The effect of pollination on floral fragrance in thistles.

We investigated postpollination changes in fragrance composition and emission rates, as well as pollinator discrimination in hand-pollinated flower heads of two thistle species: Canada thistle (Cirsium arvense) and sandhill thistle (C. repandum). Following pollination, neither species emitted any novel compounds that could function as repellents. Scent emission rates declined in pollinated plants of both species by approximately 89% within 48 hr. This decline was evident in all 13 scent components of C. arvense. Apis mellifera, the dominant pollinator in the study population of C. arvense, was nearly three times more likely to visit an unpollinated rather than a pollinated flower head. A more complex pattern was observed for C. repandum, whose scent comprised 42 compounds. Quantities of aromatic and sesquiterpenoid volatiles declined after pollination, whereas two classes of scent compounds, fatty acid derivatives and monoterperpenoids, continued to be emitted. In C. repandum, discrimination against pollinated flower heads by Papilio palamedes (its primary pollinator) was not as marked. Unpollinated control plants of both species maintained moderate levels of scent production throughout this experiment, demonstrating that senescence and floral advertisement may be delayed until pollination has occurred. We expect postpollination changes in floral scent contribute to communication between plants with generalized pollinator spectra and their floral visitors. This study provides the first field study of such a phenomenon outside of orchids.