An investigation into the rapidly induced chemical responses of Myrica gale to insect herbivory
The effect of natural herbivory on the secondary metabolism of Myrica gale was investigated. In a field experiment herbivory was found to elicit changes in both the leaf phenolics and the density of volatile oil glands. Lygocoris spinolai, a capsid bug, accounted for most of the observed herbivory. One set of plants (the controls) were kept free of insect herbivores while a second set sustained capsid bug herbivory. The terpenoid and phenolic profile of leaves from each set was obtained on a weekly basis using GLC and IIPLC respectively. GC-MS was used to identify constituents of the volatile oil. UV, ¹H and ¹³C NMR, El-MS and FAB-tlS were used to identify the phenolics. Capsid bug damage induced a cpialitative change in the phenolic content of the leaves. This induction revealed a new compound, kaempferol-3-(2,3-diacetoxy-4-p-coumaroyl)rhamnoside, which was isolated and identified. The leaves also proved to contain other flavonoids, the concentrations of which were not affected by herbivory. Artificial damage was observed to elicit a quantitative change in leaf phenolics, thus having a different effect than herbivore damage. Herbivore damage also induced quantitative changes in the volatile oil by eliciting gland production whilst having no effect on the composition of the oil. The induced flavonoid and the volatile oil were tested for fungal growth inhibitory properties. Both were found to have marked antifungal activity at low concentrations when tested against fungal species isolated from the leaves of M. gale in the field. The observed phenomena are discussed in terms of defenses against herbivores and pathogens. The relationship between the nitrogen economy of M. gale and leaf chemistry is also discussed.