Associational susceptibility in broccoli: mediated by plant volatiles, impeded by ozone.

Plant-emitted volatile organic compounds (VOCs) mediate interactions within a plant community. Typically, receiving a signal from a damaged neighbour enhances the defensive attributes of a receiver plant. The mechanisms underlying plant-plant interactions may be divided into active and passive processes, both of which involve transit of VOCs between plants and are vulnerable to environmental perturbation. Numerous studies have documented between-plant interactions, but the specific effects on a receiver plant's interactions with herbivores have received little attention. Moreover, the relative contributions of active and passive processes to plant defence and the effects of environmental pollutants on the processes have been largely unexplored. We used a system comprising Brassica oleracea var. italica (broccoli) and the specialist herbivore Plutella xylostella to test whether plants previously exposed to herbivore-damaged neighbours differed from nonexposed plants in their susceptibility to oviposition. We then investigated the roles of active and passive mechanisms in our observations and whether differences in susceptibility remained under elevated ozone concentrations. Plants exposed to herbivore-damaged neighbours were more susceptible to oviposition than plants exposed to undamaged neighbours, which indicates associational susceptibility. Mechanistically, active and passive volatile-mediated processes occurred in tandem with the passive process - involving adsorption of sesquiterpenes to receiver plants - appearing to structure the oviposition response. Exposure to ozone rapidly degraded the sesquiterpenes and eliminated the associational susceptibility. Plant volatiles have typically been thought to play roles in between-plant interactions and to promote receiver plant defence. Here, we show that receiver plants may also become more susceptible to oviposition and thus more likely to be damaged. Extensive disruption of volatile-mediated interactions by an atmospheric pollutant highlights the need to consider the pervading environment and changes therein when assessing their ecological significance.