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Re: Anthocyanin, Red plants and UV

Here is some food for thought.
As James and others have indicated, leaves are green because chlorophyl.
The green pigment may also mask other pigments. If you reduce chlorophyll,
other pigments become more visible and the leaf color can change. In part,
this is what happens to trees in the fall.

Iron, magnesium, nitrogen and sunlight are necessary for chlorophyll
production. Reduce the amount of certain nutrients and/or light, there can
be less chlorophyll. The plants will grow more slowly, but their colors may
also intensify.

The Repto bulb is said to provide UV. Daniel Larsson noticed that Reptiglo
reduced his Fe levels. When this bulb was replaced with Grolux, the Fe
returned to normal and the new shoots became green again and very slowly
the black-redness went away. Perhaps the bulb's reduction in iron might be
responsible :-)  [it would be nice to know what "Aquarium" bulbs also
provide UV in sufficient amounts to affect Fe]

I found an interesting article on leaf color with a few ideas. [See "Why
Leaves Change Color - The Physiological Basis"
<http://www.agcom.purdue.edu/AgCom/Pubs/FNR/FNR-FAQ-5.html >] It says
"sometimes conditions for chlorophyll  production less favorable. For
example, during the fall season nitrogen and phosphorus are slowly
withdrawn from the leaves of trees for storage in  twigs and branches. The
loss of these nutrients, essential for chlorophyll production, coupled with
the reduced exposure of leaves to light as days get shorter,  gradually
stops the production of chlorophyll. The green color of leaves fades as the
production of new chlorophyll diminishes and the existing chlorophyll is
degraded." Please recall previous APD discussions which mentioned low
nitrogen levels as a reason for or strategy to induce redder leaves in
Eustrallis stellata and other plants  (i.e. by limiting production of
chlorophyll which otherwise masks the other leaf pigments). The Purdue
reference also mentions that " color is influenced by cell pH.... pigments
usually are red  in acid solution and may become purplish to blue if the pH
of the cell sap is increased." .... so there are undoubtedly other factors
at play in the complex environment we call an aquarium. 

Another example of varying leaf colors are red sword plants . For some
Echinodorus hybrids, the new leaves are red while the older ones become
green. Similarly, new plantlets[say for "New Ozelot"/"Red Flame"] that
float near the surface while attached to the breeding spike are a brilliant
red color while the mother plant's leaves are much more green. I have heard
that the lack of phosphates in young terrestrial leaves (e.g. in the early
spring due to cool weather) can also lead to the production of anthocyanin.
Do the young leaves have less phosphate? Also, the distribution of
nutrients and chlorophyll may be different throughout the "entire" plant
(mother and baby).

Finally, very high light intensities may bleach chlorophyll and so retard
photosynthesis.  Plants that naturally grow in these conditions have
evolved protection devices. For example, terrestrial or emergent aquatic
plants may have thick cuticles. Underwater plants must use different
strategies. Perhaps production of extra anthocyanin or other pigments is
used by some species. Thus, the emerged leaves may use one strategy to
protect itself against UV or other wavelenghths while the submersed ones of
the same plant are using a different one. Of course, the green emergent
growth and red submersed leaves must be on the  same plant at the same
time. Otherwise, different forces may be at work....

I am sure that there is much more to say and or speculate.