Responses of Bunias orientalis to Short-term Fungal Infection and Insect Herbivory are Independent of Nutrient Supply.

Plants have to allocate their resources in both growth and defense under different environmental challenges. Several plant species have become invasive particularly in disturbed fertile habitats, which may influence their resource allocation. We studied the effects of nitrate fertilization (low versus high) on various plant responses towards a pathogenic fungus, Alternaria brassicae, and a herbivorous insect species, Mamestra brassicae, in a population of Bunias orientalis, which is invasive in parts of central Europe. Aboveground biomass and leaf trichome density were enhanced in plants under high fertilization. In contrast, the short-term fungal infection and herbivory had no effect on aboveground biomass. Leaf water, nitrogen content and glucosinolate concentrations were neither affected by fertilization nor in response to antagonist attack. The total soluble sugar content, especially fructose, as well as leaf peroxidase activity increased significantly in leaves upon fungal infection, but independent of fertilization. Larval biomass gain and herbivore survival were likewise unaffected by fertilization. Our findings highlight that under conditions of high fertilization, B. orientalis plants allocate more resources into growth and morphological defenses than chemical defenses. In contrast, induced responses to short-term antagonist attack seem independent of nitrate availability in this population.