is at 753 nm. So in the middle of the
evolution way we are still not green as we are now. Here I have to note
that bacteriochlorophyll absorbs far beyond 700 nm and the energy
absorbed
by bchl is efficient to drive charge separation. Important is that
bacteriochlorophyll in its kation state is not able drive an electron
from
water in any conditions which nature or evolution had tried. The
limitation
of electron donors, the fact that there is enough water in environkment
lead to the evolution of system which started to use water as a donor of
electrons. This had to be probably initiated by changes of pigments to
chlorophyll a which has, under certain conditions in photosystem II,
such
redox potential to drive electrons from water. And here the Qy
transition
(red absorption peak) is moved to shorter wavelengths and the overall
colour of chlorophylls to the green.
I donlt know anything about evolution of photosynthetic pigments and
some
people say that chl was before bchl but the key things are the
similarity
of synthetic pathways of porphyrins hem and chlorophyll and the need of
chlorophyll a to drive electron from water.
Regards
=46. Vacha
---------------------------------------------------
=46rantisek Vacha
Inst. Plant Molec. Biol.
Branisovska 31
370 05 Ceske Budejovice
tel. 00420-38-7775523
fax. 00420-38-41475
