Weak correlation of flower color and nectar-tube depth in temperate grasslands

Aims Nectar is one of the most common floral rewards offered to pollinators by plants. Depending on the plant species, nectar is offered openly or in tubes of various lengths restricting accessibility of this resource for flower visitors with short mouthparts. If attracting pollinators that match floral morphology increases pollination efficiency, flowers could profit from signaling nectar-tube depth to pollinators. Since flower colors are important signals in plant-pollinator communication, we investigated whether and which different chromatic or achromatic aspects of flower color might indicate nectar-tube depth or whether flower colors facilitate the differentiation between flowers with long nectar tubes by means of high chromatic uniqueness. Methods To this end, we collected flower reflectance spectra of 135 grassland plant species. We analyzed flower colors as raw reflectance spectra in principal component analysis (PCA) and in the color space of honeybees. Important Findings The correlation between flower colors and tube depths was weak. From the bee's point of view, blue flowers had on average deeper tubes than green, blue-green and UV-green flowers potentially allowing insects to predict tube depths based on blue color. Spectral purity did not correlate with nectar-tube depth, nor did the chromatic uniqueness of flower colors in the honeybee color space. Dominant wavelength showed a significant but very weak correlation with tube depth. The achromatic green contrast decreased with increasing tube depth as did brightness; thus deep tubes were less conspicuous than shallow tubes. Chromatic components resulting from PCA did not or only slightly correlate with tube depth. Our results illustrate that flower colors may have a certain potential to indicate tube depth, i.e. nectar accessibility, from a bee's perspective.