Testing the costs of defense against herbivores in ecotypes of Eruca sativa (Brassicaceae)

Plants of Eruca sativa from a desert population produce less vegetative biomass, flower earlier and possess higher density of trichomes than plants from a more mesic Mediterranean habitat. When eliciting plant defense mechanisms with methyl-jasmonate (MJ), trichome formation was exclusively enhanced in plants of the Mediterranean ecotype. To test the relative costs of defenses, ecologically-important traits were monitored in non-elicited plants of the two ecotypes and when defenses were induced by MJ. Opposing the expected tradeoffs of induced defense, MJ- elicitation significantly delayed bolting and flowering in plants of the desert ecotype but not in the Mediterranean ones. Moreover, MJ did not impose costs on fitness- related traits, i.e. fruit and seed production, nor differences between plants of the two ecotypes were found. It is suggested that early flowering in plants of the desert ecotype provide a mean to escape both the abiotic stressful environment as well herbivory, while induced defense in the Mediterranean ecotype was favored in habitats of more stressful biotic interactions. The results are discussed in light of the optimal defense theory and resource availability hypothesis.