Lack of a Tradeoff between Constitutive and Induced Defenses among Varieties of Cotton

Selecting crop plants for high yields has been associated with reduced levels of resistance against herbivores. This raises the concern that varieties selected for high levels of constitutive resistance may have reduced levels of induced defenses. Induced resistance has been hypothesized to evolve in plants when constitutive defenses are costly and not always beneficial. In this study we examined the relationship between constitutive resistance against spider mites and induced resistance against mites and verticillium wilt in ten varieties of cultivated cotton known to possess different levels of constitutive resistance. Levels of constitutive resistance against spider mites were estimated as the number of mites that were found on undamaged plants. Levels of induced resistance against mites were estimated by calculating the percent reduction in mite populations on plants that had been previously damaged relative to undamaged controls. Induced resistance against verticillium wilt was assayed by comparing the number of mitedamaged plants that recovered from exposure to the disease versus the number of undamaged control plants that recovered. We found no evidence for a negative correlation between levels of constitutive and induced resistance against mites. Similarly, we found no evidence of tradeoffs between constitutive resistance to mites and induced resistance against verticillium wilt. Selecting plants for high levels of constitutive defenses should not necessarily bring along with it the unwanted consequence of lowered levels of induced resistance.