Kolkata, West Bengal
Many people are curious. It's written in school textbooks. Yet, with wide-eyed wonder, they ask, why do flowers have so many colours? The curiosity is natural, and so is the question. But the answer isn’t simple.
If you dye a shirt red, it will become red. The same principle applies to flowers. The petals of flowers also contain dye—meaning colouring substances. These dyes or pigments are stored within the cells. Different flowers contain different pigments. The leaves, flowers, and fruits of a plant get their colour from various pigments. That’s what our eyes perceive as different colours.
But at night, in the dark, we don’t see them. When the sun rises, we do. So, we need light to see them. What kind of light? Visible light—the small portion of sunlight that we can actually see. This visible light contains seven colours: violet, indigo, blue, green, yellow, orange, and red. In Bengali, we remember them as "Beniashokola"; in English, as “VIBGYOR” an acronym made of the first letters of the colours. This pattern of colours is called a spectrum in scientific terms, more precisely, the line spectrum of sunlight.
Each of these spectral rays has a specific wavelength or frequency. Light is a kind of wave, an electromagnetic wave. Waves have peaks and troughs. The distance between two troughs is the wavelength. The wavelength of red light is different from that of violet. Red light has the longest wavelength and the lowest frequency. The higher the frequency, the shorter the wavelength. Violet has the highest frequency.
We cannot see all the wavelengths that come from the sun. We only perceive light with wavelengths between about 400 to 800 nanometres. The optic nerve at the back of our eye’s retina carries the sensation of this light to the brain.
Sunlight, which looks white, is actually a mixture of many colours (chromatic light). It’s a blend of seven different wavelengths (or frequencies). Among them, violet or red light is called monochromatic light, as each has a fixed wavelength.
So why do we see colourful flowers, fruits, and leaves in nature? When sunlight hits an object, the object absorbs several colours of the light spectrum and reflects a few back into the environment. Our eyes perceive this reflected light and our brain registers it as colour.
To return to the question: the pigments within a flower absorb certain wavelengths of sunlight. Some of those wavelengths are reflected. That reflected light is what we see as the colour of the flower. So, it is the pigments that give flowers their colour.
There are many types and names of pigments. But chemically, they are mainly of three types: porphyrins, carotenoids, and flavonoids.
Among these, the most important pigment in the porphyrin group is chlorophyll. We’re very familiar with it. Leaves are green because of chlorophyll. But the colours of flowers and fruits come from the other two types, flavonoids and carotenoids. Inside plant cells are chloroplasts, where complex chemical reactions produce chlorophyll molecules. There are various types of chlorophyll—chlorophyll a, chlorophyll b, and so on. Leaves get their green colour from chlorophyll, which also plays many other roles. It absorbs sunlight, draws carbon dioxide from the air and water from the soil, and produces glucose, the plant’s food. About 75% of all plant pigments are chlorophyll a. Chlorophyll b is more common in plants that grow in areas with less sunlight.
The main chemical responsible for colouring flowers, fruits, and vegetables is carotenoid. It produces yellow, orange, and red shades. Bright yellow-orange flowers get their colour from carotenoid compounds. Similarly, carrots and pumpkins owe their orange colour to a carotenoid called beta-carotene.
In ornamental plants, red or yellow leaves are also due to carotenoids. How? When there's very little sunlight, chlorophyll molecules start breaking down, and carotenoids form in their place. Carotenoids absorb light with wavelengths between 400 and 600 nanometres and reflect light in the yellow, orange, and red range. That’s why we see those colours.
But we can't wrap up the discussion on carotenoids just yet. There are two main types: carotene and xanthophyll. Carotene includes pigments like beta-carotene and lycopene. Lycopene is the pigment that gives flowers and fruits their red colour. Carotenoids also assist in photosynthesis. They work as antioxidants, meaning they prevent damaging oxidation reactions in plant cells.
The main compound in the flavonoid group is anthocyanidin. It’s worth mentioning briefly. It’s essentially a polyphenol compound. Most flower colours from orange and red to violet and blue are produced by anthocyanidins.
Besides pigments, other factors affect the colour of flowers, fruits, and leaves: the amount of pigment present, environmental temperature, the pH level of the soil, and genetic influences.
Specific genes determine which type of pigment will be present in a flower. The characteristics of these genes vary from species to species. That’s why even flowers from the same species can bloom in different colours.
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