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Tomato (Solanum lycopersicum) is the most cultivated vegetable crop worldwide, requiring high input of water and nitrogen. In dry regions like the Mediterranean, fresh water resources are declining due to climate change and human demand. Nitrogen leaching poses risks for our health as well as ecosystems due to eutrophication. Hence, resource input into agricultural systems has to be optimized but crop production has to stay feasible. In plants, nitrogen and water are involved in basic physiological processes and at the same time linked to defense compound production. Additionally, studies indicate that they may alter the blend of volatile organic compounds (VOC) emitted by plants. When attacked, plants emit a specific blend of herbivore-induced plant volatiles (HIPV) which can be perceived by natural enemies of the respective herbivores and facilitate prey/host detection. This indirect defense mechanism is important for biological pest control. It is known that constitutively emitted VOC blends can be affected by abiotic factors. If HIPV blends are likewise impacted, pest control through natural enemies could be less efficient (Becker et al., 2015).

Using solid-phase micro extraction (SPME) fibers we have collected volatile compounds from tomato plants (Solanum lycopersicum) receiving moderately reduced water and/or nitrogen, before and during infestation. HIPV were collected from plants infested with one of two sap-feeding (Macrosiphum euphorbiae and Bemisia tabaci) or of two leaf-chewing herbivore species (Tuta absoluta and Spodoptera littoralis) with different dietary ranges: M. euphorbiae and T. absoluta are specialists while B. tabaci and S. littoralis are generalists. Compounds were analyzed via GC-MS, chromatograms evaluated via XCMS online and results analyzed via MetaboAnalyst 3.0.

In this comparative, untargeted, multivariate analysis, we detected 54 compounds. Of these, 4 hydrocarbon monoterpenes, one oxygenated monoterpene and one hydrocarbon sesquiterpene that contributed to smaller proportions to the HIPV than to the constitutive VOC-blends. Additionally, 7 compounds contributed to higher proportions to the HIPV-blend associated with one of the herbivores. The different nutrient solutions did affect the emitted blends, especially regarding constitutive VOC and sap-feeder HIPV-blends. These results illustrate a possible bottom-up effect of plant nutrient solution on indirect plant defense, especially against sap-feeders. HIPV blends associated to leaf-chewers appear to be less susceptible to changes due to abiotic factors. Indirect defense against leaf-chewers might be under higher selective pressure for resilience due to their higher potential to quickly cause massive damage to the plant compared to sap-feeders.

• Becker, C., Desneux, N., Monticelli, L., Fernandez, X., Michel, T., & Lavoir, A.-V. (2015). Effects of Abiotic Factors on HIPV-Mediated Interactions between Plants and Parasitoids. Review Article. BioMed Research International, 2015, 1–18. https://doi.org/10.1155/2015/342982