healthmedicinet

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