Fragrance is an ancient medium of chemical communication between flowering plants and animal pollinators (Pellmyr & Thien 1986). Pollinators use fragrance for distance orientation, approach, landing, feeding, and associative learning (Williams 1983; Metcalf 1987; Dobson 1994). In turn, scent-driven pollinator preference and constancy has been invoked as an isolating mechanism for diverse angiosperm taxa (Dodson et al. 1969), particularly among sympatric, synchronously blooming species with similar floral form, coloration, and pollination mechanisms (Knudsen 1999). In this chapter, I explore the potential for odor-driven floral evolution by reviewing the physiological and behavioral responses of pollinators to floral scent.

What is fragrance? A floral scent primer

Chemical diversity and biosynthesis

Floral scents are mixtures of small, volatile organic compounds that vary in molecular weight, vapor pressure, polarity, and oxidation state (Knudsen et al. 1993). Diverse chemical classes of floral volatiles are surveyed comprehensively by Croteau & Karp (1991). The analytical methods used to collect and identify floral scent have improved dramatically over the past decade, and are discussed by Raguso & Pellmyr (1998) and Agelopoulos & Pickett (1998). Floral volatiles are produced by biosynthetic pathways, through anabolic and catabolic processes. Figure 5.1 summarizes the major biosynthetic routes to fragrance production, illustrating representative products for each pathway (Azuma et al. 1997). These multifunctional pathways also produce plant pigments, defense compounds, structural components, growth, and signaling substances (Dixon & Paiva 1995). Recent progress in fragrance biosynthesis is reviewed by Dudareva et al. (1999) and Dudareva & Pichersky (2000).