Acquired immunity to herbivory and allelopathy caused by airborne plant emissions.

Numerous plant species respond to volatile cues to adjust their defenses against herbivores. Some volatile chemicals, such as terpenoids and green leaf volatiles, that are responsible for communication between plants and arthropods are also required for intraspecific communication between plants and for coordination among branches within a single plant. We are now aware that some 'receiver' plants are able to eavesdrop on their neighbors and tailor their defenses to their current and expected risks caused by herbivores. By contrast, a suite of volatiles also serve as natural herbicides (allelochemicals) that are detrimental for receiver plants. Since various molecular and ecological mechanisms underlying these phenomena have been clarified, it is time to ask whether more plants eavesdrop on infochemical cues, and if these cues that allow them to adjust their defenses to suit their risk also increase their fitness as a result.