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Photosynthesis

Contents:

  1. Plants and light
    by Stephen Gomez <smgomez/mindspring.com> (Mon, 18 May 1998)
  2. Iron and red plants
    by "Roger S. Miller" <rgrmill/rt66.com> (Tue, 2 Nov 1999)

Plants and light

by Stephen Gomez <smgomez/mindspring.com>
Date: Mon, 18 May 1998
To: eriko/elmer.wrq.com

Erik, 

I just read your short paper on lighting.  I also read some of the other
articles on growing plants in the aquarium.  It seems the articles are
written by people with a heavy emphasis on plant nutrition and very
little on photosynthesis.  Since that's what I do I thot I would presume
to give you a little info.

Your idea to measure light per surface area is entirely correct.  What
matters is the incident light falling on the leaf surface. What you need
to measure is flux.  Most photosynthesis work measures either 

   1)          Wm-2s-1 (the -2 and -1 are superscripts)
 
or, the second method (more preferred, but harder to measure) is to
measure the energy absorbed by the pigments (Chl a, Chl b, and various
carotinoids).  The unit of measurement is the Einstein (E).  One
Einstein is the energy in one mole of light absorbed. So one E of red
light (670 nm) is 179 kJmol-1 and for blue light (430 nm) is 278
kJmol-1.  You would expect this since blue light has more energy that
red.  (For a very good explaination see J.W. Anderson & J Beardall;
Molecular Activities of Plant Cells (1991) Blackwell Scientific
Publications, Oxford. Chapter 4).  We actually measure in
microEinsteins, since an Einstein would cook a leaf.

Keeping in mind the energies above you can understand why fluorescent
light (predomninantly blue light) would do better than incandescent
light (predominantly red).  

Low light is better than high light.  If the amount of light exceeds the
ability of the plant to fix carbon then the photosystems can become
damaged by triplet oxygen.  Triplet oxygen is the bad guy in
photosynthesis and is created when the excited chlorophyll cannot pass
its electron down the electron transfer chain, so a nearby O2 gets it
and bad things happen.  It oxidizes the photosystem proteins (killing
photochemistry) and also oxidizes the lipid membrane (poking holes in
the memebrane and killing photophosphorylation) This is BAD. Too much
light and the plant turns yellow, then brown and dies.  Terrestrial
plants have many mechanisms to keep from getting fried in the noon sun,
but aquatic plants have lost many of these mechanisms.  They are
notoriusly sensitive to photodamage.  This makes sense if you think of
the environment aquatic plants evolved in.  They usually live in
brown/green water and are usually under floating plants, so by necessity
they became very good at utilizing low light, much like the plants on
the forest floor in tropical rainforests.  Direct sunlight will kills
those plants.

Your observation about peat water being yellow is interesting. 
Chlorophyll does not absorb yellow/green light, but does in the blue and
the red (that's why plants are green).  The observation that peat water
is yellow suggests that is scatters yellow light and it may absorb the
red and blue.  It would be interesting to take an absorbance spectra of
peat water.  It may be that the solution to getting enough light to
plants in peat water is to just add blue bulbs.

Well, back to the dissertation.  Thanks for the opportunity to find an
excuse for taking a short break.

Steve

----------------------------------------------------------------------
Stephen Gómez
Plant Molecular Biology Group
Dept. of Molecular, Cell, and Developmental Biology
University of California, Los Angeles 90095
(310) 825-0182
smgomez-at-mindspring.com
----------------------------------------------------------------------



Iron and red plants

by "Roger S. Miller" <rgrmill/rt66.com>
Date: Tue, 2 Nov 1999

Mark Fisher wrote:

> > > Lack of red color
> > > is often thought of as a iron deficiency.
> >
> > This is oft repeated.  Is there any reason to believe it's true?
>
> No.  Red plants get their color from a non-photosynthetic pigment called
> anthocyanin, which does not contain iron or any other metal.

[snip]

Great response Mark, thanks!

Roger Miller


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