The authors reasoned that aspen leaves that possessed high concentrations of phenolic glycosides might not need extrafloral nectaries if the chemicals directly deterred the herbivores or if these secondary compounds were also present in the nectary and had a negative effect on the very insects the plant was trying to attract. To see if this might be the case, they assessed the presence of EFNs and phenolic compounds on leaves from different shoot positions from short and tall ramets across multiple sites.
"What we found was the opposite of what we expected," said Wagner. "Instead of segregating the two defenses, aspen plants tended to express both in the same set of leaves. Other leaves on the plant were relatively poorly defended, with no EFNs and concentrations of phenolic glycosides too low to be effective against the major herbivore in our study." Indeed, while shorter ramets and younger leaves had higher levels of phenolic gylcosides, the presence of EFNs was correlated unexpectedly with higher phenolic glycoside levels when other factors (i.e., leaf position) were controlled, and damage from leaf mining insects was not related to presence of EFNs.
The authors hypothesize that leaves with EFNs might need higher levels of chemical defenses because they might attract nectar-drinking herbivores who may also lay their eggs on the leaves. However, EFNs may provide a diffuse form of indirect defense that extends to all the leaves by attracting predators of herbivores onto the aspen ramet in general.
"Our study shows that the expression of biotic defenses such as EFNs can covary with expr
|Contact: Richard Hund|
American Journal of Botany