The resource allocation hypothesis is based on the assumption that defenses are costly, but relatively few studies have quantified the reproductive price of induced defenses, which represent the best means of measuring such costs in isolation from the genotypic costs that confound research involving constitutive defenses. Jasmonic acid (JA) is a plant signal molecule involved in the defensive responses of plants. It induces many of the same chemicals that are associated with herbivore damage, and thus offers a means of inducing plants without the removal of leaf area, which incurs its own costs. In tomato plants, JA induced resistance to Manduca sexta and increased levels of two defensive enzymes, polyphenol oxidase and peroxidase. We measured the impact of JA-induced defenses in tomato, Lycopersicon esculentum (Solanaceae), on several variables associated with reproductive success: fruit number, fruit weight, ripening time, time of fruit-set, number of seeds per fruit, total seeds per plant, the relationship between fruit weight and seed number, and germination success. Plants were grown in a pest-free greenhouse and treated biweekly with solvent or with JA at either of two concentrations: 10 mM or 1 mM. The high concentration of JA led to fewer but larger fruits, longer ripening time, delayed fruit-set, fewer seeds per plant, and fewer seeds per unit of fruit weight. The reproductive impact of induction was reduced at the lower dose, but still significant; 1 mM JA resulted in delayed fruit-set and fewer seeds per unit of fruit weight, compared to control plants. Our research indicates that JA-induced defenses impose significant costs on tomato plants.

The emerald ash borer (Agrilus planipennis, EAB) is an invasive wood-borer indigenous to Asia and is responsible for widespread ash (Fraxinus spp.) mortality in the U.S. and Canada. Resistance and susceptibility to EAB varies among Fraxinus spp., which is a result of their co-evolutionary history with the pest. We characterized constitutive phenolic profiles and lignin levels in the phloem of green, white, black, blue, European, and Manchurian ash. Phloem was sampled twice during the growing season, coinciding with phenology of early and late instar EAB. We identified 66 metabolites that displayed a pattern of variation, which corresponded strongly with phylogeny. Previously identified lignans and lignan derivatives were confirmed to be unique to Manchurian ash, and may contribute to its high level of resistance to EAB. Other compounds that had been considered unique to Manchurian ash, including hydroxycoumarins and the phenylethanoids calceolarioside A and B, were detected in closely related, but susceptible species, and thus are unlikely to contribute to EAB resistance of Manchurian ash. The distinct phenolic profile of blue ash may contribute to its relatively high resistance to EAB.

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.

The activation of the phenylpropanoid pathway in plants by environmental stimuli is one of the most universal biochemical stress responses known. Induction of enzymes such as phenylalanine ammonia-lyase and peroxidase and the accumulation of such phenolics as lignin can occur in response to insect and pathogen attack, exposure to oxidizing pollutants, and mechanical stimulation, and are thought to function in the resistance of plants to damage by these stresses. I investigated whether induction of components of this generalized stress response by wind-induced mechanical stimulation could influence the resistance to pests of common bean. In greenhouse studies, exposure of 7- to 10-day-old bean seedlings to daily periods of fan-produced wind led to increased activities of peroxidase and cinnamyl alcohol-dehydrogenase and enhanced the accumulation of lignin in primary leaves of these plants. Egg production and population growth of two-spotted spider mites were reduced when offered leaves of mechanically-stimulated plants in leaf-disk and whole-plant bioassays. Infection by anthracnose after inoculation in a detached-leaf bioassay was also reduced in leaves of mechanically-stimulated plants. The consistent positive association between the enhanced activity of the lignin branch of the phenylpropanoid pathway and enhanced resistance to pests found in leaves of mechanically-stimulated plants illustrates one way in which exposure of plants to environmental stimuli that activate a generalized stress response (e.g., wind) can influence the interactions of those plants with other environmental stimuli (e.g., pests).

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to International Journal of Plant Sciences.To examine potential costs of mechanical hardening in plants, dose-dependent effects of mechanical perturbation on the growth and development of Brassica napus were investigated in a greenhouse experiment. Mechanical perturbation (stem and leaf flexure) was imposed by exposing seedlings to 0, 10, 20, 30, or 40 flexures with a bamboo rod once per day over a period of 35 d. In general, mechanical perturbation reduced plant height and root mass but increased stem diameter while decreasing pith diameter in a nearly linear dosedependent manner. In addition, mechanical perturbation delayed both cotyledon senescence and date of anthesis and also reduced flower number in a similar linear dose-dependent manner. In contrast, effects of mechanical perturbation on final leaf area, leaf mass, and stem mass were not linearly related to dose, although means of these variables were smaller in plants subjected to 40 flexures per day than in all other treatments. Dosedependent effects of mechanical perturbation on B. napus were most noticeable as plastic changes in growth form, allometry, and development; i.e., mechanical perturbation produced increasingly shorter, radially thickened, and more compact and stronger plants with increasing dose. However, dose-dependent delays in anthesis and reductions in flower number indicated that fitness of these plants in the field could be increasingly reduced by increasing exposure to mechanical perturbation, unless benefits gained through the development of a mechanically hardened phenotype counterbalanced costs to flower production.