Date: Summer 2012
I am an educator in the Nature Research Center of the North Carolina Museum of Natural Sciences. We deal with ages from toddlers to adults though our focus is often on middle through high school. 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.
As you noted the plant pigment chlorophyll best absorbs red and blue wavelengths of light but reflects green wavelengths of light, which we see. Chlorophyll uses the red and blue wavelengths of light as the best sources of light for optimizing the process of photosynthesis.
Anthony R. Brach, Ph.D.
Harvard University Herbaria c/o Missouri Botanical Garden
Many years ago this same question came up at a science teacher convention I attended. Dr. Brach's explanation above was the central theme and some of this is explained with his web resources, however, some other aspects emerged. First, black would further prevent the light from passing through the entire leaf and this would have an effect on the stomate operation on the underside of many plant species whic requires light in most cases. The second idea was that the increased heat produced by a black leaf in sunlight may dry the leaf out faster than transpiration could replace the water in the leaf which would effect photosynthesis and the leaf's structures and function. The third issue discussed was that the enzymes and the complex chemical interactions involved with photosynthesis do have a heat tolerance level and thus potentially causing a breakdown in the photosynthetic process.
A key idea to realize is that the billions of years of evolution has developed plants and leaves to function at the optimal levels. If black leaves were the best, we would see them.
Click here to return to the Botany Archives
Update: November 2011