Costs of glucosinolates in Brassica rapa: Are they context dependent?

Models predicting optimal levels of plant defense against herbivores typically include two assumptions: 1) defense is both beneficial and costly; and 2) the relationship between costs and benefits of a defense is consistent across environments. However, the expression of costs and benefits of defense may be environmentally dependent. We examined lines of Brassica rapa, previously divergently selected for the defensive trait foliar glucosinolate content. In one set of experiments (Experiment #1), plants were grown in herbivore-free and herbivore-present environments to investigate the costs and benefits of this defense. In a second set of experiments (Experiment #2), plants were grown at two nutrient levels and two temperatures to examine the environmental context of costs of defense. In Experiment #1, increased levels of damage resulted in decreased flower production and plants from high glucosinolate lines received less damage than those from low glucosinolate lines, suggesting a benefit of this defense. In this experiment no cost of defense was detected. In Experiment #2, nutrients had a significant positive effect on flower production at 23 ̊C, but not at 32 ̊C. No significant effects of glucosinolate line nor interaction between nutrient environment and glucosinolate line were detected at 23 ̊C, suggesting that no cost of defense occurred at this lower temperature. Similarly, no significant nutrient environment by glucosenolate line interaction was detected at 32 ̊C. However, a significant effect of glucosinolate line was observed suggesting that at 32 ̊C costs were incurred, but nutrient environment had no mitigating effect. While results from Experiment #1 suggested that defense was beneficial, but not costly, results from Experiment #2 suggested that costs of defense were temperature dependent. For species occupying broad geographic ranges, these findings of temperaturedependent costs are especially insightful with regard to the evolution of defense because differing geographic populations are likely to experience differing temperature environments.