artificial light please help?

Jonathan B. Marder marder at agri.huji.ac.il
Mon Nov 30 04:09:42 EST 1998


Dear Tiffany

With respect to Janice, there are a couple of things she writes which I
consider to be misleading or incorrect.

"Janice M. Glime" wrote in message
<199811241913.OAA02536 at login.ice.mtu.edu>...
>Dear Tiffany,
>  There are several things that you need to consider.  First, blue and
red
>light are the portion of the spectrum in which photosynthesis is most
>active.  That may explain why your plants grew "better" in these colors
>than in other colors.


It is a common fallacy that plants don't use green light for
photosynthesis.
The truth is that they absorb green a bit less well than blue and red,
but the green that DOES get absorbed drives photosynthesis quite well.
Furthermore, the average leaf will absorb virtually 100% of read and
blue, but also most of the green (over 90% certain thick leaves).
The only way you can observe different photosynthetic efficiencies of
different colour lights is in extremely dilute algal solutions.

>  However, the clear glass gets white light, which has all the colors,
so
>you need to consider why your beans may have grown taller in just one
>color.  I assume you had the same light source for all your treatments,
so
>those plants in jars with colored cellophane on them received less
light
>than the plants in the clear jar.  Now you must ask why they would grow
>taller with less light. Think about the way grass grows if it is under
a
>board or rug or something that blocks part or all of the light.  The
grass
>and other plants get tall and thin.  Also, non-grass plants in a field
>with tall grass will grow tall and thin.  This is a response to low
light.

This is mostly true, but light colour is probably more important than
intensity. Plants subject to artificial shade
(walls etc.) seldom respond by growing taller - unless the shading is
extreme. On the other hand, natural shade (e.g. under a tree)
selectively takes out the visible light allowing much more "far-red" to
go through. This makes plants grow taller. The light-sensing molecule
involved in this is called "phytochrome". What is interesting is that
blue light puts the phytochrome into a "state" similar to far-red
enriched light. (To get technical, that "state" is called the
photostationary state, and reflects the distribution of phytochrome
between two interconvertible forms).

>I will not tell you the whole story because your teacher probably wants
>you to try to find out what is happening, but I will tell you that this
is
>a response to a hormone called IAA. [snip]

On this I disagree. While IAA is involved in extension growth,
Gibberellin is the more important plant hormone for the effects we are
talking about here. Typically, dwarf plants are ones which don't produce
enough of their own gibberellin, but they grow tall when sprayed with
gibberellic acid. Furthermore, common chemicals like paclobutrazol
(bonsai) can be used to grow normal plants as dwarfs. It seems that the
light response described above operates via gibberellin.

>If you can find a plant physiology
>book or a general botany book, they usually have an explanation of
this....
Good advice - you should have no trouble finding explanations of
gibberellin, auxin (IAA), other plant hormones and phytochrome.

And another piece of advice. People like Janice and myself are
well-meaning, but we sometimes make mistakes, so don't take either of us
as "authorities" without checking other sources of information (like
books, which are also fallible).

All the best,

Jonathan B. Marder   <MARDER at agri.huji.ac.il>
Department of Agricultural Botany, The Hebrew University of Jerusalem
Faculty of Agriculture, P.O.Box 12, Rehovot 76100, ISRAEL
Phone: +972 8 9481918   Fax:   +972 8 9467763
Web page:    http://www.agri.huji.ac.il/~marder





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