MadSci Network: Botany

Re: How/why do colored lights affect the growth of plants?

Date: Tue Apr 4 18:05:23 2000
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 954822200.Bt

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.  


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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