Divergence in floral scent profiles among and within cultivated species of Phlox

Variation in floral scent profiles within and among plant species may be attributed to a number of different phenomena, including selection by pollinators or enemies, direct or indirect artificial selection in horticulture, stochastic processes such as genetic drift, and pleiotropy between scent and other floral traits, including floral color. The genus Phlox represents a unique opportunity to study scent variation across multiple species using horticultural specimens derived, in part, to generate color diversity. In this study, we assessed floral scent variation among horticultural cultivars of nine different Phlox species and the related species Polemonium caeruleum, as well as differences in scent variation between wild-originated and horticultural Phlox drummondii color morphs using dynamic headspace extraction and gas chromatography–mass spectroscopy. We identified significant variation in floral scent among species of Phlox, between color morphs pooled across species, and significant scent variation between cultivars showing different floral colors within the species P. subulata, P. stolonifera, P. carolina, and P. drummondii. We also uncovered significant differences between the volatile profiles of wild-originated and horticultural specimens of P. drummondii. Color morphs of horticultural P. drummondii differed in their scent, while morphs from wild-originated plants did not. This study documents rampant variation in floral scent composition within and among Phlox species and their horticultural cultivars. Species-level differences are likely to have been shaped by differences in selection regimes (both artificial selection in cultivation and natural selection in wild progenitors) and/or stochastic processes among groups. Differences among color morphs suggest some possibility of pleiotropy between color and scent. Species with particularly high levels of between-cultivar variation further highlight the potential for pleiotropic interactions between color and scent phenotypes, although no clear patterns connecting pigment to production of particular scent compounds emerge. Our finding that horticultural P. drummondii shows greater variation in scent profiles among color morphs suggests that either selection in wild populations minimizes scent differences between color morphs or relaxation of selection/incidental artificial selection on scent in horticultural settings allows accumulation of variation between color morphs.