Costs of glucosinolates in <i>Brassica rapa</i>: Are they context dependent?

Abstract

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 temperature-dependent costs are especially insightful with regard to the evolution of defense because differing geographic populations are likely to experience differing temperature environments.