Volatile organic compounds (VOCs), such as terpenoids, green leaf volatiles, and benzenoids, are emitted by the vegetative parts of plants (leaves and bark), and they are thought to play a role in defense against pathogens and herbivores via cytotoxicity, direct repellence of herbivores, or indirect defense (attraction of herbivore’s enemies). Current research on the defensive roles of VOCs mostly focuses on model plant species in laboratory settings, and little is known about their importance in non-model species or in situ. The optimal defense theory (ODT) states that if VOCs play a role in plant defense, then the VOC blend of different vegetative parts should vary with the differential predation pressure imposed upon them and with the value of the plant part. Hence, more valuable organs should be protected against a broad range of predators, and should emit a more diverse blend of VOCs. We characterized the composition of the full blend of VOCs released from bark and leaves after mechanical damage for individuals of 206 tree species sampled in situ in the rainforests of French Guiana. Here, we present the results from 55 common species for which we sampled 2-6 individuals per species. As different herbivore communities are associated with bark and leaves, we expected VOC composition to differ between them, and as wood is more valuable to tree species, we expect bark to emit a higher diversity of compounds.
Results/Conclusions
We discovered that tropical trees release an incredible diversity of VOCs, especially in the sesquiterpenes group. There was little intraspecific variation in VOC composition, and the total number of VOCs released varied dramatically across species. For most species (40 of 55), the number of VOCs released by the bark was greater and includes more sesquiterpenes than in leaves. Across all individuals, the compositional overlap in VOCs between the bark and leaves was always less than 50%. In light of the recent literature about the defensive role of VOCs, we argue that VOCs and especially sesquiterpenes may be more important than previously thought in tropical tree anti-herbivore strategies.
