Beta carotene Function

gerald deitzer gd3 at umail.umd.edu
Wed May 14 13:20:10 EST 1997


Paul J. Strieleman wrote:
> 
> I have a plant related question I can't find a good answer too.   If the
> function of beta carotene is an accessory pigment for photosynthesis,
> why is it in such high concentration in carrots (a root) or squash (a
> fruit)?  I assume it has some other function in the plant but can't find
> an answer.  If you have time to ponder this I would appreciate it.
> 
> Thanks
> 
> Paul

Carotenoid pigments are synthesized in chloroplasts and do serve as
accessory pigments in photosynthesis.  Perhaps more importantly, they
serve as protective pigments preventing chlorophyll from being
photooxidized.  When chloroplasts degenerate during senescence, the
chlorophyll is degraded, leaving the carotenoids behind as the major
pigments in the plastid.  This is why leaves turn yellow or red in Fall
before they drop.  There is no function to this, it is simply a
consequence of normal senescence.  

The carotinoids that accumulate in flower petals of many species
accumulate in chromoplasts, which are plastids that either derive from
pre-existing chloroplasts, or from pro-plastids that produce
chromoplasts directly. These evolved to function as attractants for
insect pollinators that can see the reflected ultra-violet light.  The
same is true for the carotinoids in fruits like squash and tomato that
have evolved to attract herbivors (fruigivors?) that aid in seed
dispersal.  

Why carotenoids, especially beta-carotene, accumulates in storage roots
like carrot is a very good, but unfortunately, unanswerable question. 
Probably these roots are capable of producing normal chloroplasts, but
lack chlorophyll due to the lack of light and so produce chromoplasts
instead.  Normally roots  produce leucoplasts, which are plastids that
develop from the same pro-plastids in the dark that produce chloroplasts
in leaves in the light.  Leucoplasts do not contain either chlorophyll
or carotenoids, but accumulate large quantities of starch.  Unlike
chlorophyll in angiosperms, the synthesis of carotenoids does not have a
direct requirement for light.  However, their synthesis is generally
stimulated by light, which may be why roots do not ordinarily contain
chromoplasts and make leucoplasts instead. 

Hope this helps and if I said anything that is not accurate, I would
appreciate hearing about it before I teach it again in my Light and
Plant Development course.

Gerry Deitzer
Associate Professor of Horticulture
Department of Natural Resource Science and Landscape Architecture
University of Maryland
College Park, MD 20742-4452



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