Cotton plants defend themselves against feeding injury from the beet armyworm, Spodoptera exigua, by direct and indirect (incurred through natural enemies of the herbivore) resistance, both of which can be systemic and inducible. Numerous studies havedemonstrated that plant signals are transmitted upward in the cotton plant, but no attempts have been made to examine the potential that the systemic response is transported downward in cotton, although such movement has been demonstrated in the lima bean. Therefore, we examined the direction of systemically induced resistance in cotton using the beet armyworm as eliciting herbivore. In addition, we evaluated the roles that herbivore density and duration of herbivory play in the process of induction and the systemic response in cotton plants. Plants should be favored to respond differentially to variations in the magnitude and duration of defoliation because resistance induction is assumed to be costly. Results confirmed upward transmission of inducedsystemic resistance (ISR) in cotton, as bioassay S. exigua caterpillars reared on both young, expanding leaves and mature leaves following previous S. exigua herbivory on lower leaves performed worse than those on control plants. ISR was not observed inleaves lower than actual damaged leaves, however, as bioassay S. exigua caterpillars raised on leaves immediately below the damaged leaves performed equally well in comparison with their counterparts from control plants. The duration of feeding did notaffect the magnitude of ISR, provided that the feeding damage was kept at the same level. The feeding damage following 1 d herbivory was not statistically significant than that following 3 d herbivory. Therefore, the levels of ISR of 1 and 3 d herbivorydid not differ. The relationship between amount of feeding damage inflicted within the same period of time and magnitude of ISR in young, expanding leaves was best expressed as quadratic. The magnitude of ISR increased as feeding damage mounted before reaching a peak value, then it leveled off or even decreased as feeding damage kept increasing.
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