Wednesday, September 17, 2014

Autumn Leaves: More Than Just Pretty Colors

This post originally appeared on the Telegram and Gazette 9/2012

The green tree leaves of summer are already starting to give way to the bright yellows and reds of autumn. We should have a brilliant display of colors throughout the fall.

As you may remember from your high school science class, what gives green leaves (and green plants in general) their color is a compound called chlorophyll, which absorbs light energy from the sun. As the summer comes to an end, the change in temperature and change in the amount of daylight trigger processes in the leaves that cause chlorophyll to break down. As this inducer of green color disappears, the color effects of other compounds, carotenoids and anthocyanins specifically, are unmasked. Carotenoids are the compounds that give carrots their orange color and bananas their yellow color. Anthocyanins can give plants bluish, purplish, or reddish tints. Red cabbage, cranberries, and red raspberries are just a few examples of produce that have high levels of anthocyanins.

For the most part, scientists thought that the changing of leaf color in autumn was simply an effect of the disappearance of chlorophyll and signaled that the leaves were about to fall.  Over the past several years, however, researchers have found that the appearance of yellow, orange, and red leaves may have additional ecological impacts.

In 2005, researchers Martin Schaefer and Gregor Rolshausen proposed that the changing leaf color actually acts as a defensive signal against consumption by herbivores (plant-eating organisms).  The "Defense Indication hypothesis," as they termed it, is based on their own work as well as on observations that support their ideas, but were made by other researchers.  Their hypothesis (or, idea that will be tested through experiments and observations) is based on the fact that the signaling pathway that causes the production of anthocyanins also causes the production of defensive compounds to which herbivores have an aversion. After enough time, it is thought that herbivores learn to associate the defensive compounds with the colored leaves and avoid them altogether.

The very hungry caterpillar ate lots of stuff, but not orange, red, or yellow leaves.
Further, the biochemical pathways that cause chlorophyll to break down become active along with pathways that cause the production of compounds called anti-feedants, which make the leaves difficult to digest. If herbivores repeatedly consume autumn-colored leaves and then become sick due to the anti-feedants, they learn to associate the red, yellow, and orange colors with a negative eating experience and avoid those colored leaves in the future.

While the primary cause of autumn leaf colors is the loss of chlorophyll, this paper discusses just one example of how the color change has a significant impact on other organisms. Like so many things in nature, one change often has the potential to ripple through the environment and bring about widespread ecological effects.

Contributed by:  Kelly Hallstrom
Visit Kelly’s blog, You Don’t Have To Be A Rocket Scientist
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Schaefer HM, & Rolshausen G (2006). Plants on red alert: do insects pay attention? BioEssays : news and reviews in molecular, cellular and developmental biology, 28 (1), 65-71 PMID: 16369938

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