MadSci Network: Botany

Re: Do different colors of light affect the growing rate/ability of plants?

Date: Wed Oct 3 00:15:25 2001
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1001462360.Bt

Plants can use all colors of light in photosynthesis although green and yellow 
light are used less efficiently, often around 60% relative to red and blue if 
red and blue are considered 100%. Plants also use light in several other ways. 
Blue light is the trigger for phototropism, an example of which is the bending 
of plant shoots tpward light in a window. The plant pigment phytochrome is 
triggered by red and far red light.  Phytochrome is involved in several plant 
processes such as etiolation (the extreme elongation of plant shoots when grown 
in the dark) and photoperiodism (daylength effects on plant flowering and 
dormancy). The study of light effects on plant shape is termed 
photomorphogenesis, and the Plant Photobiology website cited below has a 
detailed introduction on the subject.

If you grew plants without blue light, you would expect them not to exhibit 
phototropism. Similarly, plants grown with no red light may be etiolated even 
though they would have sufficient nonred light for adequate photosynthesis.

It is difficult for plant scientists, and basically impossible for students, to 
grow plants under different light colors and assure that the plants get the 
same amount of light from each color. Expensive equipment such as quantum 
sensors, light meters, and spectroradiometers are required to do this. If you 
do not have the same amount of light from each color, then you cannot tell if a 
reduction in plant growth with certain colors is caused by less light or by the 
color of light.

One type of fluorescent lamp, termed Daylight, is very similar to sunlight in 
its color composition. The common Cool White flourescent has less red and far 
red than sunlight which makes plants slightly shorter. Incandescent lamps have 
much more red and far red than sunlight. High pressure sodium lamps are high in 
yellow wavelengths. 

The Plant PhotoBiology website contains a lot of info including spectral output 
by various electric lamps. Plant light questions come up a lot on if you search 
the Madsci archives you can find many other answers on plant light.



Photoperiodism and Phytochrome

Plant PhotoBiology

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