Pollination modes and divergent flower traits in three species of Plantago subgenus Plantago (Plantaginaceae)

Different pollination modes have been reported from the genus Plantago, including cleistogamy, autogamy, ambophily and anemophily. However, these categories are no fixed units, but broad overlap exists between them. For several widely distributed P. species rigorous tests of the pollination mode are missing. Therefore, we analysed six floral traits and conducted pollination experiments of three common P. species to quantify the different pollination modes and their impact on seed production. Plantago australis turned out to be obligately cleistogamous with extremely low pollen grain production. Instead, P. major is an autogamous species with chasmogamous flowers and the ability to increase seed production by wind-mediated outcrossing. However, its short style-stigma complexes underline that seed production is mainly induced by active selfing. Finally, P. media is pollinated by wind and insects. Its long, conspicuously collourd filaments and high pollen grain production attract bees but also allow wind pollination. Seed production of P. media increased by 42%, when insects in addition to wind had access to the flowers. Cleistogamy and ambophily evolved from wind-pollinated ancestors in P., probably in parallel to the colonization of new habitat types.