How much pigment should flowers have?:Flowers with moderate pigmentation have highest color contrast

Floral pigments are a core component of flower colors, but how much pigment a flower should have to yield a strong visual signal to pollinators is unknown. Using an optical model and taking white, blue, yellow and red flowers as case studies, I investigate how the amount of pigment determines a flower’s color contrast. Modeled reflectance spectra are interpreted using established insect color vision models. Contrast as a function of the amount of pigment shows a pattern of diminishing return. Low pigment amounts yield pale colors, intermediate amounts yield high contrast, and extreme amounts of pigment do not further increase, and sometimes even decrease, a flower’s color contrast. An intermediate amount of floral pigment thus yields the highest visibility, a finding that is corroborated by previous behavioral experiments on bees. The implications for studies on plant-pollinator signaling, intraspecific flower color variation and the costs of flower color are discussed

Surprising absence of association between flower surface microstructure and pollination system

The epidermal cells of flowers come in different shapes and have different functions, but how they evolved remains largely unknown. Floral micro-texture can provide tactile cues to insects, and increases in surface roughness by means of conical (papillose) epidermal cells may facilitate flower handling by landing insect pollinators. Whether flower microstructure correlates with pollination system remains unknown. Here, we investigate the floral epidermal microstructure in 29 (congeneric) species pairs with contrasting pollination system. We test whether flowers pollinated by bees and/or flies feature more structured, rougher surfaces than flowers pollinated by non-landing moths or birds and flowers that self-pollinate. In contrast with earlier studies, we find no correlation between epidermal microstructure and pollination system. The shape, cell height and roughness of floral epidermal cells varies among species, but is not correlated with pollinators at large. Intriguingly, however, we find that the upper (adaxial) flower surface that surrounds the reproductive organs and often constitutes the floral display is markedly more structured than the lower (abaxial) surface. We thus conclude that conical epidermal cells probably play a role in plant reproduction other than providing grip or tactile cues, such as increasing hydrophobicity or enhancing the visual signal

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 investigates 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 used calculations 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 reanalysed 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 reanalysis of published datasets revealed that values for colour contrast between flowers and their background is 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 - which both 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

Coloration of the Chilean Bellflower, <i>Nolana paradoxa</i>, interpreted with a scattering and absorbing layer stack model

An absorbing-layer-stack model allows quantitative analysis of the light flux in flowers and the resulting reflectance spectra. It provides insight in how plants can optimize their flower coloration for attracting pollinators. The coloration of flowers is due to the combined effect of pigments and light-scattering structures. To interpret flower coloration, we applied an optical model that considers a flower as a stack of layers, where each layer can be treated with the Kubelka-Munk theory for diffusely scattering and absorbing media. We applied our model to the flowers of the Chilean Bellflower, Nolana paradoxa, which have distinctly different-colored adaxial and abaxial sides. We found that the flowers have a pigmented, strongly scattering upper layer, in combination with an unpigmented, moderately reflecting lower layer. The model allowed quantitative interpretation of the reflectance and transmittance spectra measured with an integrating sphere. The absorbance spectrum of the pigment measured with a microspectrophotometer confirmed the spectrum derived by modeling. We discuss how different pigment localizations yield different reflectance spectra. The absorbing layer stack model aids in understanding the various constraints and options for plants to tune their coloration

Conical epidermal cells cause velvety colouration and enhanced patterning in <i>Mandevilla</i> flowers

The majority of angiosperms have flowers with conical epidermal cells, which are assumed to have various functions, such as enhancing the visual signal to pollinators, but detailed optical studies on how conical epidermal cells determine the flower's visual appearance are scarce. Here we report that conical epidermal cells of Mandevilla sanderi flowers effectively reduce surface gloss and create a velvety appearance. Owing to the reduction in surface gloss, the flower further makes more efficient use of floral pigments and light scattering structures inside the flower. The interior backscattering yields a cosine angular dependence of reflected light, meaning that the flowers approximate near-perfect (Lambertian) diffusers, creating a visual signal that is visible across a wide angular space. Together with the large flowers and the tilted corolla tips, this generates a distinct visual pattern, which may enhance the visibility to pollinators

The thermal ecology of flowers

BACKGROUND: Obtaining an optimal flower temperature can be crucial for plant reproduction because temperature mediates flower growth and development, pollen and ovule viability, and influences pollinator visitation. The thermal ecology of flowers is an exciting, yet understudied field of plant biology. SCOPE: This review focuses on several attributes that modify exogenous heat absorption and retention in flowers. We discuss how flower shape, orientation, heliotropic movements, pubescence, coloration, opening-closing movements and endogenous heating contribute to the thermal balance of flowers. Whenever the data are available, we provide quantitative estimates of how these floral attributes contribute to heating of the flower, and ultimately plant fitness. OUTLOOK: Future research should establish form-function relationships between floral phenotypes and temperature, determine the fitness effects of the floral microclimate, and identify broad ecological correlates with heat capture mechanisms

