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Last Updated: Aug 13, 2008 - 9:46:29 AM |
The spiciness is a defense mechanism that some peppers develop to
suppress a microbial fungus that invades through punctures made in the
outer skin by insects. The fungus, from a large genus called Fusarium,
destroys the plant's seeds before they can be eaten by birds and widely
distributed.
"For these wild chilies the biggest danger to the seed comes before
dispersal, when a large number are killed by this fungus," said Joshua
Tewksbury, a University of Washington assistant professor of biology.
"Both the fungus and the birds eat chilies, but the fungus never
disperses seeds – it just kills them."
Fruits use sugars and lipids to attract consumers such as birds that
will scatter the seeds. But insects and fungi enjoy sugars and lipids
too, and in tandem they can be fatal to a pepper's progeny.
However, the researchers found that the pungency, or heat, in hot
chilies acts as a unique defense mechanism. The pungency comes from
capsaicinoids, the same chemicals that protect them from fungal attack
by dramatically slowing microbial growth.
"Capsaicin doesn't stop the dispersal of seeds because birds don't
sense the pain and so they continue to eat peppers, but the fungus that
kills pepper seeds is quite sensitive to this chemical," said
Tewksbury, lead author of a paper documenting the research.
"Having such a specific defense, one that doesn't harm reproduction or
dispersal, is what makes chemistry so valuable to the plant, and I
think it is a great example of the power of natural selection."
The paper is published the week of Aug. 11 in the online Proceedings of
the National Academy of Sciences. Co-authors are Karen Reagan, Noelle
Machnicki, Tomás Carlo, and David Haak of the University of Washington;
Alejandra Lorena Calderón Peñaloza of Universidad Autonoma Gabriel Rene
Moreno in Bolivia; and Douglas Levey of the University of Florida. The
work was funded by the National Science Foundation and the National
Geographic Society.
The scientists collected chilies from seven different populations of
the same pepper species spread across 1,000 square miles in Bolivia. In
each population, they randomly selected peppers and counted scars on
the outer skin from insect foraging. The damage was caused by
hemipteran insects – insects such as seed bugs (similar to aphids and
leaf hoppers) that have sucking mouth parts arranged into a beaklike
structure that can pierce the skin of a fruit.
The researchers found that not all of the plants produce capsaicinoids,
so that in the same population fruit on one plant could be hotter than
a jalapeño while fruit from other plants might be as mild as a bell
pepper. But there was a much-higher frequency of pungent plants in
areas with larger populations of hemipteran insects that attack the
chilies and leave them more vulnerable to fungus.
The scientists also found that hot plants got even hotter, with higher
levels of capsaicinoids, in areas where fungal attacks were common. But
in areas with few insects and less danger of fungal attack, most of the
plants lacked heat entirely. In those areas, chilies from the plants
that did produce capsaicinoids had a lot less kick because they only
produced about half the capsaicinoids as the plants did in areas where
fungal attack was common.
Using chemical substances as a defense is not unique to peppers.
Tomatoes, for example, are loaded with substances that give their
unripened fruit a decidedly unpleasant taste, allowing the seeds a
chance to mature and be dispersed. But unlike peppers, tomatoes and
most other fruits lose their chemical defenses when the fruit ripens.
That is a necessary step, scientists believe, because otherwise the
fruit would not be consumed by birds and other animals that disperse
the seed. The problem with that strategy, Tewksbury said, is that it
leaves the fruit exposed to fungal attack.
"By contrast, peppers increase their chemical defense levels, or their
heat, as they ripen. This is a very different model and peppers can get
away with it because birds don't sense pain when they eat capsaicin,"
Tewksbury said. "I think a lot of plants would love to come up with
this way of stopping fungal growth without inhibiting dispersers. It's
just very hard to do."
The fact that chilies have capsaicin could be the reason humans started
eating the peppers in the first place, he said. Chili peppers and corn
are among the earliest domesticated crops in the New World.
"Before there was refrigeration, it was probably adaptive to eat
chilies, particularly in the tropics," Tewksbury said. "Back then, if
you lived in a warm and humid climate, eating could be downright
dangerous because virtually everything was packed with microbes, many
of them harmful. People probably added chilies to their stews because
spicy stews were less likely to kill them."
All chilies originated in South America, and wild chilies now grow from
central South America to the southwestern United States. Explorers
carried the plants back to Europe, but they were not widely used there.
>From Europe, chilies made their way to Asia and Africa, where they have
become a common ingredient in nearly every tropical cuisine.
"In the north, any adaptive benefit to using eating chilies would be
much smaller than at the equator because microbial infection of food is
less common and it's easier to keep food cold. Maybe that's why food in
the north can be so boring," Tewksbury said.
"Along the equator, without access to refrigeration, you could be dead
pretty quickly unless you can find a way to protect yourself against
the microbes you ingest every day.
Source:Ocnus.net 2008
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