Costs of induced responses and tolerance to herbivory in male and female fitness components of wild radish

Theory predicts that plant defensive traits are costly due to trade-offs be tween allocation to defense and growth and reproduction. Most previous stud ies of costs of plant defense focused on female fitness costs of constituti vely expressed defenses. Consideration of alternative plant strategies, suc h as induced defenses and tolerance to herbivory, and multiple types of cos ts, including allocation to male reproductive function, may increase our ab ility to detect costs of plant defense against herbivores. In this study we measured male and female reproductive costs associated with induced respon ses and tolerance to herbivory in annual wild radish plants (Raphanus rapha nistrum). We induced resistance in the plants by subjecting them to herbivo ry by Pieris rapae caterpillars. We also induced resistance in plants witho ut leaf tissue removal using a natural chemical elicitor, jasmonic acid; in addition, we removed leaf tissue without inducing plant responses using ma nual clipping. Induced responses included increased concentrations of indol e glucosinolates, which are putative defense compounds. Induced responses, in the absence of leaf tissue removal, reduced plant fitness when five fitn ess components were considered together; costs of induction were individual ly detected for time to first flower and number of pollen grains produced p er flower. In this system, induced responses appear to impose a cost, altho ugh this cost may not have been detected had we only quantified the traditi onally measured fitness components, growth and seed production. In the abse nce of induced responses, 50% leaf tissue removal, reduced plant fitness in three out of the five fitness components measured. Induced responses to he rbivory and leaf tissue removal had additive effects on plant fitness. Alth ough plant sibships varied greatly (49-136%) in their level of tolerance to herbivory, costs of tolerance were not detected, as we did not find a nega tive association between the ability to compensate for damage and plant fit ness in the absence of damage. We suggest that consideration of alternative plant defense strategies and multiple costs will result in a broader under standing of the evolutionary ecology of plant defense.