The suns energy emission varies by
wavelength. You are right that the sun
gives off the most amount of its energy as visible
light in the green region of the spectrum (483-520
nm). All plants on Earth, even the single-celled
plants that grow in the ocean, contain
chlorophyll-a as their main light-absorbing
pigment. Plants have other pigments for absorbing
light as well, including chlorophyll-b,
chlorophyll-c and pigments known as
carotenoids, but chlorophyll-a remains
the main light-absorbing pigment.
Chlorophyll-a absorbs light throughout the
visible spectrum, but mostly in the blue and red
regions and very little in the green region. In
fact, of all the many pigments that plants use to
absorb light, none of them absorb much green
To us it might seem inefficient that plants
don't take advantage of the one part of the
spectrum that the sun emits most of its energy in.
This is actually a form of protection.
Chlorophyll-a and other pigments are easily
destroyed by too much energy, and when the
pigments break down and stop absorbing light
entering the plant, that energy can cause damage
to other plant tissues as well, including the
plants' DNA. Think of it as a sort of plant
sunburn. Plants have elaborate mechanisms to
repair DNA that has been damaged by too much sun
energy, but these repair mechanisms are costly,
and require extra nutrients. A plant that is
stressed by too little nutrients or too little
water can actually die from excess sun exposure.
Plants have adapted to balance their need for
the suns energy with their need to protect
themselves from sun damage by using regions of the
spectrum that are not as abundant. In general,
light absorbed in the blue region is used for
plant growth and light absorbed in the red
and far red regions are used as cues for
flowering or orienting (that is, bending
leaves and stems toward or away from light,
growing tall to escape shading in a forest, etc).
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