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If the sun's light peaks in the green, why do plants prefer to reflect green light (giving them their green color)? And in particular why do they prefer to absorb red light and with that not efficiently utilizing the sun's radiation?
Question Date: 2004-01-07
Answer 1:

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 light.

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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