The ecology and evolution of pollen odors

The literature is reviewed and new evidence presented that pollen produces odors, which serve multiple functions in pollination and defense. Pollen od or, which originates from pollenkitt, comprises volatiles that belong to th e same chemical classes found in flower scents, that are in species-specifi c mixtures, and that contrast with odors of other floral parts. Pollen can also take up volatiles from surrounding floral odors, but this adsorption i s selective and varies among species. Pollen odors are more pronounced in i nsect- than bird- or wind-pollinated plants, suggesting that volatile emiss ion evolved in part under selection to attract pollinators. Pollen-feeding insects can perceive pollen odor and use it to discriminate between differe nt pollen types and host plants. Pollen odor influences bee foraging, inclu ding the location of pollen sources, discrimination of flowers with differe nt amounts of pollen, and host-plant recognition by pollen-specialist speci es. In the few wind-pollinated plants studied, odors of male flowers or pol len are comparatively high in alpha-methyl alcohols and ketones; these vola tiles may serve in pollen defense, with some known to repel insects. Pollen odor often includes chemicals with documented defense activity, which is p robably aimed mainly at nonpollinator pollen-feeding insects and pathogens; an involvement in pollen allelopathy is also possible. Pollen volatiles co mprise chemically diverse compounds that may play multiple roles, and their emission in pollen odor undoubtedly evolved under the principle, and often conflicting, selective pressures to both protect the male gametophyte and increase its dispersal by animals.