Induced Resistance and Susceptibility to Herbivory: Plant Memory and Altered Plant Development

Systems of induced resistance for both vertebrates and invertebrates possess a memory; individuals that are attacked by a particular pathogen are able to respond more quickly and more effectively the second time they encounter that pathogen. Many plants have been found to be more induced by repeated herbivory than by a single bout of herbivory. In these experiments the frequency and magnitude of damage were confounded so that it was not clear if memory was involved of if more overall damage amplified the induced response. We conducted an experiment to test the memory of induced resistance in cotton to repeated attacks by spider mites. We held the overall amount of damage (herbivore—days) constant but varied the number of times that plants were exposed to damage. We did not find that plants damaged a second time responded more effectively than those damaged only once. Power analysis indicated that a large of memory was unlikely to have been overlooked in these experiments if it existed. Following observations that different types of damage caused different responses in mountain birch (Haukioja et al. 1990), we damaged either cotyledonary leaves or apical buds of young cotton plants and challenged them with spider mites. We found that damage to cotyledons induced resistance but damage to apical buds induced susceptibility. Damage to cotyledons accelerated the rate of abscission of cotyledons, a favored tissue for spider mites; damage to buds delayed abscission of cotyledons. Cotyledon abscission was sufficient to induce resistance but was not necessary condition since induced resistance has been observed when cotyledon abscission was not found. Our results were consistent with the hypothesis that damage to these two tissues differentially affected hormonal regulation as well as the alternative hypothesis that these forms of damage had opposite effects on plant ontogeny.