Why are the inner and outer sides of many flower petals differently coloured?

The flower perianth has various, non-mutually exclusive functions, such as visual signalling to pollinators and protecting the reproductive organs from the elements and from florivores, but how different perianth structures and their different sides play a role in these functions is unclear. Intriguingly, in many species there is a clear colour difference between the different sides of the perianth, with colour patterns or pigmentation present on only one side. Any adaptive benefit from such colour asymmetry is unclear, as is how the asymmetry evolved. In this viewpoint paper, we address the phenomenon of flowers with differently coloured inner and outer perianth sides, focusing on petals of erect flowers. Guided by existing literature and our own observations, we delineate three non-mutually exclusive evolutionary hypotheses that may explain the factors underlying differently coloured perianth sides. The pollen-protection hypothesis predicts that the outer side of petals contributes to protect pollen against UV radiation, especially during the bud stage. The herbivore-avoidance hypothesis predicts that the outer side of petals reduces the flower's visibility to herbivores. The signalling-to-pollinators hypothesis predicts that flower colours evolve to increase conspicuousness to pollinators. The pollen-protection hypothesis, the herbivore-avoidance hypothesis, and the signalling-to-pollinators hypothesis generate largely but not entirely overlapping predictions about the colour of the inner and outer side of the petals. Field and laboratory research is necessary to disentangle the main drivers and adaptive significance of inner-outer petal side colour asymmetry.

Predictive Links between Petal Color and Pigment Quantities in Natural Penstemon Hybrids

Flowers have evolved remarkable diversity in petal color, in large part due to pollinator-mediated selection. This diversity arises from specialized metabolic pathways that generate conspicuous pigments. Despite the clear link between flower color and floral pigment production, quantitative models inferring predictive relationships between pigmentation and reflectance spectra have not been reported. In this study, we analyze a dataset consisting of hundreds of natural Penstemon hybrids that exhibit variation in flower color, including blue, purple, pink, and red. For each individual hybrid, we measured anthocyanin pigment content and petal spectral reflectance. We found that floral pigment quantities are correlated with hue, chroma, and brightness as calculated from petal spectral reflectance data: hue is related to the relative amounts of delphinidin vs. pelargonidin pigmentation, whereas brightness and chroma are correlated with the total anthocyanin pigmentation. We used a partial least squares regression approach to identify predictive relationships between pigment production and petal reflectance. We find that pigment quantity data provide robust predictions of petal reflectance, confirming a pervasive assumption that differences in pigmentation should predictably influence flower color. Moreover, we find that reflectance data enables accurate inferences of pigment quantities, where the full reflectance spectra provide much more accurate inference of pigment quantities than spectral attributes (brightness, chroma, and hue). Our predictive framework provides readily interpretable model coefficients relating spectral attributes of petal reflectance to underlying pigment quantities. These relationships represent key links between genetic changes affecting anthocyanin production and the ecological functions of petal coloration.

Caution with colour calculations: spectral purity is a poor descriptor of flower colour visibility

BACKGROUND

The colours of flowers are of key interest to plant and pollination biologists. An increasing number of studies have investigated the importance of saturation of flower colours (often called 'spectral purity' or 'chroma') for visibility to pollinators, but the conceptual, physiological and behavioural foundations for these metrics as well as the calculations used rest on slender foundations.

METHODS

We discuss the caveats of colour attributes that are derived from human perception, and in particular spectral purity and chroma, as variables in flower colour analysis. We re-analysed seven published datasets encompassing 774 measured reflectance spectra to test for correlations between colour contrast, spectral purity and chroma.

MAIN FINDINGS AND CONCLUSIONS

We identify several concerns with common calculation procedures in animal colour spaces. Studies on animal colour vision provide no ground to assume that any pollinator perceives (or responds to) saturation, chroma or spectral purity in the way humans do. A re-analysis of published datasets revealed that values for colour contrast between flowers and their background are highly correlated with measures for spectral purity and chroma, which invalidates treating these factors as independent variables as is currently commonplace. Strikingly, spectral purity and chroma - both of which are metrics for saturation and are often used synonymously - are not correlated at all. We conclude that alternative, behaviourally validated metrics for the visibility of flowers to pollinators, such as colour contrast and achromatic contrast, are better in understanding the role of flower colour in plant-pollinator signalling.

Achromatic Cues Are Important for Flower Visibility to Hawkmoths and Other Insects

Studies on animal colour vision typically focus on the chromatic aspect of colour, which is related to the spectral distribution, and disregard the achromatic aspect, which is related to the intensity (“brightness”) of a stimulus. Although the chromatic component of vision is often most reliable for object recognition because it is fairly context independent, the achromatic component may provide a reliable signal under specific conditions, for example at night when light intensity is low. Here we make a case for the importance of achromatic cues in plant-pollinator signalling, based on experimental data on naïve Deilephila elpenor and Macroglossum stellatarum hawkmoths, optical modelling and synthesising published experiments on bees, flies, butterflies and moths. Our experiments show that in ecologically relevant light levels hawkmoths express a strong preference for brighter stimuli. Published experiments suggest that for flower-visiting bees, butterflies, moths and flies, achromatic cues may be more important for object detection than often considered. Our optical modelling enabled disentangling the contribution of pigments and scattering structures to the flower’s achromatic contrast, and illustrates how flower anatomy and background are important mediating factors. We discuss our findings in the context of the often-assumed dichotomy between detection and discrimination, chromatic versus achromatic vision, and the evolution of floral visual signals.