Floral scent and pollinator visitation in relation to floral colour morph in the mixed-mating annual herb Collinsia heterophylla

Even though floral scent is of major importance for pollinator attraction, it is less investigated than other floral traits. Previous studies suggest the importance of joint exploration of olfactory and visual floral cues to understand plant–pollinator interactions. We investigated flower scents in Collinsia heterophylla, a bee-pollinated, annual herb a with mixed-mating system combining self- and outcross-pollination. In Collinsia, floral size and development variation is related to mating system, ranging from large-flowered mixed-mating species to small-flowered self-pollinated species. However, to our knowledge, flower scent has not been described in any species in the genus. We also studied whether flower-emitted volatiles were coupled to presence versus absence of a coloured band on the upper lip within a population in C. heterophylla, and if these colour morphs affected pollinator visitation. We performed headspace collections of volatiles in the greenhouse from potted flowering plants, and compared these to controls in the bud stage. Flower-specific volatiles were highly dominated by terpenoid compounds typical of bee-pollinated plants, such as β-myrcene, (Z)- and (E)-ocimene and sesquiterpenes (E)-α-bergamotene and β-sesquiphellandrene. The aliphatic ester methyl hexanoate was also prominent, together with additional esters, whereas methyl cinnamate constituted the only aromatic compound. Floral colour morphs showed no qualitative difference in volatiles, but the coloured morph produced significantly higher quantities for seven of the 26 individual flower compounds. A field experiment performed within a natural population, using behavioural observations and florescent dyes dusted on the flowers, could not detect any differences in pollinator visitation between colour morphs. We conclude that C. heterophylla flowers emit volatile compounds commonly associated with attraction of their most important pollinators. It would be highly interesting to explore the function of floral scent for pollinator attraction and relate floral scent to mating system variation across Collinsia for a better understanding of pollinator influence on floral evolution.