Parasitic Weed Sniffs Out Prey, Study Says

Sean Markey
for National Geographic News

September 29, 2006

Scientists have now proved what gardeners suspected all along: Weeds really can sniff out tomato plants.

Seedlings from the parasitic plant called dodder track chemical aromas to locate their preferred host plants, a new study says.

"Sniffing" the air, the seedlings can even identify preferred tomato-plant hosts over less desirable ones such as wheat, researchers from Pennsylvania State University in University Park report in today's issue of the journal Science. (Related:"Weird Plants Taking Root in Everyday Gardens" [August 2003].)

"This is a pretty cool example of plants behaving in a way that people think only animals behave," ecologist Richard Karban told the journal. Karban, of the University of California, Davis, was not involved in the study.

Keen Sense of "Smell"

Dodders, or Cuscuta, are known by a variety of nicknames including strangleweed and witch's shoelaces. They rank among the U.S. Department of Agriculture's list of top ten weeds.

The pest costs tomato growers in California alone about four million dollars (U.S.) a year, experts say. It will attack carrots, onions, and alfalfa, among other crops (related photo: farmer weeding crops).

The morning glory relative has no real roots, and unlike most other plants, it cannot produce food through photosynthesis.

Instead, the spaghetti-like weed smothers its host plant, injecting syringe-like needles into stems and leaves to suck out water and nutrients.

"It's kind of like a leach," said study co-author Mark Mescher, a Penn State chemical ecologist.

Because dodder saplings have only a limited amount of food in their seeds, the saplings must quickly find a host plant or die.

Mescher, fellow Penn State chemical ecologist Consuelo de Moraes, and graduate student Justin Runyon suspected that the weeds found their marks using airborne chemical cues.

So the researchers placed seedlings in vials of water and found that 80 percent of the weeds grew toward a nearby tomato plant.

In a second test, dodder seedlings were put in an open-air chamber with two tunnels. One tunnel held four fake tomato plants, the other four genuine plants. Seventy-seven percent of the dotter seedlings grew toward the real plants.

To test whether the weeds were indeed "sniffing" plant aromas and not responding to shade or other light cues, the team then replaced the real and fake tomato plants with aromatic chemicals—one containing the scent of real tomato plants, the other an unrelated solvent.

Again, the dodder seedlings overwhelmingly zeroed in on the real-tomato smells.

The team also experimented with wheat plants to learn if dodder seedlings could choose between a desirable host and a less desirable one.

"We offered them tomato plants and wheat plants, and they overwhelmingly chose the tomato plants," de Moraes said.

The tests even revealed a possible practical application: An airborne chemical from wheat repels dodders and could help combat the parasite.

Chemical Language

Scientists have known for some time now that plants use a variety of airborne chemicals as lures or repellents to encourage or discourage a variety of animal behaviors.

"We showed a few years ago that plants produce different chemicals during the day and at night," study co-author Mescher said.

"The nighttime [chemicals] can actually be repellents to the moths that lay the eggs that become caterpillars," which in turn munch plant leaves.

Alternatively, some plants emit daytime chemicals that act like fast food ads to lure beneficial insects. The bugs help rid the plant of unwanted pests, such as leaf-munching caterpillars.

"There are a lot of complex interactions between insects and plants that are known to be mediated by these sorts of volatile, airborne chemical cues," Mescher said.

The latest study, he adds, is the first to show that these aromatic signals also can govern interactions between plants and other plants.

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