Help! Middle School Plant/Light/Color Project

Ross Koning koning at ECSUC.CTSTATEU.EDU
Mon Dec 8 10:20:48 EST 1997


At 9:01 AM -0500 12/5/97, bbekstrand at hotmail.com wrote:
>Hello all,
>
>I'm hoping someone out there who understands photosynthesis can help me
>make sense out of data on my daughter's project.  She grew radishes under
>fluorescent light (plant light) where 2 seeds were put in a container
>which was wrapped and covered with colored cellophane.

Barry,
One design flaw here was the size of the sample.  I don't
know the viability of your seeds...fresh seeds from a reliable
seed company can germinate at 85% but sometimes lower.  When
you plant only two seeds, you are taking chances.  The chance
of both seeds failing to germinate under GREAT conditions would
be 0.15 * 0.15 =3D 0.0225.  This 2% chance is usually OK...it
should happen one time in 44 tries.  But if your seeds are not
great or your conditions (light, temperature, water, pests, etc.)
are not great and the percentage germination is only 50% then
your chances of both seeds failing jumps to 0.25.  In other words
a failure in one container out of four!  This just due to chance
alone...not your treatments. It is VERY important to keep sample
size large enough to overcome such difficulties.  Someone mentioned
replication...that is a critical element of careful science.

>She had 6
>containers: clear (control), red, green, purple, green on top of red, and
>green on top of purple.  She used 2 seeds in each container in an attempt
>to guarantee germination if it was possible without ending up with a
>bunch of plants in each container.

Actually the bunch would have been a better idea (see above).
Alternatively one might have germinated the seeds in white
light (no covers!), thinned the sprouts down to a standard
number (at least five) in each container, and then put on the
covers.

>From what we found in encyclopedias, we understand that plants use
>certain wavelengths in photosynthesis.        Our encyclopedia states that
>plants need wavelengths in the red and violet portions of the spectrum.
>From this, our expectation was that the red covered and purple covered
>plants would thrive, assuming the cellophane acts as a filter, only
>letting through red and violet wavelengths respectively.

I'm so glad you did some reading before coming to the net.
This shows a lot of initiative that is really encouraging.
Some of us are thinking that students aren't getting the
idea of print as a medium of information exchange.

The source you read may not have given you ALL the information
you need.  While it is true that green is probably the least
effective color of light to drive photosynthesis (much of the
green light is refelected to our eyes which is why the leaves
appear to be green to us), light of that color CAN be used to
drive photosynthesis...it simply is absorbed by pigments other
than chlorophyll at first.  These "antenna" pigments then transfer
the energy of that light to chlorophyll.  Some of that energy
is lost in the transfer (thanks to the 2nd law of thermodynamics),
so the energy ending up in the reaction center chlorophyll is
actually lower (equivalent to a red wavelength).  Even chlorophyll
itself absorbs some green light...it just is NOT very efficient
compared to more extreme wavelengths in the visible spectrum.

Now here's the important point...this kind of discussion in
comparing colors ASSUMES that you are supplying equal photon
flux density for each of the color treatments.  In science we
generally manipulate only ONE variable at a time...in studying
wavelength/color we keep all other variables constant (ie
photon flux density or what we might perceive as "intensity").
That leads us to your results...

>Our results do not support this at all, however.  The red and the purple
>covered containers did not grow a plant in either at all, nor did the
>green over purple container.  The clear, green, and green over red
>containers did grow plants.  Initially, all of these plants looked
>similar, but after a couple of weeks it is obvious the clear plant is
>doing best.

You might guess that if these were all placed under the same lamp,
that, depending on the density (darkness) of the cellophanes that
you used, some of the containers would have to be closer to the
lamp (the dark filters) while others (especially the "clear" cover)
would have to be farther away from the lamp to control the photon
flux density ("intensity") of light hitting the seeds.  My guess
is that you had the containers at the same distance from the lamp.
Thus in addition to the color variable, your seeds were exposed to
various photon flux densities ("intensities").  The plant in the
clear-covered container had the greatest photon flux density, so
it grew quite well.  To make this project more "fair" you would
have to back this container away from the lamp!  Remember too that
the green wavelengths CAN be effective in driving photosynthesis,
and that the seeds of this treatment are receiving a wide ranges
of wavelengths, thus more chances to absorb light energy and do
photosynthesis!

Maybe if you think about filters you understand what I am saying...
The green filter allows green light to pass to the inside of
the container...but at what efficiency?  Remember the filter
looks green from above!  Aha! Some of the green light is also
being reflected by the filter!  So only a fraction of the total
light hitting the filter is passing...true EVEN of the green light!
So to make sure the seeds under the green filter were receiving
the same PFD as those under the colorless filter, you would need
to move this container closer to the lamp.

=46inally, green cellophane is not a "true" as you might guess.
Do other colors penetrate?  Yes!  If your green cellophane isn't
too dense you might notice while looking through it that you
can detect colors (especially bright colors) in the environment
with your own eyes.  If you can tell colors while looking through
the cellophane, then you know you are dealing with a filter that
(while accentuating green transmission relative to other colors)
leaks light of other colors as well.

Such issues have to be taken into account before you interpret
your data.  It makes good science though, to do this before you
you set up your project.  An excellent experience for your
daughter might be to use a light meter (old-fashioned camera
accessory) to measure the "intensity" of light passing through
the cellophanes.  Maybe add additional layers of the cellophanes
that are too-light in color.  Move closer or farther from your
lamp until the light hitting the "soil" beneath the filter is
the same in "intensity."  Then repeat the project with these
layers and distances (and a larger number of plants).  Science
is a recursive process and this extra experience would show
your daughter (and her teachers) that she understands this
fact and could make a BETTER project out of her first attempt.

So many people think science is an all-or-nothing, there is
only one answer, proposition...and this is not true.  There
is science and there is better science.  And science is NOT
the answers you find but IS the process you use to find those
answers.

Good luck, and I hope you DO choose to repeat the project
with control for PFD.  I'd be happy to reply to another
email if you would like to write again.

ross

>The only conclusion I can really make is that the clear plant thrived
>because it got wavelengths from across the spectrum as required.  But why
>did the green plant grow and not the red or purple?  Is my encyclopedia
>wrong, or do I misunderstand the process?  Should I surmise the red and
>purple seeds didn't germinate due to the color of light, or is this
>experimental coincidence?  Any guess why green over red would grow, but
>not green over purple (these were wrapped 1 color on top of the other,
>thinking it would be a double filter, so to speak, expecting it to knock
>out everything needed).
>
>Any help is appreciated.  We are rapidly approaching time for conclusions,
>and I don't want her to think the old man is lost! :)
>
>Thanks and regards,
>
>Barry Ekstrand
>
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________________________________________________________________
Ross Koning                 | koning at ecsu.ctstateu.edu
Biology Department          | http://koning.ecsu.ctstateu.edu/
Eastern CT State University | phone: 860-465-5327
Willimantic, CT 06226 USA   | fax: 860-465-4479
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