Coloration of flowers by flavonoids and consequences of pH dependent absorption

Flavonoid pigments are key determinants of flower colors. As absorption spectra offlavonoids are known to be severely pH-dependent, cellular pH will play a crucial rolein flower coloration. The flavonoids are concentrated in the vacuoles of the flowers’epidermal cells, and thus the pigments’ absorption spectra are modulated by thevacuolar pH. Here we study the pH dependence of flavonoid absorption spectra inextracts from flowers of two poppy speciesPapaver dubium(red) andMeconopsiscambrica(orange), and a white and redMandevilla sanderivariety. In the red poppyandMandevillaflowers, absorption spectra of the cyanidin- and pelargonidin-basedanthocyanins peak in the blue-green-wavelength range at low pH, but exhibit a distinctbathochromic shift at higher pH. This shift to longer wavelengths is not found for theblue-absorbing nudicaulin derivatives ofM. cambrica, which have a similar absorptionspectrum at low and high pH. The pH-dependent absorption changes of the whiteM. sanderi’s flavonoid remained restricted to the UV. An analysis of the spectra withlogistic functions suggests that the pH-dependent characteristics of the basic states offlavonols and anthocyanins are related. The implications of tuning of pH and pigmentabsorption spectra for studies on flower color evolution are discussed

Pollinator or pedigree:Which factors determine the evolution of pollen nutrients?

A prime example of plant-animal interactions is the interaction between plants and pollinators, which typically receive nectar and/or pollen as reward for their pollination service. While nectar provides mostly carbohydrates, pollen represents the main source of protein and lipids for many pollinators. However, the main function of pollen is to carry nutrients for pollen tube growth and thus fertilization. It is unclear whether pollinator attraction exerts a sufficiently strong selective pressure to alter the nutritional profile of pollen, e.g., through increasing its crude protein content or protein-to-lipid ratio, which both strongly affect bee foraging. Pollen nutritional quality may also be merely determined by phylogenetic relatedness, with pollen of closely related plants showing similar nutritional profiles due to shared biosynthetic pathways or floral morphologies. Here, we present a meta-analysis of studies on pollen nutrients to test whether differences in pollen nutrient contents and ratios correlated with plant insect pollinator dependence and/or phylogenetic relatedness. We hypothesized that if pollen nutritional content was affected by pollinator attraction, it should be different (e.g., higher) in highly pollinator-dependent plants, independent of phylogenetic relatedness. We found that crude protein and the protein-to-lipid ratio in pollen strongly correlated with phylogeny. Moreover, pollen protein content was higher in plants depending mostly or exclusively on insect pollination. Pollen nutritional quality thus correlated with both phylogenetic relatedness and pollinator dependency, indicating that, besides producing pollen with sufficient nutrients for reproduction, the nutrient profile of zoophilous plants may have been shaped by their pollinators' nutritional needs

Atmospheric H₂S exposure does not affect stomatal aperture in maize

Main conclusion: Stomatal aperture in maize is not affected by exposure to a subtoxic concentration of atmospheric H2S. At least in maize, H2S, thus, is not a gaseous signal molecule that controls stomatal aperture. Abstract: Sulfur is an indispensable element for the physiological functioning of plants with hydrogen sulfide (H2S) potentially acting as gasotransmitter in the regulation of stomatal aperture. It is often assumed that H2S is metabolized into cysteine to stimulate stomatal closure. To study the significance of H2S for the regulation of stomatal closure, maize was exposed to a subtoxic atmospheric H2S level in the presence or absence of a sulfate supply to the root. Similar to other plants, maize could use H2S as a sulfur source for growth. Whereas sulfate-deprived plants had a lower biomass than sulfate-sufficient plants, exposure to H2S alleviated this growth reduction. Shoot sulfate, glutathione, and cysteine levels were significantly higher in H2S-fumigated plants compared to non-fumigated plants. Nevertheless, this was not associated with changes in the leaf area, stomatal density, stomatal resistance, and transpiration rate of plants, meaning that H2S exposure did not affect the transpiration rate per stoma. Hence, it did not affect stomatal aperture, indicating that, at least in maize, H2S is not a gaseous signal molecule controlling this aperture