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.

Differences in growth and competition between plants of a naturalized and an invasive population of Bunias orientalis.

The global shift of species' distributions has led to high numbers of noninvasive naturalized plants and the accumulation of invasive species within ecosystems. Competition between species may influence population dynamics, but little is known about the impacts of competition between conspecifics of naturalized and invasive populations. We investigated several plant traits at initial growth and regrowth following artificial defoliation in intra and interpopulation competition. Therefore, we used plants of Bunias orientalis from one noninvasive naturalized and one invasive population grown alone or in competition of two or three. Plants from the naturalized population were expected to be less competitive than plants from the invasive population, reflecting their differential impact in the introduced range. Independent of status, intrapopulation competition was expected to have less negative impacts on plants than interpopulation competition. Our results show that competition impacted mostly growth- rather than physiology-related traits. The relative magnitude of intra and interpopulation competition differed among plant traits at the first and second harvest. Plants of the invasive population outperformed the naturalized population by allocating relatively more resources to the aboveground biomass and producing more and longer leaves particularly when grown in competition against two plants. Moreover, plants of the invasive population were more competitive, which may influence their successful establishment and range expansion in the introduced range, but growth patterns differed after artificial defoliation. Although evolution of intrapopulation competition in naturalized and invasive ranges may be expected, interpopulation competition seems to adversely impact the performance of the naturalized plant population of B. orientalis studied here. Apart from the status (naturalized vs. invasive), other factors may have had an influence on plant performance. Thus, further research is needed with more naturalized and invasive populations to test the generality of our findings and to isolate the specific mechanisms driving differences in competitiveness.