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

Abstract

Models predicting optimal levels of plant de-fense 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 envi-ronments to investigate the costs and benefits of this defense. In a second set of experiments (Experiment #2), plants were grown at two nu-trient levels and two temperatures to examine the environmental context of costs of defense. In Experiment #1, increased levels of damage re-sulted 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 glucosi-nolate line nor interaction between nutrient en-vironment and glucosinolate line were detected at 23˚C, suggesting that no cost of defense oc-curred at this lower temperature. Similarly, no significant nutrient environment by glucose-nolate 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 sug-gested that costs of defense were temperature dependent. For species occupying broad geo-graphic ranges, these findings of temperature-dependent costs are especially insightful with regard to the evolution of defense because dif-fering geographic populations are likely to ex-perience differing temperature environments.