Floral odor bouquet loses its ant repellent properties after inhibition of terpene biosynthesis.

In their natural environment, plants are synchronously confronted with mutualists and antagonists, and thus benefit from signals that contain messages for both functional groups of interaction partners. Floral scents are complex blends of volatiles of different chemical classes, including benzenoids and terpenoids. It has been hypothesized that benzenoids have evolved as pollinator attracting signals, while monoterpenoids serve as defensive compounds against antagonists. In order to test this hypothesis, we reduced terpene emission in flowers of Phlox paniculata with specific biosynthetic inhibitors and compared the responses of Lasius niger ants to natural and inhibited floral scent bouquets. While the natural odors were strongly repellent to ants, the bouquets with a reduced emission rate of terpenoids were not. The loss of the flowers' ability to repel ants could be attributed predominantly to reduced amounts of linalool, a monoterpene alcohol. Flying flower visitors, mainly hoverflies, did not discriminate between the two types of flowers in an outdoor experiment. Since individual compounds appear to be capable of either attracting pollinators or defending the flower from enemies, the complexity of floral scent bouquets may have evolved to allow flowers to respond to both mutualists and antagonists simultaneously.