Plants delight us with their diverse spectrum of colours, the pigments and processes that create them, and the functions which they provide.
While the major colour associated with plants is green, they also provide an extensive palette of shades and hues. At the end of what has been a depressing and drab year, it seemed a good time to reflect on the chromatics of plant biology.
Starting at the high frequency end of our visible spectrum, violet leaves are an unfortunate sight for researchers who forget to water their Arabidopsis trays. The soluble pigments responsible for such a deep colour are anthocyanins1, of which several hundreds of variations exist, each one with a slightly different absorption spectrum. Their biosynthesis from the amino acid phenylalanine is induced by stress, and they are thought to protect the leaves from further oxidative damage. Plants also use anthocyanins to give their leaves, flowers or fruits a purple pigmentation, perhaps because insect pollinators have greater sensitivity to short-wavelength light. These plants are appreciated by horticulturalists because they add a touch of contrast in a predominantly greenish garden.
Indigofera tinctoria is an unremarkable leguminous shrub that grows worldwide in hot climates. But this plant contains a treasure for which humans have fought and travelled for millennia: a deep blue pigment for dyeing textiles. The indigo that can be extracted from the leaves after fermentation and precipitation originates from an indole precursor and was first named in India, from where it was mostly imported along the ancient Silk Road. Indigo can also be extracted from other plants, but in ancient times the pigment was so rare and expensive that it was reserved for the aristocracy. It became much more popular once it was used to colour a sturdy cotton cloth called denim, and once an industrial synthesis method was developed in the late 1800s.
Colours are not always due to the spectral absorption properties of a chemical. Thin repetitive microstructures, like millions of small mirrors, can reflect specific light wavelengths by refraction and interference, depending on the viewing angle. These are called structural colours and are present in butterfly wings, bird feathers and also a few plants. The otherworldly glittering metallic blue of the fruits of the shrub Viburnum tinus are due to multiple consecutive layers of lipids embedded in the cell wall of epidermal cells2. And the iridescent, structural blue from the leaves of shade-loving Begonia species comes from the spacing of thylakoids inside the chloroplasts that increase the efficiency of photon capture in low light environments3.
In the middle of our visible spectrum sits green. Forests, grasslands and fields are mostly vast seas of green. The simple response to the question ‘why are leaves green?’ is that photosynthetic pigments such as chlorophylls and carotenoids use blue and red, and that the green light, being unused, is reflected away. As with most simple answers, it is only partially right. Green light is used by plants4, and the addition of a green wavelength can increase yields. In fact, many ‘non-photosynthetic’ pigments can absorb different types of light and can pass on the absorbed energy to chlorophyll. Cryptochromes, well known as blue-light-sensing proteins, can also detect green light.
Leaves turning yellow are generally a bad sign for houseplants and indicate a serious pest, nutritional or watering problem. But dying leaves are not always evidence of our bad home gardening skills. Autumn’s foliage colours are a sign of the reassuring permanence of yearly season cycles. Around September in the Northern Hemisphere, the shorter days send a signal to the deciduous trees. As the sunlight dims, green chlorophyll is degraded and recycled. The underlying pigments then gradually appear — it is the turn of yellow xanthophylls to shine. The leaves fall in a few weeks and winter arrives. Christmas is just around the corner.
One fruit often associated with Christmas are oranges. Once quite rare, they could only grow in southern latitudes, or when protected behind tall glass panels in luxurious orangeries. Sweet oranges are not a natural fruit but a hybrid between two citrus species, pomelo and mandarin5. Curiously, the fruit gave its name to the colour, which comes from carotenoid pigments that themselves got their names from a completely different crop. Today the worldwide production of citrus species is menaced by a bacterial disease which takes its name from a colour. In English, ‘citrus greening disease’ describes the fruits of affected plants which become stunted and unable to ripen after the vascular network of the tree is altered. Its name in Chinese, ‘huánglóngbìng’, means ‘yellow dragon disease’.
Another Christmas tradition is to offer poinsettias to friends and families. Native to southern Mexico, the latex of these members of the genus Euphorbia was used by the Aztecs as a medicine. In winter, the shrub flowers and develops modified leaves that can be a stunningly bright red. The contrasting combination of green leaves and scarlet bracts makes for perfect Christmas colours. It is also said that the shape of these red leaves is reminiscent of the star of Bethlehem, another link with the winter holiday.
The rainbow, a rare but inspiring natural wonder, has long been used as a symbol of finding peace and harmony in diversity. Colourful rainbow flags are hoisted to show pride and claim an identity, from the LGBTQ+ community to Andean peoples. As we close the door on an extremely unusual and chaotic year that has seen a tragic global pandemic kill over one million people, lockdowns, wars, terrorism, worsening economic conditions, conspiracies, contested elections and more, we sincerely hope that the rainbow will prove to be the emblem of 2021.
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Xu, Q. et al. Nat. Genet. 45, 59–66 (2013).