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RE: Iron and red plants

> Isn't anthocyanin formation  mainly the result of sugar breakdown
> (oxidation) in response to light? 

Close.  Anthocyanin formation begins when an enzyme, phenylalanine
ammonialyase, is expressed after exposure to UV-B radiation.  This enzyme
begins the process by removing the ammonia group (ammonialyase means "to
lose ammonia") from phenylalanine, which is an amino acid, not a sugar.  The
actual synthesis of anthocyanin occurs many steps later, but the pathways
(and the responsible genes) have been pretty well identified.

> Does the "sunscreen" effect pertain to the anti-oxidant 
> properties of cyanin compounds? Do red plants use these pigments to absorb

> the effects of excess light to protect chlorophyll instead of using
carotene or
> xanthophyll?

The "sunscreen" effect is due to its strong absorption of UV radiation.
Anthocyanins are thought to have many functions, most notably as insect
attractors for flowers (pollination) and animal attractors for fruits (i.e.,
seed dispersion).   Carotenoids and xanthophyll are photosynthetic pigments
with absorption peaks that differ from chlorophyll--they can capture light
energy that chlorophyll cannot.  These pigments take advantage of the
available light energy that can be used in photosynthesis.

I have to wonder why some aquatic plants are red.  UV protection would be
less important to a submersed plant than an emersed, or terrestrial plant,
and I doubt that it has an "attractor" function.  Perhaps the red color is
simply a "leftover" from its terrestrial origins?