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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