In a world of white,flower colour matters: A white–purple transition signals lack of reward in an alpine Euphrasia

The New Zealand alpine flora displays a range of unusual characteristics compared with other alpine floras, in particular the high frequency of species with small white flowers. The presence of both white and bright purple flowers on the same plant in the New Zealand alpine annual creeping eyebright (Euphrasia dyeri Wettst.) provides an ideal opportunity to investigate the significance of flower colour in an environment where coloured flowers are rare. The relationships among flower age, gender phase, reward availability and petal colour were assessed in natural populations of E. dyeri. The effect of pollination on flower colour was tested using hand pollination of bagged flowers. Direct observations and videos of flowers were used to assess patterns of flower visitation by native and introduced pollinators. Unpollinated white E. dyeri flowers changed from white to purple within 6 days. However, pollination of white flowers triggered a significantly faster colour change, typically within 1–2 days. White flowers had receptive stigmas, large amounts of lipid‐rich pollen and small amounts of nectar, whereas stigmas of purple flowers are not receptive and flowers did not provide pollen or nectar rewards. Flowers were mainly visited by native syrphid flies. Both native syrphids and introduced Bombus bees showed a marked avoidance of purple flowers, tending to preferentially visit white flowers. Our study suggests that flower colour change from white to bright purple in E. dyeri functions to direct pollinators to rewarding, receptive flowers. As many Euphrasia L. species are described as having variably coloured flowers, this mechanism may be more widespread in the genus. Furthermore, our results add to the growing evidence that the dominance of white flowers in the New Zealand alpine is not simply due to a lack of colour discrimination among pollinators.