Effects of nutrient and CO2 availability on tolerance to herbivory in Brassica rapa

The ability of plants to recover from herbivore damage and maintain their fitness depends on physiological mechanisms that are affected by the availability of resources such as carbon and soil nutrients. In this study, we explored the effects of increased carbon and nutrient availability on the response of rapid cycling Brassica rapa to damage by the generalist herbivore, Trichoplusia ni (Noctuidae), in a greenhouse experiment. Using fruit mass as an estimate of plant fitness, we tested three physiological models, which predict either an increase or a decrease of tolerance to herbivory with increasing resource availability. We used leaf demography to examine some plausible mechanisms through which resource availability may affect tolerance. Our results contradict all models, and, rather, they support a more complicated view of the plasticity of resource uptake and allocation than the ones considered by the models tested. Fruit mass was negatively affected by herbivore damage only under elevated CO₂, and only for certain harvest dates. Increased CO₂ had no effect on the number of leaf births, but it decreased leaf longevity and the total number of leaves on a plant. Nutrient addition increased the number of leaf births, leaf longevity and the total number of leaves on a plant. We conclude that a shortening of the life span of the plants, brought about by elevated CO_2, was responsible for a higher susceptibility of plants to herbivore damage under high CO₂ concentration.