Why Do Buttercups Light Up Your Chin?

A favourite childhood game is to hold a buttercup (Ranunculus repens) under the chin, and if it glows yellow, you’re said to like butter. I remember playing this game with classmates in the sports field behind our primary school in England.

“Do you like butter?
Hold this buttercup under your chin;
If your chin turns yellow,
Then you do!”

Behind this simple reflection of yellow light lies a fascinating botanical secret. The buttercup’s petals are expertly designed by nature to reflect light in a very specific way, resulting in its characteristic luminescent yellow sheen. This not only inspired a childhood tradition but also serves a critical role in the buttercup’s lifecycle. As we delve into the anatomy of the buttercup petal, the layers and intricacies that contribute to this luminous phenomenon unravel, revealing the science behind the glow.

Scientists at the University of Cambridge have shone a light on how the buttercup produces its characteristic yellow glow. Buttercup petals were photographed with an Olympus SZX16 stereomicroscope equipped with an Olympus DP70 digital camera (Olympus, Tokyo, Japan) and a Zeiss Universal Microscope (Zeiss, Oberkochen, Germany) with a Mueller DCM510 camera (Mueller Optronic, Erfurt, Germany).

Why do buttercups glow?

When light hits the buttercup petal, it first passes through the outer transparent epidermal cells. Once it reaches the lower starch layer, the light interacts with both the starch granules and the pigment-containing cells. The starch granules effectively reflect light due to their high refractive index, while the pigments in the chromoplasts absorb certain wavelengths of light, particularly in the blue-green spectrum. This absorption makes the reflected light appear more yellow.

This combination of reflection by the starch granules and selective absorption by the pigments is what gives buttercup flowers their characteristic bright, glossy, and yellow appearance, which is believed to be an adaptation for attracting pollinators. Below we explain these three critical layers which together, produce the characteristic yellow glow.

Epidermal layer:

The epidermal layer is an ultrasmooth, transparent layer with pigments that absorb blue-green light, leaving longer-wavelength yellow light to reflect back to the eye [1, 2, 3, 4, 5, 6]. Anchored lightly to a starch layer below, the epidermal layer has pockets of air between it and the starch, giving the petals their glossy sheen. The epidermal layer of cells has two extremely flat surfaces which reflect light. One is the top of the cells, and the other exists because the epidermis is separated from the lower layers by an air gap[4]. The reflection of light by the smooth surface of the cells and the air layer effectively doubles the gloss of the petal.

Starch layer:

The starch layer lies below the epidermal layer and is made up of starch-containing parenchyma cells (cells in plants that are involved in storage, photosynthesis, and regeneration, and are known for their thin cell walls and large central vacuoles) [1]. The starch layer has a scattering effect, which helps to reflect any light that isn’t absorbed by the pigments in the epidermal layer or the light that is bounced back [2,4].  Electron micrographs demonstrated that the upper epidermis is essentially a thin plate separated from the starch layer by an air space [6].

Mesophyll layer:

The mesophyll layer in buttercup petals is primarily concerned with the display of colour and optical properties that are crucial for attracting pollinators. This layer contains a high concentration of carotenoids, a pigment which gives the petals their characteristic yellow colour. Additionally, the cells in the mesophyll layer of buttercup petals are loosely packed and may contain air spaces. These air spaces, along with the pigments and cell structures, contribute to the complex interaction with light, which not only results in the vibrant colour but also adds to the glossy and sometimes iridescent appearance of the petals.

What is the purpose of shiny petals?

The high density of pigments in this layer also plays a role in heat absorption, and aids in thermoregulation, benefiting the reproductive success of the flower. Combined with the starch granules also present in this layer, which reflect light due to their high refractive index, the pigmented layer contributes to the buttercup’s vibrant and glossy appeal to pollinators.