|MadSci Network: Botany|
This question comes up quite frequently. It's very difficult to explain results of this kind of experiment because the amount of light may also vary as well the color. Therefore, you don't know whether any differences in plant growth are due to color or to the amount of light. To make sure each color provides the same amount of light, a plant scientist would use a quantum sensor to measure the photosynthetically active radiation (PAR) to be sure each color provided the same PAR. PAR is a measure of the number of photons (light particles) with wavelengths of 400 to 700 nanometers. Another problem is that colored cellophane may not provide a pure color. The human eye is a poor judge of color because it is much less sensitive to blue and red wavelengths than to green and yellow. If your school owns a spectrophotometer you could make a transmission spectrum to see what wavelengths of light your cellophane actually transmits. Roll up up piece of cellophane and place it in the test tube or cuvette in the spectrophotometer. Start at 400 nanometers and measure the percent transmission every 5 or 10 nanometers up to 700. You will need to rezero the spectrophotometer with an empty test tube for each wavelength. Graph your data with percent transmission on the vertical axis and wavelength on the horizontal axis. Sunlight has roughly equal amounts of all colors of light which should affect your hypothesis. Light colors from 700 to 400 nanometers roughly follow the mnemonic ROY G BIV - red, orange, yellow, green, blue, indigo, violet. The wavelengths that correspond to colors are roughly as follows: red is 760 to 650, orange is 650 to 600, yellow is 600 to 560, green is 560 to 500, blue green is 500 to 470, blue is 470 to 430 and violet is 430 to 340. Plant species vary somewhat but most plants reflect slightly more green than other wavelengths. That's why they appear green to our eyes. However, the rate of photosynthesis with green light is often 60% or more of the rate with blue or red light. Salisbury and Ross (1985) has a couple graphs of photosynthesis rate versus light wavelength. In one graph of 22 crop species, the relative rate of photosynthesis averages about 65% between 400 and 500 nanometers, about 75% between 500 and 600 nanometers and about 90% between 600 and 680 nanometers. There is a sharp dropoff above 680. Light color has other effects on plants as well. Blue light is required for phototropism. The ratio of red to far-red light affects elongation. Sun plants that receive a lot of far-red light, as occurs in shade, will grow taller. This is an adaptation to help them grow up into the light. References Bickford, E.D. and Dunn, S. 1972. Lighting for plant growth. Kent, Ohio: Kent State University Press. Salisbury, F.S. and Ross, C. 1985. Plant Physiology. Belmont, CA: Wadsworth.
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