Re: [AGA Member] Plant Pigmentation Discussion (somewhat long)

["Phil Edwards" <biotypical@hotmail.com>]

Roger Miller wrote:

<quote>
> North sky light is blue, so the light in a shaded area open to the
> north might be blue, but that doesn't describe a forest understory.
<end quote>

Yes, for a temperate forest, where the density of vegitation and leaves is 
less than a jungle/rainforest you're very likely right.

<quote>
Also from reading now long ago, the light that penetrates to a forest 
understory is heavily green, not blue.  Blue light is used by the overstory 
plants.
<end quote>

I've been told by people who have been to the tropical jungles that there is 
almost no light in the understory/ground level.  In a jungle {aquatic or not 
:)} all of the overstory plants use Red-Violet light.  Taking into account 
the density of green reflecting leaves in the canopy, very little green 
light makes it into the understories of a jungle.  This is also true of red 
light and the spectra more to the left side of our visable spectrum.  This 
is caused by the longer wavelength and decrease in penetrating power of 
those spectra.  The farther right we go on the spectrum the shorter the 
wavelength and higher the penetrating power of the light.

A forest/jungle canopy is almost totally analagous with water.  The farther 
down one goes the more attenuated the spectrum is and the more blue the 
light gets.  I'm told that the understory of say the Amazonian rainforest is 
actually almost black from the lack of light due to absorption through the 
various layers of foliage.

<quote>
> From what I've read, the thinking on the red underside in understory jungle
> plants is that the color lets them scavenge *red* light by reflecting the
> light back up into the leaf.  It seems that if the pigment were there to
> capture blue light that it would be on the top of the leaf.
<end quote>

Your assessment of blue absorbing pigment is logical and makes sense to me.  
However, taking into account though that red light is the first to be 
attenuated/absorbed not many understory plants have much of an opportunity 
to access/absorb red light.  Thereby reducing the need for a plant to adapt 
special means of reflecting and absorbing it.  Every plant hopes to make it 
up into the sun one day whether by climbing or when whatever's shading it 
falls down and uncovers it.  That being the case most plants continue to 
have an upper side with pigments ready to take advantage of any increase in 
light and change in spectra.  Given that blue light penetrates the best it 
makes sense for a plant to have those pigments which absorb non-blue light 
above the blue absorbing pigment in the leaf.  This strategy gives each leaf 
the best opportunity to absorb every scrap of available light it can.

Cryptocoryne wendtii is a perfect example of this.  On the average, under 
low light conditions they tend to have a darker color on the upper sides of 
the leaf with a fairly red/burgandy underside.  Since the plant doesn't need 
much pigment to absorb red-green light there isn't much pigment for those 
spectra.

It doesn't hurt the plant much to produce those pigments (blue light passes 
through them easiest), and they could potentially be useful, there are some. 
 That's why our plants often have a rusty-brown color on the upperside of 
their leaves.  They have a mix of (lots of) red pigment mixed with a little 
green, orange, and yellow for good measure.  Red + Green = Brown and (Red + 
Orange-Yellow) + Green = Rusty-red + Green = Rusty-Brown.

All of you math teachers out there take that down!  I never thought I'd be 
using math and art to explain a botanical concept.  LOL!


After thinking about this it also explains, to me at least, why red light 
tends to encourage leggy growth and blue tends to encourage shorter, bushier 
growth.  Please forgive me if I'm rehashing old material.

When a plant is exposed to light with a higher red quality it believes that 
it's near the top of a canopy and needs to continue growing as all as it can 
as quickly as possible to prevent itself from being overgrown.  Hence the 
longer internodal length and/or long petioles in our plants.

The converse is true for light with a higher blue quality.  Plants receiving 
this light are led to believe that they're already overshadowed and have 
less need to spend energy growing tall when growing wider will be of more 
benefit.  Broad leaves and wider spread allows the plant to gather more 
light, which it needs to compensate for the lack of light it believes it's 
receiving.

I think that does it,
Phil

BTW, this is fun!  Thanks for making me use my head Roger.

_________________________________________________________________
Get MSN 8 Dial-up Internet Service FREE for one month.  Limited time offer-- 
sign up now!   http://join.msn.com/?page=dept/dialup

------------------
To unsubscribe from this list, please send mail to majordomo@thekrib.com
with "Unsubscribe aga-member" in the body of the message.  Archives of
this list can be found at http://lists.thekrib.com/aga-member/