Does competition magnify the fitness costs of induced responses in Arabidopsis thaliana? A manipulative approach.

Studies have increasingly shown that the constitutive or induced expression of resistance in plants is costly to fitness in the absence of enemy attack. If such costs are based on resource allocation tradeoffs, it has been hypothesized that resource limitation associated with plant competition increases the fitness costs of resistance. In two greenhouse studies, I examined the expression and costs of induced responses in pot-grown Arabidopsis thaliana grown alone or surrounded by six intraspecific neighbors. In the first study, I manipulated the expression of systemic acquired resistance (SAR) with the application of salicylic acid and monitored peroxidase activity and total seed production in treated plants. I used five lines of A. thaliana that varied in their competence to express SAR in this study. In the second study, I manipulated the expression of induced resistance (IR) with the application of jasmonic acid and monitored trypsin inhibitor activity and total seed production in treated plants. I used two lines of A. thaliana that varied in the competence to express IR in this study. Lines varied in their chemical and fitness responses to hormone treatment and competition in each study, but the application of wound-related hormones significantly increased levels of chemical defenses and reduced total seed production by an average of 15% overall in both studies. Competition reduced peroxidase activity by 6% in the first study, had no effect on trypsin inhibitor levels in the second study, and reduced total seed production by an average of 35% overall in both studies. However, there was no interaction between competition and hormone treatment on total seed production in either study. Induced responses were costly to fitness in A. thaliana, but competition had no effect on the magnitude of costs.