Abstract
Plant-emitted volatile organic compounds (VOCs) play important roles in plant-insect interactions. Atmospheric pollutants such as ozone (O3) can react with VOCs and affect the dynamics and fidelity of these interactions. However, the effects of atmospheric degradation of plant VOCs on plant-insect interactions remains understudied. We used a system comprising Brassica oleracea subsp. capitata (cabbage) and the specialist herbivore Plutella xylostella to test whether O3-triggered VOC degradation disturbs larval host orientation, and to investigate the underlying mechanisms. Larvae oriented towards both constitutive and larva-induced cabbage VOC blends, the latter being the more attractive. Such behaviour was, however, dramatically reduced in O3-polluted environments. Mechanistically, O3 rapidly degraded VOCs with the magnitude of degradation increasing with O3 levels. Furthermore, we used Teflon filters to collect VOCs and their reaction products, which were used as odour sources in behavioural tests. Larvae avoided filters exposed to O3-transformed VOCs and spent less time searching on them compared to filters exposed to original VOCs, which suggests that some degradation products may have repellent properties. Our study clearly demonstrates that oxidizing pollutants in the atmosphere can interfere with insect host location, and highlights the need to address their broader impacts when evaluating the ecological significance of VOC-mediated interactions.
Highlights
Volatile organic compounds (VOCs) emitted by plants are known to serve as important infochemicals in multitrophic interactions[1]
When offered a choice between volatile organic compounds (VOCs) emanating from undamaged and P. xylostella-infested plants, larvae showed a strong preference for the latter (Fig. 2b; P < 0.001). Both Principal Component Analysis (PCA) and independent t-tests revealed that the VOC blends emitted by infested plants differed significantly from those emitted by undamaged plants both qualitatively and quantitatively
Our results reveal that higher O3 concentrations, in excess of 50 ppb, break down several plant VOCs, alter the ratio of components in the blend, and eventually reduce the ability of herbivores to orient towards the VOC blend emitted from their host plants
Summary
Volatile organic compounds (VOCs) emitted by plants are known to serve as important infochemicals in multitrophic interactions[1]. Despite the numerous studies on plant VOCs as efficient mediators of interactions between plants and their surroundings, the implications of air pollution for these chemically mediated interactions are often ignored and have only recently begun to be more appreciated[6,7,8,9] Both theoretical and empirical studies have indicated that pollinator foraging distance can be dramatically reduced with increasing concentrations of atmospheric oxidants[3,10,11,12]. Some studies have shown that plants are capable of taking up and metabolizing oxygenated VOCs formed during the photochemical oxidation of certain monoterpenes[18,19,21] In these studies, several genes related to plant defence against oxidative stress were found to be up-regulated when plants were exposed to methyl vinyl ketone and macroclein[18,22], which accounts for more than 80% of reaction products of isoprene in the first phase of reaction in the atmosphere. We attempted to separate the effects of O3-initiated loss of putative bioactive volatiles from the effects of O3-initiated reaction products, SOA particles
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