Foliar Terpene Chemotypes and Herbivory Determine Variation in Plant Volatile Emissions

Plants that synthesize and store terpenes in specialized cells accumulate large concentrations of these compounds while avoiding autotoxicity. Stored terpenes may influence the quantity and profile of volatile compounds that are emitted into the environment and the subsequent role of those volatiles in mediating the activity of herbivores. The Australian medicinal tea tree, Melaleuca alternifolia, occurs as several distinct terpene chemotypes. We studied the profile of its terpene emissions to understand how variations in stored foliar terpenes influenced emissions, both constitutive and when damaged either by herbivores or mechanically. We found that foliar chemistry influenced differences in the composition of terpene emissions, but those emissions were minimal in intact plants. When plants were damaged by herbivores or mechanically, the emissions were greatly increased and the composition corresponded to the constitutive terpenes and the volatility of each compound, suggesting the main origin of emissions is the stored terpenes and not de novo biosynthesized volatiles. However, herbivores modified the composition of the volatile emissions in only one chemotype, probably due to the oxidative metabolism of 1,8-cineole by the beetles. We also tested whether the foliar terpene blend acted as an attractant for the specialized leaf beetles Paropsisterna tigrina and Faex sp. and a parasitoid fly, Anagonia zentae. None of these species responded to extracts of young leaves in an olfactometer, so we found no evidence that these species use plant odor cues for host location in laboratory conditions.