MadSci Network: Botany

Re: I am doing a project on the effects of colored lights on bean sprouts

Date: Fri Jan 27 10:35:38 2006
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1138332246.Bt

There are two problems with colored cellophane for plant light experiments: 

1. Just because cellophane looks green doesn't mean that it screens out all but
green wavelengths.  A similar problem exists for other colors of cellophane. 

2. There is no simple way to determine if each color of cellophane transmits the
same amount of PAR (Photosynthetically active radiation). 

The human eye is not a very good light color or light intensity sensor. The
human eye is more sensitive to some colors (green and yellow) than others (red
and blue). If your school has a spectrophotometer, such as a Spec 20, you could
run a transmission spectrum for each color of cellophane to access problem #1.
Make a measurement at 5 or 10 nanometer intervals between 400 and 700 nanometers
wavelength. Graph the % transmission on the vertical axis and wavelength on the
horizontal axis. 

The approximate wavelength (nm) for various colors are as follows:
Blue 475
Green 510
Yellow 570
Orange 590
Red 650

It is a widespread misconception that plants reflect all green light and will
not photosynthesize with green light. Leaves often absorb over half of the green
wavelengths and use them in photosynthesis (Hershey 2004). Look at the
photosynthetic action spectra in the three articles. Figure 7 in Balegh and
Biddulph (1970) shows that the reflection of green light from bean leaves is
about 5% and photosynthesis at 510 nm (green) is almost the same as at 475 nm


Balegh, S.E. and Biddulph, O. 1970. The photosynthetic action spectrum of the
bean plant. Plant Physiology 46(1): 15.

Clark, John B. and Lister,Geoffrey R. 1975a. Photosynthetic action spectra of
trees I. Comparative photosynthetic action spectra of one deciduous and four
coniferous tree species as related to photorespiration and pigment complements.
Plant Physiology 55(2): 401406.

Clark, John B. and Lister,Geoffrey R. 1975b. Photosynthetic action spectra of
trees II. The relationship of cuticle structure to the visible and ultraviolet
spectral properties of needles from four coniferous species. Plant Physiology 
55(2): 407413.

Hershey, D.R. 2004. Avoid misconceptions when teaching about plants. An original article.

What Wavelength Goes With a Color?

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