Subject: Soil seed banks - germination of weed seeds

From: "Lawrence F. London, Jr." <[email-masked]>
Date: Fri, 31 Oct 1997 23:27:28 -0500 (EST)

I found this in my archives tonight & thought it was interesting.
My question is: Would the shade of green of different plant's
leaves matter with regard to filtering sunlight and changing the
ratio of red to far red light thus inhibiting the germination of weed
seeds in the soil beneath them?
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 Lawrence F. London, Jr. - InterGarden - Venaura Farm
 mailto:[email-masked] - mailto:[email-masked]
 http://sunSITE.unc.edu/InterGarden
 http://sunSITE.unc.edu/InterGarden/permaculture.html
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Date: Sun, 21 Sep 1997 15:39:09 EDT
From: "E. Ann Clark, Associate Professor" <[email-masked]>
To: sanet <[email-masked]>
Subject: Re: soil seed bank

Joel:  a fascinating area.  Have a look at a book by Mary 
Allessio Leck, V. Thomas Parker, and Robert L. Simpson, called Soil 
Seed Banks.  1989.  Academic Press.  All kinds of neat stuff relating 
to your questions, including species specific predation.  Chapter by 
Cavers and Benoit includes a table on numbers of viable weed seeds in 
arable soils (range is 250 to 130,000 seeds/meter squared, with most 
in the range of 15-20,000).  

Chapter by Baskin and Baskin (major contributors to this literature) 
discusses one particularly intriguing aspect - that "leaf-filtered 
sunlight inhibits germination of nondormant seeds".  I've looked into 
this and was fascinated to realize that the ratio of red:far-red 
light (which affects the function of phytochrome, a key developmental 
hormone) changes upon filtration through a vegetative canopy.  In 
practical terms, this means that keeping the land covered by living 
plant material REPRESSES germination of weed seeds - leaving the land 
open and bare ENCOURAGES weed seed germination.  Residue doesn't work 
- has to be living stuff, I think.  

Thus, the focus of organic farmers in keeping the soil covered at all 
times is revealed as having yet another beneficial effect - weed 
control.  

This kind of stuff was studied in depth in the UK about 10-20 years 
ago.  H.A. Roberts was a major contributors.  Ann

[email-masked]
Dr. E. Ann Clark
Associate Professor
Crop Science
University of Guelph
Guelph, ON  N1G 2W1
Phone:  519-824-4120 Ext. 2508
FAX:  519 763-8933

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From: [email-masked]
Date: Sat, 1 Nov 1997 11:28:42 -0500 (EST)

>seeds in the soil beneath them?

Well I don't know about the biology part, but the optics part I can answer.
 First, the use of color in this regard is a common mistake.  Color is a
psychological phenomenon and while it is excited by physical phenomena,
wavelengths, the correlations are loose at best.  For example, you can see
colors that have no corresponsing wavelength--that do not appear in the
spread when white light goes through a prism.  You can also see colors even
when the wavelengths are not present, due to our capacity to process limited
information to reconstruct the spread of wavelengths where white-light
illumination present. 

In this case, it means that you can't look at a leaf and estimate how much
red passes through.  It is known that chlorophyll is one of the best
reflectors of infrared.  The major effect of this would be to tend to reflect
infrared back to the soil as sunlight contains little and cooler bodies, such
as the earth, tree trunks, etc., all emit various wavelengths.  However there
is a continuous spectrum of infrared in sunlight, of course, and so one can
diagnose disease and pest damage in corn fields from space satellites using
infrared imaging.

In some color schemes, red is said to the be complementary to green, meaning
that a "pure" red object (meaning that the hue is desaturated) relects little
green light and a "pure" green object reflects little red light.  So red
reflected by the understory, for example natural mulch which has often a high
red component, would be absorbed.  

Bear in mind that in the human eye, red an green are not compliments but two
of the three color-senstive types of cones, one for wavelengths perceived as
red, one for wavelengths perceived as green, and one for wavelengths
perceived as blue, all reflected from a white surface in a color-neutral
setting.  In human vision, the complement of reflected red light is cyan, not
green.  (blue-green, meaning that both "blue" and "green" receptors are
stimulated.)  Moreover, the "color" sensitve cones overlap in sensitivity, so
that some wavelengths stimulate "red-senstive" and "green-sensitive" cones
equally.  (These are perceived as yellow.) A further complication comes from
the fact that the perception of the "same yellow" can be generated by equal
stimulation of red and green receptors by quite different wavelengths of
light.  The percenption comes from both types of cones being stiumlated
simultaneously, not from a particular wavelength.  And this is before the
signal gets processed by varous nerve bundle arrangements on the way to the
brain.  By the time the brain gets through with what it gets, it is
astonishing that we can actually get a reproducable result by behaving as if
those these sensations, eg color, had any connection to the physical world.

The problem comes when we have a real-life situation in which some light
transmits through the leaves--presumably more red as green is absorbed--and
some light reflects from the leaf surface, which will be mostly green with
little red reflected.  Again, we are using somewhat inaccurate shorthand here
meaning light that in the spectrum, as from a prism, looks red.  There is a
fair correspondence with red and green between what we see and wavelength
distribution of what I call spectral colors, but it is only approximate.)

I would be fascinated and surprised to find out if any of this mattered as
much as a whole percentage point in the proclivity of even one individual
seed to germinate.  First, not all seeds are light sensitive.  Second, there
are many, many other factors.  I would be interested to know what sorts of
seeds exhibit sensitivity to wavelength as a germination impetus or
inhibitor, and what sorts of plants these are--what their role is ecologically
 and what climates they represent. Weeds that germinate in response to light
could, presumably, be more sensitive to "red" light as that is more likely to
dominate that the passage through soil.  Likewise, plants of tropical
ecosystems, particularly species that exploit random disturbances, could
conceivably benefit by being sensitive to shifts in the color spectrum, as
well as intensity, of overstory light.  These could signal that one level or
other of the canopy has an opening.  "No understory plants need apply.,"

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since
1982), Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
courses via email.  Next live training:  Paraguay, Sept. 1998. Internships
available. Copyright, 1997, Dan & Cynthia Hemenway, P.O. Box 52, Sparr FL
32192 USA  [email-masked]  

We don't have time to rush.

From [email-masked]  Sat Nov  1 13:01:29 1997
Date: 	Sat, 1 Nov 1997 12:55:14 -0500 (EST)
Reply-To: [email-masked]
Sender: [email-masked]
From: "Lawrence F. London, Jr." <[email-masked]>
To: [email-masked]
Subject: Re: Soil seed banks - germination of weed seeds

> hormone) changes upon filtration through a vegetative canopy.  In 

I assume, possibly incorrectly, that the darker green the color,
the more leaf surface area there is, the greater the change in 
the ratio of red to far red light will occur?