Plants that depend on insect and other animal pollinators to carry their pollen from one plant to another are using more than their scent, colour and nectar to attract attention.
Botanist Dr. Beverley Glover and physicist Professor Ullrich Steiner say we should also pay attention to sophisticated structural mechanisms. "
Botanists have long known that there are many tricks with which plants attract pollinators but not all are visible to the human eye," she explained.
"We need to look at flowers like an insect looks at them, using sophisticated optical instruments and measuring ultraviolet (UV) reflection."
Taking a physical optical approach to understanding flowers is a new field, as Steiner explained: "Using modern optical methods such as spectroscopy with high spatial resolution we have been able to study the optical function of surface structures on plant petals and discover something new about how they give rise to structural colour in flowering plants."
"Botanists have long known that there are many tricks with which plants attract pollinators but not all are visible to the human eye," she explained.
"We need to look at flowers like an insect looks at them, using sophisticated optical instruments and measuring ultraviolet (UV) reflection."
Structural colour is the generation of a visible colour independently of chemical pigments by influencing the behaviour of light. A physical structure within the petal reflects a narrow bandwidth of light wavelength, allowing all other wavelengths to pass through to the interior, where they are absorbed. The effect is often a more intense and pure colour than a pigment creates.
Dr Silvia Vignolini, who works on the project in the Department of Plant Sciences and the Cavendish Laboratory, has been characterizing the incidence of structural colour among species of flowering plants. In recently published findings, she showed that the bright and glossy appearance of a buttercup's petals is the result of interplay between two extremely flat surfaces in the epidermal layer of the petal. Reflection of light by the smooth surface of the cells and the air layer below the epidermis effectively doubles the gloss of the petal and reflects a significant amount of UV light.
Many pollinators, including bees, have eyes sensitive in the UV region.
"To stand out against the green background, some flowers reflect light at a wavelength best suited to the photoreceptors in a pollinator's eye," explained Vignolini. In the case of some daisies, for example, the researchers have shown that the outer ring of ray florets (petal-like structures) reflects UV, whereas the inner ring does not; this, they speculate, could result in a 'bulls-eye' effect that draws the pollinator towards the centre.
Vignolini's work has demonstrated that some plants produce iridescence through ordered striations in the plant epidermis which, rather like the grooves of a CD, have the effect of creating colour through interference. Remarkably, bumblebees can learn that the shifting color is a signal of a rewarding flower and can remember that signal even when presented with novel flower colors. The results so far indicate that iridescence has evolved several times in the flowering plant kingdom and is more phylogenetically widespread than previously realized.
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