Chemical ecologists in the college have found that the language of chemistry among plants goes both ways. As well as producing volatiles as signals, plants have the ability to "listen." In recent studies, Tumlinson discovered that when corn seedlings were exposed to volatiles from neighboring plants that were damaged by insects, the corn seedlings primed themselves for a possible attack. "You know that leafy, green odor you get when you crush a leaf in your hand?" he says. "We found in our lab that healthy corn seedlings next to plants being damaged by caterpillars can detect those leafy odors and somehow--we don't yet know how--prime their defenses." When the corn plants themselves were attacked by caterpillars, they mounted a stronger defense, releasing twice as many compounds as the corn plants that had not been exposed to these odors.

In other studies of "listening" plants, researchers found that plants produce chemical defenses a full day earlier when they had been "warned" chemically by neighboring plants under stress. "We're looking at the genes that are turned on to create that response and how we could turn the response into a visual cue such as a color change," says Appel. "And this brings us back to the idea of sentinel plants. There is widespread concern about genetically modified crops--but if you interspersed listening plants, or sentinel plants, throughout the field, and if they changed color in response to the neighboring plants' problems, you could exclude the sentinel plants from the food chain by not harvesting them. Then you'd have a true sentinel telling you the other plants are having a problem."

Another entomologist, Consuelo De Moraes, showed that plants produce different signals in response to the different caterpillar species that feed on them and thus attract the appropriate natural enemies. "Plants are not just saying, 'Yes, I am damaged,' they are providing specific information about who is damaging them. It is a really intricate system," says De Moraes.

While most plant studies in the lab and in the field are conducted during the day, De Moraes and her colleagues at Penn State also have made an important nighttime discovery: Tobacco plants release different volatiles at night than they do in daylight. "We actually thought volatiles were released only during the day," says De Moraes. "So this discovery of nocturnal odors came as a surprise." De Moraes and research associate Mark Mescher determined that tobacco plants being attacked by caterpillars release chemicals at night that repel night-flying moths searching for places to lay their eggs. The blend of odors released at night deters female moths from laying eggs on those plants.

"The plant sends out a chemical signal to the moth to stay away," says Mescher. "These odors are the plant's way of defending itself at night." For a female moth, identifying a plant already crawling with caterpillars has several advantages. She can help her off spring avoid competition from other caterpillars and steer clear of besieged plants that will have already mobilized chemical defenses, making them less palatable and attracting natural enemies of the caterpillars.

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