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An electronic nose that smells plants' pain

Device can detect distress signals from plants that are harmed, under attack

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2:13pm, October 17, 2008
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It turns out the best way to hear a plant scream is to smell it.

Scientists are using an electronic nose tailored to eavesdrop on plants that have been damaged or are under attack. The nose successfully discriminated among the various distress signals different plants emit, depending on the pests plaguing them — discerning, for example, a tobacco hornworm attack from assault by powdery mildew.

The electronic nose, or e-nose, could be a valuable tool for monitoring pests in commercial greenhouses, allowing for better targeted pest control, researchers report in an upcoming Environmental Science & Technology. A tailored e-nose could “nip the problem in the bud,” says Nigel Paul, of Lancaster University in England, who led the research.

Currently, someone skilled at identifying pests must do a walk through of the greenhouse and hope to visually detect a disease or attacker, Paul adds.

Previous research trained e-noses — which are small, portable machines — on things like spoiled milk or library paper contaminated with fungal growth. This e-nose is the first designed for sniffing out the volatile, or easily evaporated, compounds plants make.

Traditionally, these analyses are done with gas chromatography, which is expensive, time-consuming and requires bulky devices and skilled analysts. The technique with an e-nose is a nice, quick and dirty test that, once trained, can recognize the specific signature of a plant’s response to a pest.

Led by Paul, the researchers tweaked a commercial e-nose so that it could detect the volatiles emitted from cucumber, pepper and tomato plants. The technology uses an array of polymer sensors that interact with the volatile compounds collected from around a damaged leaf still on the plant. The interaction of the volatiles and the sensors causes a change in electrical resistance that is analyzed using specific software.

The 13-sensor e-nose distinguished between the volatiles emitted by cucumber leaves damaged by a hole punch (to mimic pruning or clipping) versus those damaged by spider mites. With tomato leaves, the nose could tell hornworm damage from powdery mildew from hole-punched leaves.

There was some overlap in the compounds emitted from the mechanically wounded leaves and pest-wounded plants, but fine-tuning is the next step, Paul says.

“This is a very good study,” comments Natalia Dudareva of Purdue University in West Lafayette, Ind. A biochemist and molecular biologist, Dudareva says the nose would be useful in her experiments, giving her a quick indication of the presence or absence of particular plant compounds. With more research and refinement, the e-nose could be tuned to identify a variety of volatile bouquets, such as perfectly ripe fruits or designer floral scents, Dudareva says.

Scientists have described about 1,700 volatile compounds made by different plants, a clever communication strategy for organisms that can’t move and don’t have mouths. The flirty coo emitted by a flower that’s trying to attract pollinators may be a perfume of 100 of these compounds. Plants also emit volatiles when stressed, including several rallying cries. Some of the cries stimulate production of defensive compounds in nearby plants, and others may attract enemies of the pest that is attacking the plant.

“They can function in different ways, they can intoxicate or repel,” says Dudareva.

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