Green flowers need yellow to get noticed in a green world.

Background and aims: Flower colour is a key feature in plant-pollinator interactions that make the flowers visible amid the surrounding green vegetation. Green flowers are expected to be scarcely conspicuous to pollinators; however, many of them are visited by pollinators even in the absence of other traits that might attract pollinators (e.g., floral scents). In this study, we investigate how entomophilous species with green flowers are perceived by pollinators.

Methods: We obtained reflectance spectra data of 30 European species that display green or green-yellow flowers to the human eye. These data were used to perform spectral analyses, calculate both chromatic (colour contrast against the background) and achromatic (colour contrast that relies on the signals from the green-sensitive photoreceptors) cues, and model colour perception by hymenopterans (bees) and dipterans (flies).

Key results: The visibility of green flowers to bees and flies (i.e., their chromatic contrast values) was lower compared to other floral colours commonly pollinated by these insects, whereas green-yellow flowers were as conspicuous as the other flower colours. Green flowers with low chromatic contrast values exhibited higher achromatic contrasts, which is used to detect distant flowers at narrow visual angles, than green-yellow flowers. Additionally, the marker points (i.e., sharp transition in floral reflectance that aid pollinators in locating them) of green and green-yellow flowers aligned to some degree with the colour discrimination abilities of bees and flies.

Conclusions: We found that many entomophilous green and green-yellow flowers are conspicuous to bees and flies through their chromatic or achromatic contrasts. While acquiring pigments like carotenoids, which impart a yellowish hue to flowers and enhances their visibility to pollinators, could increase their conspicuousness, the metabolic costs of pigment production, along with the use of alternative strategies to attract pollinators, may have constrained carotenoid emergence in certain lineages of green-flowered species.