|MadSci Network: Botany|
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 (blue). References Balegh, S.E. and Biddulph, O. 1970. The photosynthetic action spectrum of the bean plant. Plant Physiology 46(1): 1–5. 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): 401–406. 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): 407–413. Hershey, D.R. 2004. Avoid misconceptions when teaching about plants. An actionbioscience.org original article. What Wavelength Goes With a Color?
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