A phenotypic trade-off between constitutive defenses and induced responses in wheat seedlings

Abstract

In plant defense theory, a trade-off between constitutive and induced defenses in plants is expected on the basis of cost-saving arguments. Thus, assuming that investment in defense is costly to the plant in terms of reduced growth and/or reproduction, induced responses would be a less expensive alternative when constitutive defenses are low. Conversely, plants with high levels of constitutive defenses would gain little from inducible defense systems. Despite this rationale, such a trade-off has rarely been detected. The present study reports the occurrence of a phenotypic trade-off between constitutive and aphid-induced levels of secondary metabolites in wheat seedlings. The trade-off was detected after examining the variation in induced responses as a function of the variation in constitutive defenses generated by plant ontogeny (8 ontogenetic stages, from 5 to 26 days after germination) and the environment (different regimes of temperature, light, and nutrient availability). Plant growth rate and the rate of change of constitutive levels of defense were identified as correlated traits potentially mediating the negative association between induced responses and constitutive defenses. The possible generalization of this result is discussed. Results are also discussed invoking efficacy issues as well as a hypothesis that variability in plant quality may function per se as a resistant factor against herbivores. It is suggested that both induced responses and increased rates of change of constitutive defenses may provide such beneficial variability.