Plants are known to mount a defensive response when exposed to volatile chemicals from other plants, but the critical concentration required for this response is not known. We showed that intermittent exposure over a period of 3 weeks to trace amounts (less than 140 pptV) of green leaf volatiles emitted by a freshly damaged Arabidopsis plant induced physiological (defensive) responses in undamaged neighbouring plants. These results demonstrated that plants can respond to long-term repeated exposures to subcritical amounts of chemical signals.

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