Hi, I’m Holly Strand.
As early as 4th grade, children learn that leaves are green because of the high concentration of chlorophyll relative to other pigments. And that autumn leaves turn color because the breakdown of chorophyll molecules unmasks the yellow, orange and red pigments that remain.
This simple and satisfying explanation has existed for awhile. However, research in the past 15 years has uncovered a much more complicated story. Leaf physiologists jumped from the question, “What makes autumn leaves so colorful?” And started to ask the evolutionary question: “What’s the purpose behind this autumn color fest?” “What’s in it for the tree itself?”
The group of pigments that produce yellow or orange are called carotenoids. The carotenoids are in the leaf throughout the growing season but only become apparent with the breakdown of chlorophyll into colorless metabolites. However, the red pigments—or anthocyanins– are produced in the leaf AFTER much of the chlorophyll is lost. This active production of new anthocyanins led scientists to believe that the pigment must be performing a critical function.
Just what function is still not known but many hypotheses exist. One is that anthocyanins act as a sunscreen shielding leaf tissues against the harmful effects of sunlight. The risk of sun damage is particularly high in fall because of the lack of chlorophyll protection and because there is increased light due to a thinning canopy. Without sun damage, the leaves can continue to absorb nutrients right up to the end.
Another possibility is that anthocyanins function as an antioxidant shield. Plant chloroplasts produce fewer free radicals when shielded from green wavelengths of light. Anthocyanins absorb the green wavelengths. Without anthocyanin, a surplus of superoxides could cause damage to the plant cell structure.
Lastly, there is the coevolution hypotheses. Aphids and other insects feed, carry viruses and bacteria. So red coloration was an adaptation developed by trees designed to lighten their insect load. The red leaves signal to insects that the tree is not a suitable host. The coevolution theory is supported in part because trees with autumn colors tend to be the ones with a history of interaction with aphids. Also, experiments have shown that most aphids strongly prefer green leaves over red leaves. In some cases anthocyanin may be positively correlated with a form of chemical defense.
However, when considering this last theory, remember that species perceive color differently. To the human eye, a tree may be a brilliant autumn shade of red. To the aphid, that same tree might be an indistinguishable gray. Further the insect may be avoiding red leaves because of differences in texture, taste, scent, texture or something else that is important to an aphid.
Whatever the reason is, I hope you are enjoying Utah’s colorful reds and yellows.
For links to current research and a Forest Service update on fall color around the state, go to www.wildaboututah.org.
For Wild About Utah, I’m Holly Strand.
Images: Courtesy and Copyright 2003 Michael Kuhns, Extension.usu.edu
Text: Holly Strand
Sources & Additional Reading
USDA Forest Service Fall Colors web site for the Intermountain Region https://www.fs.usda.gov/detail/r4/recreation/?cid=FSBDEV3_016189
Archetti, M., Doring, T. F., Hagen, S. B., Hughes, N. M., Leather, S.
R., Lee, D. W., Lev-Yadun, S., Manetas, Y., Ougham, H. J., Schaberg,
P. G., Thomas, Howard (2009). Unravelling the evolution of autumn
colours: an interdisciplinary approach. Trends in Ecology and
Evolution, 24, (3), 166-173. https://digitalcommons.fiu.edu/cas_bio/38/
Chittka L, Döring TF (2007) Are Autumn Foliage Colors Red Signals to
Aphids? PLoS Biol 5(8): 187. https://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0050187
Lee, David and Kevin S. Gould Why Leaves Turn Red. American
Scientist, Volume 90. 524-531 https://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/leaves/2002_11_leaf_article.pdf
Gunnell, JayDee, Reese, Julene, Ask a Specialist: What Causes the Fall Leaves to Change Color?, USU Cooperative Extension, https://extension.usu.edu/htm/news-multimedia/articleID=18662