The diversity of flower colour: how and why?

The diversity of flower colour has astonished artists, gardeners and scientists for centuries. Flowers generate colour by reflecting only a subset of the wavelengths which make up white light, resulting in a coloured appearance. This is achieved either through the use of chemical pigments which absorb certain wavelengths, or by the use of structures which reflect only certain wavelengths. Chemical colour has been well studied in plants, and the three major pigment groups are flavonoids, carotenoids and betalains. Spatial and temporal regulation of the synthesis of these pigments gives pattern and depth of colour to the flower. Combinations of pigments can result in variations in final flower colour, while the addition of metal ions and the alteration of cell pH can also infl uence the final wavelengths absorbed by pigments. Focussing light into the pigment-containing regions of the cell, using specialised cell shapes, also influences intensity of flower colour. Structural colour, including iridescence, is produced independently of pigment colour, and can overlay it. Flower colour itself is viewed as an advertisement to attract pollinating animals to the rewards (usually nectar) contained within the flower. This article concludes with an analysis of the long-running debate over whether specific flower colours attract specific pollinators, or whether all colours are simply different ways of attracting a wide variety of animals.