Extensive population-level sampling reveals clinal variation in (R)-(-)-linalool produced by the flowers of an endemic evening primrose, Oenothera harringtonii

The study of floral trait diversity has a long history due to its role in
angiosperm diversification. While many studies have focused on visual
traits including morphology and color, few have included floral scent
despite its importance in pollination. Of the studies that have included
floral scent, sampling has been limited and rarely explores variation at
the population level. We studied geographic variation in the flowers
of Oenothera harringtonii, a rare plant endemic to a vulnerable
shortgrass prairie habitat, whose population structure and conservation
status are well studied. The self-incompatible flowers of O.
harringtonii open at dusk, produce nectar and a strong fragrance,
and are pollinated by hawkmoths. We collected floral trait (morphology,
scent chemistry and emission rates) data from 650 individuals from 19 wild
populations to survey floral variation across the entire range of this
species. Similarly, we collected floral data from 49 individuals grown in
a greenhouse common garden, to assess whether variation observed in the
field is consistent when environment factors (temperature, watering
regime, soil) are standardized. We identified 35 floral volatiles
representing 5 biosynthetic classes. Population differentiation was
stronger for floral scent chemistry than floral morphology.
(R)-(−)-linalool was the most important floral trait differentiating
populations, exhibiting clinal variation across the distribution
of O. harringtonii without any correlated shifts in
floral morphology. Populations in the north and west produced
(R)-(−)-linalool consistently, those in the east and south largely lacked
it, and populations at the center of the distribution were polymorphic.
Floral scent emissions in wild populations varied across four years but
chemical composition was largely consistent over time. Similarly, volatile
emission rates and chemical composition in greenhouse-grown plants were
consistent with those of wild populations of origin. Our data set, which
represents the most extensive population-level survey of floral scent to
date, indicates that such sampling may be needed to capture potentially
adaptive geographic variation in wild populations.