People and animals are not the only ones who can smell. Plants are also able to perceive odors（气味） , but they process them in a very different way . While insects or mammals smell odors within a second of exposure, plants require much longer exposure times to respond. Then using this information, they react effectively to the odors emitted by infected or infested¹ neighboring plants in order to increase their resistance to pest insects or disease. The new study by Sarai Giron-Calva, from the Department of Genetic² Engineering at CINVESTAV-Irapuato in Mexico, and colleagues is published online in Springer's Journal of Chemical Ecology.  Odors emitted by stressed plants can induce（诱导） resistance to disease in healthy neighbors. While this so-called 'talking trees' phenomenon was identified more than 20 years ago, the mechanism³ by which plants smell and process odors remains⁴ unclear.

To establish both the dose of odors and length of exposure which are required to induce resistance against bacterial⁵ infection in plants, the authors exposed lima bean plants to different concentrations of two odor compounds -- nonanal and methyl salicylate (MeSA). Both compounds are known to enhance resistance to bacterial disease in lima beans. They then measured the plants' resistance to the pathogen, after both 6 and 24 hours.

In the case of nonanal, plant resistance was significantly increased after both 6 and 24 hours. Furthermore, there was no difference in induced resistance between the two concentrations of the odor. In contrast, the authors found that the lima bean plants mounted a high level of resistance to bacterial infection after exposure to MeSA over an entire day, but not after short exposure of 6 hours. After 24 hours of exposure, the low concentration of MeSA was as effective at inducing resistance to the pathogen（病原体） as the higher concentrations.

The researchers conclude: "In contrast to insect or mammals which can respond to odors within a second of exposure, the 'nose' of plants works in a fundamentally different way to perceive odors. Plant to plant signaling in this instance is likely to involve the accumulation of odors in the receiving plant over a long period of time."