Re: color of light and color of plant leaves

["James Purchase" <jppurchase at rogers_com>]

Moon wrote:
<<  (like, with over 90% efficiency for blue light and
 only slighty less for red-violet light). >>
I think I need an update, what is red violet light? Red and violet are at
opposite ends of the spectrum. You might have blue/violet light or
blue/green
or red/orange but not red/violet.

Oops! You're correct - I plead typo!

I've found a table of the various photosynthetic pigments and their major
absorption maximums in "Photosynthesis, 6th Edition" by Hall & Rao (1999.
Cambridge University Press):

Chlorophylls:
Chlorophyll a .... 420nm and 660nm .... in all higher plants and algae
Chlorophyll b .... 435nm and 643nm .... in all higher plants and green algae
Chlorophyll c .... 445nm and 625nm .... in diatoms and brown algae
Chlorophyll d .... 450nm and 690nm .... in red algae

Carotenoids:
beta-carotene .... 425nm, 450nm, 480nm ... in higher plants and most algae
alpha-carotene ... 420nm, 440nm, 480nm ... in most plants and some algae
Luteol ........... 425nm, 445nm, 475nm ... in green and red algae and higher
plants
Violaxanthol ..... 425nm, 450nm, 475nm ... in diatoms and brown algae

Phycobilins (water soluble):
Phycoerythrins ... 490nm, 546nm, 576nm ... in red algae and some
cyanobacteria
Phycocyanins ..... 618nm ................. in some red algae and
cyanobacteria
Allophycocyanins . 650nm ................. cyanobacteria and red algae

These maxima were recorded with the pigments dissolved in organic solvents.
If a different solvent is used, the numbers may be slightly different (for
example, the red absorption maxima for chlorophyll a and chlorophyll b are
663nm and 645nm respectively when dissolved in acetone).

I've also found reference to the fact that there are multiple forms of
chlorophyll a, some of which are found in deep water algae species
(specially adapted to absorb in blue light).

What does all of this mean for our aquarium plants? Probably not a heck of a
lot - we are not illuminating our tanks with light filtered through narrow
band filters and have very little control of the precise frequency of the
light that we do provide. Additionally, we are also lighting our tanks for
the visual effect perceived by the human eye - and any tank lit with ONLY
light of 420nm and 660nm would look pretty strange to the human eye. Of
course, I'm talking about my eye - you're free to decide for yourself what
is "pleasing" to you. I don't really think that the plants care too much, so
long as they have enough light.

White (continuous spectrum) light of appropriate intensity ought to provide
a happy medium - for both our eyes and the plants. Any tanks that I have
ever had which received some (SOME!) natural sunlight, even filtered thru
window glass, have had better plant growth (both in amount AND in "color")
than tanks under any form of artificial illumination, from any bulb that
I've tried over the years. Mother Nature knows her stuff! That's why I tend
to favour lights with a full spectrum that mimics natural sunlight as
closely as possible (high CRI) over specialty tubes with strange peaks in
only certain areas.

I had also said that one possible explanation for the occurance of red
coloring in some plants was as a form of protection from intense light.
Karen took exception to the statment, pointing out that many red aquarium
plants are green when grown emersed (and under higher light levels). I
didn't say that it was the ONLY reason, nor that it was true in ALL cases,
but I stand by my original statement - it is one possible explanation for
the occurance of red coloration as the auxilliary pigments increase to
protect the plant from damage caused by intense light.

If Karen, or anyone else, has an explanation for the red coloration of
aquatic plants under low or moderate light levels, I'd be glad to hear it.
Could it be that under lower light levels, the wider absorption bands of the
various carotenoids offer the plant the chance to trap more of the limited
amount of solar radiation that is available?

The presence, and relative abundance, of any of these pigments is under the
control of the plant, not the aquarist, except as a result of the plant
adapting to the light and other environmental conditions that are being
provided. Humans might carefully select plants which have a genetic habit of
producing more of the yellow and red pigments (and thus have more "colorful"
leaves) but the plant's main objective is to capture light energy, not to be
"decorative" to our eyes. Beauty is in the eye of the beholder.

Daniel Larson also had a follow up question:
----
>The wavelengths our
>eyes are most sensitive to are not the same ones most useful for
>photosynthesis, but you can kill two birds with one stone (light) by using
>full spectrum bulbs that will provide all of the colors of the visible
>spectrum in a manner similar to sunlight.

Including UV-A and UV-B?
----

What are you asking? UV-A and UV-B are NOT part of the visible spectrum (not
for human eyes, anyway). The normal human eye is capable of "seeing"
electromagnetic radiation (light) over the range of approximately 400nm -
700nm. Note the use of the words "normal" and "approximately" - every eye is
slightly different and every person's ability and perception of "light" and
"color" might be slightly different.

But, in general, most people can "see" light of different wavelengths
(energy levels) as follows:

red ..... 800nm - 650nm
orange .. 640nm - 590nm
yellow .. 580nm - 550nm
green ... 530nm - 490nm
blue .... 480nm - 460nm
indigo .. 450nm - 440nm
violet .. 430nm - 390nm

Electromagnetic radiation which falls outside of this range is invisible to
humans (but some animals can see it). UV-a and UV-b fall outside of the
range of what our eyes can "see". The UV-a range is 320nm - 400nm while the
UV-b range is 280nm - 320nm. Note that none of the photosynthetic pigments
have major absorption peaks anywhere near these ranges (there are weak
absorption bands in the UV-a range for chlorophyll-a).

That doesn't mean that plants don't react and respond to radiation in these
ranges - it just isn't involved (beneficially) with photosynthesis. However,
excess UB-b radiation can cause severe damage to living cells, so its
probably not something that you want a lot of. If you want to read about the
dangers of UV-b on plants, consult "Aquatic Photosynthesis" by Falkowski and
Raven. Their explanation is far too technical and detailed for our purposes
here. Why the damage occurs is a contentious issue but there is general
agreement that damage does occur with exposure to high levels of UV light.

I think that we ought to keep in mind that there are many processes going on
in plants other than photosynthesis which are affected by light - plants are
very complex. Perhaps the visual coloration of our aquatic plants (redness)
is a side issue, related to one of those processes and not directly related
to phosynthesis?


James Purchase
Toronto