[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Plant Pigments

I'm new to the list and I'd like to thank everyone for the great info.

There has been a lot of traffic lately about plant pigments and
photosynthesis, and I got curious and dug into a text:  "Biochemistry",
by Mathews and van Holde (1990)  My summary:

There are two chlorophylls (a and b) which serve as the primary arbiters
of photosynthesis.  Chlorophyll is indispensible to the process as it
occurs in higher plants, algea, and some of the systems of cyanobacteria,
because it is the final "sink" into which the light energy is dumped
before it is used in the reactions which produce chemical energy for the

However, there are many other non-clorophyll pigments that participate
in the initial gathering of light energy which is then turned over to
clorophyll for use in the production of energy.  The main "accessory
pigments" are the carotenoids, phycoerythrin, and phycocyanin, but there
are probably a few others out there too.  The purpose of these pigments is
to provide a different absorbtive spectra from that of chlorophyll, in
order to expand the energy gathering capabilities of the plant.  (Why use
only one part of the spectrum when you can use them all!)

Chlorophyll a and b absorb the red and blue-violet portions of the visual
spectrum.  Beta-carotene absorbs in the blue portion of the spectrum plus
a little of the green, and beta-cerotene is just one of the carotenoids so
I'm sure together they have an even broader range.  Phycoerythrin 
absorbs blue-green-yellow, and phycocyanin absorbs the yellow to orange.
Together, these pigments cover the entire visual spectrum, allowing the
plants to be much more efficient at a given light level.  It is the
accessory pigments which give plants their "fall" colors, as chlorophyll
ceases to be produced.

Knowing this, one might expect plants to be black (absorbing everything)
which is the color my plants were until I started reading this list.
(Actually I guess it was more a nice brown ;) )  The reason that they are
often green is that chlorophyll is still the dominant pigment in most
species, but that doesn't mean those other pigments aren't being used to
some varying degree!  Obviously, red plants have really cranked up their
levels of the pigments that absorb in the yellow-blue range for whatever
survival advantage.  I have heard of plants that live under forest
canopies that have cranked up acessory pigments to grab all the light that
filters through the (green) trees.

This might explain the observations that have been posted about R.
macranda losing its red color when placed in bright sun:  In the
(relatively) dim lights of an aquarium, being able to use more of the
spectrum provided may be very advantageous to this species.  But outdoors,
the intensity may be so high that efficiency of use no longer becomes the
limiting factor to growth, so the plant stops bothering to make so much of
its accessory pigments and just uses chlorophyll.

All this makes me a little nervous about anyone trying to sell me
lightbulbs that are "better" than a bulb that mimics, as closely as 
possible, natural daylight.  Many of the "grow" bulbs seem to have spectra
that mimic the absorption of chlorophyll, which of course leaves out all
the accessories.  For green plants it probably makes no difference, but I
would never want to try to grow red (or other colored) plants exclusively
under such lamps.  Anyone out there tried this?  It just seems that plants
have adapted to a certain solar spectrum over the millenia, and we should
try to duplicate it as closely as possible, IMHO. 

Whew!  That was a long one!  I guess I figured I'd start this out in a big