Some plant leaves are blue, but have no blue pigments. How does that work?

All leaves are green. Or are they? Lundquist et al recently surveyed a diversity of plant species and found at least 130 have iridescent blue leaves, making them more common than you might think. These ‘living jewels’, widely distributed in the dark understories of tropical rainforests, have photonic crystals in their leaves that produce intense, vivid metallic blue hues. 

Most of the visual world is coloured by pigments – reds, yellows, blues and greens — that are produced by molecules reflecting back colour. For example, chlorophyll is the pigment that makes leaves appear green. But, not all biological colours are pigment-based.

Some colours, called structural colours, arise when the shape of an animal or plant cell can bend incident light to create iridescence. The dazzling colours we see on bird feathers and on the backs of beetles and butterflies are examples of structural colours in animals. In plants, structural colours are found in flowers, fruits and even leaves, but until now it has been difficult to study its prevalence across plant genera. As Lundquist et al report:

“Most species are not in cultivation and are widely distributed across the tropics, often as narrow endemics restricted to inaccessible locations such as the more remote Venezuelan tepuis.” 

Examples of leaves that appear blue thanks to structural colour. Lundquist et al. 2024.

To overcome this obstacle, and perform the first broad investigation of photonic structure in leaves, Lundquist et al gathered living specimens from Brunei as well as dried material from the herbarium at Royal Botanic Gardens, Kew in the United Kingdom. They specifically hunted for nanostructures called helicoidal cell wall layers that are made up of cellulose molecules arranged in a helical screw shape. These helicoidal structures are common in leaf epidermis and can act as photonic crystals to produce structural colour.  

Herbariums, with their brown, dry leaf samples, may not seem like the most logical place to study plant colour, but helicoidal layers, if present, stay intact during the drying process and their reflectance can be studied after re-hydration with water overnight. Lundquist et al. measured whether the samples transmit either left- or right-handed circularly polarized light and observed helicoidal structures using electron microscopy.  

To their surprise, they could detect the helicoidal structures in 20 genera and they could recover the colour recorded when the herbarium sample was first collected:

“The ability of herbarium specimens to recover structural colour upon hydration was not anticipated prior to this study … yet most of the leaf samples investigated here regained the approximate leaf colours stated on the herbarium labels.” 

Most of the species reflected blue light, but green and/or violet light was also common.

Lundquist et al. predominantly found the helicoidal structures in ferns and monocots (Orchidaceae family and three Poales families, Cyperaceae, Eriocaulaceae and Rapateaceae).

Additionally, based on the phylogeny of these species, Lundquist et al. determined that helicoidal structures have evolved at least 38 times, in a remarkable example of convergent evolution. Lundquist et al. speculate that these structures may have an adaptive benefit for photoprotection and/or minimizing herbivory by confusing insect predators. 

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Lundquist, C.R., Rudall, P.J., Sukri, R.S., Conejero, M., Smith, A., Lopez-Garcia, M., Vignolini, S., Metali, F. and Whitney, H.M. (2024) “Living jewels: iterative evolution of iridescent blue leaves from helicoidal cell walls,” Annals of Botany, p. mcae045. Available at: https://doi.org/10.1093/aob/mcae045.

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