What is the optimal wavelength of light for growing plants

Ross Drewe nospam at nospam.com
Sat Oct 18 06:18:44 EST 1997

  >Plants, for the most part, are uninterested in red and infra-red
  >radiation. They do their photosynthesis thing with the light from
  >the sun; in order to do this most efficiently, they are most
  >responsive to light at the wavelengths of the peak output from the
  >sun, namely the middle of the visible spectrum, i.e. green -- not
  >coincidently the color of peak visual acuity of the human (among
  >others) eye, which is as efficient as it is because it, like plants,
  >is at its best for the wavelengths most available in sunlight.

This posting appears to express what the author would like to be true,
not what actually is true.

It is *very* well established that photosynthesis in all higher plants
uses mostly blue and red light - exactly the opposite of what is
claimed above. Jim Ivey is correct in saying that the absorption peaks
for chlorophyll are in the blue and red. I would like to add that the
actual effectiveness of the light of different wavelengths in
photosynthesis does not necessarily coincide with the absorption
spectra of the chlorophylls. It is influenced by other light absorbing
pigments and varies from plant to plant. It is expressed as an
'activity spectrum' and has to be experimentally determined for each
species (by measuring photosynthetic rates). However, it remains true
that it is the red and blue parts of the spectrum which are used.

  >The same evolutionary pressure has made eyes and plants most
  >sensitive to green light, which is why the grass is green.

This is also nonsense. If there is any truth in the argument, it would
be the other way around - we would see best in the green because plants
have been using chlorophyll for much longer than we have been evolving.
As it happens, we do NOT have maximum visual sensitivity for the
midgreen, where chlorophyll reflects best (see below). In view of the
complexity of the photosynthetic pathways and the limited number of
photosynthetic pigments which appear to work (across the whole animal
kingdom), it appears that evolution has NOT been free to match the
maximal visual sensitivity with the colours in the environment. 

  >they are most responsive to light at the wavelengths of the PEAK
  >output from the sun, namely the middle of the visible spectrum,
  >i.e. green

Wrong. The peak energy in sunlight is in the blue, not the green. The
spectrum of sunlight above the atmosphere is reasonably approximated by
black body radiation at around 5800K, over an extended range (say,
0.2-2 microns). At the earth's surface, it is modified by atmospheric
absorption - it lacks a lot of the UV, especially the shorter
wavelengths, and there are sharply defined chunks out of the infra-red.
However, the bulk of the energy is still in the blue at ground level.

Furthermore, there is no reason why evolution should put the peak
sensitivity at the peak of the sunlight spectrum. In fact, it should
logically be the other way round. Increasing the sensitivity of the eye
in spectral regions which are relatively deficient in sunlight would be
the obvious way to maximise information across the widest possible
spectrum. And this does happen in nature: many animals make good use of
a visual range which, for them, extends into the UV. For some animals,
information from the UV is critical to some of their normal behaviours.
It's the information that counts, not how brightly the scene is lit.
There may even be animals that use infra-red to some extent in vision,
despite its low photon energies - I don't know any off hand, perhaps
someone else can give examples.

  >i.e. green -- not coincidently the color of peak visual acuity of
  >the human (among others) eye,

Wrong again.   In sunlit conditions ('photopic' conditions) our
wavelength of maximum sensitivity is more towards the yellow (around
555nm) than the midgreen typical of sunlit plants.

Ross Drewe

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