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iLabs: Black Plants, Part I — One Answer to the Question I Couldn’t Answer

August 5, 2012

In the  previous post I mentioned I couldn’t answer a visitor question AND I could find no useful answers anywhere on Google. So I went to NEWTON and got one answer. Here’s the story:

The question:

If black absorbs all color wavelengths and thus their energy, and plants need that energy,  why aren’t plants black instead of green?

My query to NEWTON:

I was recently asked by a visitor why plants are not black instead of green. We both understood WHY plants are green in color, ie absorbing blue and red wavelengths and reflecting green. But the idea was, if absorbing some energy is good, why not absorb it all and have the plants be black? Then they would get more energy and possibly be even more efficient.

NEWTON’S answer, via Jim Tokuhisa, Ph.D., Assistant Professor of Horticulture, Virginia Tech

Dear Deb,

You and your visitor posed a wonderful question because it is quite logical.  The limitation that prevents black plants is in the nature of the conversion of radiant light energy into chemical energy and thinking about light energy as both a wavelength and as a discrete packet of energy.  The energy of light (remembering that this energy is different from heat) is “harvested” by capturing it in the movement of electrons in chlorophyll and other pigments and transferring the energy to other molecules.  These pigments are designed to absorb specific wavelengths (or specific packets or quanta) of light energy which are the blue and red wavelengths.  The molecules that take the energy from the pigments are designed to accept specific sizes or “packets” of energy.  To capture all light energy would require a variety of pigments that are each designed to absorb a specific wavelength or quantum of light energy and be able to transfer it to other acceptor molecules that also are capable of accepting specific packets of energy.  So black plants would need to have many different pigment molecules and different acceptor molecules to absorb all of the light energy.  An analogy would be the different power requirements for electrical equipment.  Our electrified life would be much simpler if we could buy just AA batteries or plug all of our electronic toys into a single power supply.  Plants have a simple power supply and have developed ways to make certain that the power supply works and is not easily overloaded and damaged by to much or to little light.

Jim Tokuhisa, Ph.D.
Assistant Professor of Horticulture
Virginia Tech

In my next post, more about NEWTON…..and then, Black Plants, Part II — NASA’s Take on Extraterrestrial Plant Colors

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