Effects of direct foliar uptake of gaseous nitrogen dioxide on plant-herbivore interactions

Background/Question/Methods Vegetation is a significant sink for atmospheric reactive N, and in N-limited ecosystems, foliar N uptake may represent an important pathway of N assimilation. Foliar uptake of gaseous reactive N may have extended ecosystem consequences by altering consumption rates and population dynamics of herbivorous insects, but the extent to which N deposition-induced changes in herbivory might lead to changes in plant and ecosystem C and N dynamics is currently unknown. We conducted an enriched 15N experiment in a custom-built hydroponics-fumigation system that manipulated the concentrations of nitrogen supplied to the leaves (as 15NO2-enriched air) or to the roots (as nitrate), in order to experimentally distinguish the relative impacts of leaf and root available N sources on plant-herbiviore interactions. Using tobacco and specialist tobacco hornworm herbivores as a model system, we examined the effects of variation in foliar N uptake on rates of herbivory and plant biochemistry, including levels of leaf protein and phytochemicals such as nicotine, protease inhibitors and phenolics. Our objective was to test the impact of foliar N uptake on herbivores and plants in order to understand the implications of increased reactive N deposition for trophic interactions and nutrient flow in terrestrial ecosystems. Results/Conclusions Total biomass accumulation was higher in plants exposed to high levels of root N supply (both NO3- and NH4+) regardless of NO2 exposure. NO2-derived nitrogen in leaf tissue varied between 8 and 16%, with foliar NO2 uptake highest in plants supplied with low levels of NO3-. Herbivore performance and leaf chemistry both varied as a function of the interactive effects of exposure to gaseous NO2 and root available NO3- concentrations. Results suggest that foliar NO2 uptake contributes a significant proportion of nutritive N to plant metabolism and may lead to altered rates of herbivory as a direct function of changes to leaf chemistry. Herbivory may thereby diminish the role of terrestrial vegetation as a sink for atmospheric N and could alter ecosystem fluxes of N. Our study contributes significant insights to our understanding of how producers and consumers will respond to increased reactive N inputs in terrestrial ecosystems.