When we talk about colour, began Mahua Ghara, an alumna of the Centre for Ecological Sciences at IISC, “what we essentially address is the way light falls on an object”.
Mahua, who was at Goobe’s Bookstore to talk about the colours of flowers in spring, continued, “Some of this light is absorbed and some, reflected. It then goes into the eye and the brain processes the image that hits the eye. As we know, whites reflect all the colour, black absorbs all the colour and yellow reflects yellow. This is how we see and animals may see more or less of the colour spectrum that we see,” said Mahua who worked on fig-wasp interaction at IISC before pursuing a post-doctoral programme at the Tel Aviv University in Israel. As part of her research there, she worked on the colours of Royal Iris flowers.
This being springtime in Bengaluru, there is a burst of colour on the trees with all the tabebuias flowering. Why are there so many colours? “This is because there are different animals which are useful to plants, in helping them reproduce by spreading the pollen. Colours are the plant’s way of attracting insects or animals, ranging from a honeybee to a hummingbird, a fly or a butterfly. Colour is something that an animal can see from a long way off, helping the animal find what it is seeking. Once the animal goes to the flower, it is rewarded with nectar and pollen. Bees, for instance, then remember the flower and go back to the hive to bring the other bees there.”
Each species is able to see different sets of colour, enabling them to identify the source of nectar more easily.
“Depending on the insect variety, the vision of each species could be slightly different. Bees see shades of blue, green and yellow, they don’t see red. Birds on the other hand, can see shades of blue, green, yellow and red. And so bees don’t visit red flowers while birds visit red flowers. We don’t know too much about the vision range of other animals because it’s hard to verify with experiments.”
Research has shown that bees can see in ultraviolet, which means they can see something that humans cannot. This, explained Mahua, enables them to see structures and patterns on flowers that are not visible to humans without a UV filter. These markers help the bee identify the region of the flower where the nectar is stored.
What about night blooming flowers, that are generally white or cream in colour and are significantly more fragrant?
“This is part of the plant’ strategy because during the day there are so many different colours and there is so much competition for the limited number of insects available to pollinate. Nocturnal pollinators don’t need colour because they use fragrance to attract nocturnal insects.”
Though there are many colours during pollination in the beginning, observed Mahua, insects tend to prefer certain colours in flowers which are then propagated more often.
“This is how genetic variation and selection take place for a particular colour. And pollinators go to flowers not just for nectar and food, they also sometimes seek shelter in the flowers from cold nights and they get a heat reward, in terms of warmth from the flower. They also sometimes lay eggs inside the flower, as in the case of the fig-wasp system.”
Flowers, of Lantana variety, for example, sometimes change colour, often when they are in bunches, observed Mahua, either to signify that they have already been pollinated or to simply attract attention better.
“A tree with flowers of different colours definitely attracts more pollinators. And at other times, the change in colour may simply happen due to abiotic or non-biological factors such as change in soil conditions. It could also be a strategy to fend off animals that may eat them.”
Where do flowers get their colours from? “Plants have a class of pigments like humans have melanin pigment. One of most popular kinds of pigments are flavonoids, among which anthocyanins are responsible for colours such as yellow (found in grapes), as well as pink, purple or orange. Carrots, on the other hand, have carotenoid pigments and beetroots and bougainvilleas have betalins.”
Colours in flowers could also be a result of combinations of these various kinds of pigments or the amount of pigment present in the flower. Mahua explained that a change in Ph or acidity levels or the presence of metal ions could also lead to differences in colour. Another major contributor to colour in flowers are cell shapes.
“If the flower has conical cells, for instance, it may be darker. If the cells are more flat, the colour tends to be lighter. When the shape of the cell changes, there is a difference in the angle at which light falls on the flower and so there is a difference in colour. Colour is not always due to the presence of a pigment.”
Cell shapes, added Mahua, as in the case of the peacock feather, can create different shades, even metallic tints or iridescence, which is observed in certain orchid species. These are structural colours.
