Effects of ozone and ammonium sulfate on cauliflower: Emphasis on the interaction between plants and insect herbivores

Abstract Ammonium sulfate [(NH 4 ) 2 SO 4 ] deposition and elevated ozone (O 3 ) concentrations may negatively affect plants and trophic interactions. This study aimed to evaluate for the first time the interactive effects of high (NH 4 ) 2 SO 4 load and elevated O 3 levels on cauliflower ( Brassica oleracea L.) under field conditions. Cauliflower seedlings were treated with 0 (AS0) or 50 (AS50) kg ha −1 (NH 4 ) 2 SO 4 and exposed to ambient (AOZ, ≈20 ppb) or elevated (EOZ, ≈55 ppb) O 3 for about one month, in a Free Air O 3 Concentration Enrichment (FACE) system. The oligophagous diamondback moth ( Plutella xylostella Linnaeus, 1758) showed a clear preference towards the seedlings treated with AS50, which intensively grazed. Plant-herbivore interactions were driven by (NH 4 ) 2 SO 4 availability, rather than O 3 , via increased nitrogen content in the leaves. Further laboratory bioassays were followed to confirm the validity of these observations using polyphagous Eri silkmoth larvae ( Samia ricini ) as a biological model in a standardized experimental setup. Choice assays, where larvae could select leaves among leaf samples from the different experimental conditions, and no-choice assays, where larvae could graze leaves from just one experimental condition, were conducted. In the choice assay, the larvae preferred AS50-treated leaves, in agreement with the field observations with diamondback moth. In the no-choice assay, larval body mass growth was inhibited when fed with leaves treated with EOZ and/or AS50. Larvae fed with AS50-treated leaves displayed increased mortality. These observations coincide with higher NO 3 and Zn content in AS50-treated leaves. This study shows that plant-herbivore interactions can be driven by (NH 4 ) 2 SO 4 availability, independently of O 3 , and suggests that high N deposition may have severe health implications in animals consuming such plant tissues. Key message : Plant-herbivore interactions are driven by high (NH 4 ) 2 SO 4 availability, independently of O 3 .