Inducible defenses by their nature are phenotypically variable traits. To assess the degree of genetic control over induction and therefore the availability of these traits for selection, we measured the levels of genetic variation and genetic correlation for induced and constitutive furanocoumarins in two populations of wild parsnip, Pastinaca sativa L., near Urbana, Illinois, USA. Leaflets of greenhouse—grown from families of each population were artificially damaged. After 48 h undamaged and damaged leaflets were analyzed for amounts of furanocoumarins, secondary compounds with known insecticidal properties. The same plants were subsequently used in a transplant experiment to determine the frequency of attack by leaf—feeding herbivores for each population in each habitat. While individuals from both populations experienced similar frequencies of attack within each location, the overall infestation rate was much higher at the Race Street location (41%) than at the Phillips Tract location (5%). The two populations contained significant amounts of genetic variation for constitutive and induced levels and induced increments of certain furanocoumarins, and differed significantly in the constitutive levels of two of the furanocoumarins. The Phillips Tract population, which experiences a low probability of herbivory, contained significantly lower constitutive amounts of furanocoumarin. The results of these studies are consistent with the interpretation that these populations have divergent defense strategies that are in accordance with their probabilities of being attacked. This study provides the first evidence that selection can act on inducibility of plant chemical defenses.
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Zangerl et al. (1990) studied this question.