Chlorophyll and Green Light

David R. Hershey dh321 at PGSTUMAIL.PG.CC.MD.US
Sun Nov 3 23:39:11 EST 1996


Chlorophyll absorption spectra indicate a very low absorption of green
compared to red or blue wavelengths. However, the photosynthesis action
spectrum of an intact leaf indicates the rate of photosynthesis is roughly
60% as much with green light as with red and it may actually be higher
than with blue (see Salisbury & Ross, Plant Physiology, 3rd, p. 185) Thus,
leaves can use green light fairly effectively in photosynthesis. Some of
the absorption may be due to accessory pigments. Chlorophyll in an intact
leaf can also absorb green light much more effectively than the
chlorophyll absorption spectrum (chlorophyll extract in a
spectrophotometer) indicates. One reason is that although green light is
absorbed with low efficiency, it has many chances to be absorbed because
it is repeatedly reflected from cell to cell by the complex leaf geometry
so it has many chances to be absorbed. Such geometry effects do not occur 
with chlorophyll extract in a spectrophotometer tube. This provides an 
excellent illustration of how in vitro can differ markedly from in vivo. 
Unfortunately, biology textbooks usually just publish the in vitro 
chlorophyll absorption spectrum rather than the in vivo photosynthesis 
action spectrum.

Thus, the common idea that leaves are green because they reflect ALL green
light is incorrect. Most leaves reflect relatively more green light
relative to red/blue wavelengths and appear green to our eyes. An
exception is the blue Colorado spruce (Picea pungens 'Glauca') with bluish
needles. The sensitivity of our eyes might have something to do with it
too because our eyes are most sensitive to 550 nm wavelengths and much less 
sensitive to red or blue wavelengths. 

******************************************************************
David R. Hershey
					
Snail mail: 6700 Belcrest Road #112, Hyattsville, MD 20782-1340

Adjunct Professor, Biology/Horticulture Department
Prince George's Community College, Largo, MD 20772-2199

Email: dh321 at pgstumail.pg.cc.md.us
*******************************************************************


On 1 Nov 1996 ceumb at stolaf.edu wrote:

> Dear Plant Edders,
> 
> A friend asked me a question the other day about chlorophyll.  He was
> wondering why blue and red but not green were absorbed.  After some
> reading and chatting with colleagues I came to the short answer that
> chlorphyll was so efficient with red and blue that there was no
> evolutionary advantage in absorbing green and/or that changes needed
> to absorb green would mean loss of efficiency absorbing blue or red.
> 
> Does this answer make any sense esp. since some bacteria use rhodopsin
> to capture green light for photosynthesis?
> 
> Thanks,
> Charles Umbanhowar
> St. Olaf College
> ceumb at stolaf.edu
> 
> 
> 
> 
> 
> 



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