Polymorphism in plant defense against herbivory: Constitutive and induced resistance in Cucumis sativus

Theory predicts that plant resistance to herbivores is determined by both g enetic and environmentally induced components. In this study, we demonstrat e that the phenotypic expression of plant resistance to spider mite herbivo ry in Cucumis sativus is determined by genetic and environmental factors an d that there is an interaction between these factors. Previous feeding by s pider mites induced systemic resistance to subsequent attack over several s patial scales within plants, reducing the population growth of mites compar ed to that on control plants. Effects of induction were effective locally o ver the short term, but resulted in local increased susceptibility to spide r mite attack after several days. However, this local induced susceptibilit y on the damaged leaf was associated with induced resistance on newer leave s. Induced resistance was correlated with increases in cucurbitacin content of leaves, but was not associated with changes in the density of leaf tric homes. Induced resistance to herbivory was not detected in plants of a geno type lacking constitutive expression of cucurbitacins, which were in genera l highly susceptibile to mite attack. Allocation trade-offs between growth and defense are often invoked to explain the maintenance of variation in th e levels of plant resistance. Contrary to current thinking, neither constit utive nor herbivore-induced plant resistance were associated with reduction s in plant allocation to root and shoot growth. However, plants that had hi gh levels of induced resistance to spider mites were the most susceptible t o attack by a specialist beetle. Such ecological trade-offs between resista nce to generalist herbivores and susceptibility to specialist herbivores ma y be important in the maintenance of Variation of plant resistance traits. In summary, C, sativus exhibits strong genetic variation for constitutive a nd induced resistance to spider mites, and this variation in resistance is associated with ecological trade-offs.