Exogenous Application of Methyl Jasmonate Increases Emissions of Volatile Organic Compounds in Pyrenean Oak Trees, Quercus pyrenaica

Abstract

The tri-trophic interactions between plants, insects, and insect predators and parasitoids are often mediated by chemical cues. The attraction to herbivore-induced Plant Volatiles (HIPVs) has been well documented for arthropod predators and parasitoids, and more recently for insectivorous birds. The attraction to plant volatiles induced by the exogenous application of methyl jasmonate (MeJA), a phytohormone typically produced in response to an attack of chewing herbivores, has provided controversial results both in arthropod and avian predators. In this study, we examined whether potential differences in the composition of bouquets of volatiles produced by herbivore-induced and MeJA-treated Pyrenean oak trees (Quercus pyrenaica) were related to differential avian attraction, as results from a previous study suggested. Results showed that the overall emission of volatiles produced by MeJA-treated and herbivore-induced trees did not differ, and were higher than emissions of Control trees, although MeJA treatment showed a more significant reaction and released several specific compounds in contrast to herbivore-induced trees. These slight yet significant differences in the volatile composition may explain why avian predators were not so attracted to MeJA-treated trees, as observed in a previous study in this plant-herbivore system. Unfortunately, the lack of avian visits to the experimental trees in the current study did not allow us to confirm this result and points out the need to perform more robust predator studies.

Keywords: avian olfaction; defense against herbivory; foraging; herbivore-induced plant volatiles.

Figure 1

Experiment design showing the approximate location of an experimental branch at each of the focal trees (a), three respective treatments as they looked like in the 14 days long predation experiment—Control, herbivore-induced, MeJA-treated (b), and appearance of the branches during the measurement of volatile compounds (c). The measurement started 62 h after the beginning of the experiment, upon removal of the organza bag and real caterpillars from herbivore-induced treatment. Volatiles were then collected for 75 min. After that, the organza bag and live caterpillars were returned to their respective treatments. For the MeJA application (b), the organza bag was always removed, MeJA sprayed on the leaves inside, and the bag returned to the branch.

Figure 2

The total amount of volatiles (ng) emitted by Control (n = 15), herbivore-induced (n = 15), and MeJA-treated (n = 15) Pyrenean oaks at a study site in an oak forest of Guadarrama Mountains, Spain. Treatments with significantly different amounts of volatiles are marked by asterisks (*** p < 0.001, ** p < 0.01); results of chisq test. White circles show VOCs emitted by each individual tree.

Figure 3

Mean change in the VOCs emission between MeJA-treated, herbivore-induced, and Control trees of Pyrenean oak. The x-axes show the individual compounds, and y-axes show the relative amount (ng; in a and b with different y-axis scales) of emitted VOCs. MeJA-treated and herbivore-induced trees released significantly higher amounts of Bourbonene (a) compared to control trees. MeJA-treated trees emitted more Trans-β-Ocimene and β-Ocimene (b) than the control trees. MeJA-treated trees did not produce Isolongifolen in contrast to the control and herbivory-infested trees (a). Treatments with significantly different amounts of volatiles are marked by asterisks (*** p < 0.001, * p < 0.05, ^• p = 0.05); results of chisq test.