What is the optimal wavelength of light for growing plants

Michael L Roginsky d_micro at ix.netcom.com
Fri Oct 24 16:03:23 EST 1997


In <01bcdbb7$6179c680$2a9d0ccb at rd> "Ross Drewe" <nospam at nospam.com>
writes: 
>
>  >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
>
To make it simple, look at the info provided by GE and Sylvania on
their electro-luminescent tubes used to start plant seedlings indoors.
Very little in the red and below bandwidht, increasing with shorter
wavelenghts into the blue and above. Cheers.....Micro.



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