Hi.

Looking at a graph of light absorption, chlorophyll (a+b) appear to
absorb at a peak of 642 and 662 nm in the red region.

I'd guess that the action-spectrum would show maximum action at some
point between these two wavelengths, however the graph I have shows
the peak activity at around 690nm. What's the deal?

If you had to use a red light with one specific wavelength, what would
it be?

Thanks.

(flopmaster) wrote in message . com...
Hi.

Looking at a graph of light absorption, chlorophyll (a+b) appear to
absorb at a peak of 642 and 662 nm in the red region.

I'd guess that the action-spectrum would show maximum action at some
point between these two wavelengths, however the graph I have shows
the peak activity at around 690nm. What's the deal?

If you had to use a red light with one specific wavelength, what would
it be?

Thanks.

Accessory pigments. Chlorophyll a and b are not the only pigments
involved in capturing photons, and the process is not so simple as
direct absorption of a photon by a cholorphyll molecule. There's a
useful discussion, including a description of the Emerson effect, at
http://www.biologie.uni-hamburg.de/b-online/e24/24c.htm

If you were forced to use a single wavelength of light, it would be
somewhere between 670 and 700 nm; but the Emerson effect predicts far
better results with two wavelengths together, especially 670 and 700
nm.

--
Chris Green

What kind of plant do you have and what is your goal that involves
using a single wavelength?

A photosynthesis absorption spectrum is artificial because chlorophyll
is extracted into an organic solvent, which shifts the absorption peak
compared to in vivo. It is an in vitro spectrum which also excludes
the accessory pigments, such as carotenoids, that in the intact leaf
capture light energy at wavelengths chlorophyll absorbs poorly and
channels it to chlorophyll.

The in vitro chlorophyll extract also lacks the complex chloroplast
and leaf structure found in the leaf. Salisbury and Ross's Plant
Physiology text suggests that chlorophyll in vivo has a much higher
absorption of green and yellow wavelengths than the in vitro
absorption spectrum because photons not absorbed the first time can be
"repeatedly reflected from chloroplast to chloroplast" and get
multiple chances for absorption. They suggest the actual absorption of
green and yellow wavelengths by chlorophyll in vivo is about 50%
compared to under 5% in the absorption spectrum.

Salisbury and Ross gives a graph summarizing action spectra of 22 crop
species, and the peak is 100% at between 600 to 640 nm with a blue
peak of between 60 to 75% at around 450 nm. However, the average
between 500 and 600 nm is actually above the blue peak. Plants with
bluish leaves, such as blue Colorado spruce (Picea pungens 'Glauca'
and related cultivars) may have no blue peak.

Other pigments, such as anthocyanins, may absorb or reflect some
wavelengths making them unavailable for photosynthesis, which can also
alter the action spectrum.

David R. Hershey




(flopmaster) wrote in message . com...
Hi.

Looking at a graph of light absorption, chlorophyll (a+b) appear to
absorb at a peak of 642 and 662 nm in the red region.

I'd guess that the action-spectrum would show maximum action at some
point between these two wavelengths, however the graph I have shows
the peak activity at around 690nm. What's the deal?

If you had to use a red light with one specific wavelength, what would
it be?

Thanks.