The effects of soil nitrogen availability on the defensive chemistry of an invasive plant species, Linaria dalmatica

Soil nitrogen availability plays an important role in plant allocation to defensive chemistry.  According to popular theories in plant evolutionary ecology, such as the carbon-nutrient and growth-differentiation balance hypotheses, carbon-based defense compounds should decrease as nitrogen availability increases. Greenhouse and garden experiments were conducted to examine the effects of soil nitrogen enrichment on carbon-based anti-herbivore compounds found in an invasive plant species, Linaria dalamtica (Scrophulariaceae). In both experiments, plants were grown at one of four levels of nitrogen availability. Nitrogen was added as ammonium nitrate at levels that simulated observed rates of nitrogen deposition. In both experiments, increased nitrogen availability resulted in a significant decrease in iridoid glycoside concentrations. Nitrogen enrichment yielded an approximate 30% reduction in defense compounds for plants grown in the greenhouse experiment. Such changes in defensive chemistry may alter the ability of L. dalmatica plants to defend against herbivore attack. Further, this decrease in defense compounds may have important implications for a specialist herbivore (and biological control agent) of L. dalmatica, Calophasia lunula (Lepidoptera: Noctuidae), which has been shown to sequester iridoid glycosides for its own protection. Future research will address the effects of soil nitrogen enrichment on multi-trophic level interactions as mediated by changes in plant and herbivore chemistry.