Why Do Some Plants Have Coloured Leaves?

There are many leaves in the plant kingdom that defy the natural green to splash colors of all hues. Purple-leaved Fennel, Purple Passion Plant, Snake Plant, Calathea, Aluminium Plant, African mask plant, and the all-season crotons! All of them embellish themselves with colours so exotic that one cannot pass them by without noticing. It is common that the new shoots and leaves of many plants brightly parade in purple or red. The shoots however get clad in green as they grow. We all know that the plants bear multi-coloured flowers to attract pollen-carrying insects and birds to facilitate reproduction. However, the leaf colour does not have such a crucial role to serve. Nature has some very different purposes in mind, it seems.

The shoots of many plants are either red or purple. The colour works as an invisible cloak that protects these delicate plant parts from sun damage. Anthocyanin is the pigment that gives leaves red or purple colour. It is the sunscreen of these newborn beauties and scientists believe that chlorophyll literally hides behind this mask to protect itself from sharp sunlight. There is still much ambiguity surrounding the reason why shoots or leaves of plants have colours other than green. One hypothesis is that the leaves that bask in strong sunlight are those that display non-green colours. Another theory is that anthocyanins are strong antioxidants and they help the plants to remain winter-ready. Then there are the tropical rainforest plants that have leaves with purple undersides the utility of which to the plant is still not clearly known.

Scientists say that the red or purple colour hides the shoots and leaves from the prying eyes of the predators which include pests as well as bigger herbivores. Studies have shown that red leaves get less devoured by herbivores when compared to green ones. Phenolic, which is a resin present in high concentrations in red or purple leaves, could be another reason why herbivorous insects and animals do not want to eat them. It is also postulated that purple undersides of certain tropical leaves have the same function- to protect them from fungal attacks, as it is a big threat in the humid microclimate of a tropical forest. The herbivorous insects are attracted to green colour than red or purple. Hence, colours other than green reduce insect attacks.

The differently coloured leaves still can carry out photosynthesis as efficiently as their green counterparts because they have a sufficient quantity of chlorophyll in them though masked by the colour pigments.

Come Fall, trees such as Maples, Aspens, and Sourwood literally start a riot of colours. It is basically in preparation for the ensuing period of dormancy made necessary by a hostile climate. What happens at the molecular level is that the anthocyanin pigment acts as a shade under which the chlorophyll is broken down and nitrogen from it is reabsorbed by the plant for surviving the hibernation time. There are also other factors that impact the fall colours of tree leaves such as temperature, humidity, and sunlight. Very low temperatures close to the freezing point in Autumn cause bright red colour in Maple leaves. A cloudy sky on the other hand will enhance the saturation of fall foliage colouration.

Different pigment combinations impart different shades to leaves. Good saturation of both anthocyanin and carotenoid results in brown colour. This is the case when the leaf is senile or fallen. Above all, based on the pH value of the anthocyanin pigment, the leaf exhibits colours ranging from red to blue. Bilirubin, the pigment that gives the yellow colour to the skin, eyes, (and urine of those who suffer from jaundice), is also present in some plants and gives a yellow hue to plant parts. For example, the seeds of the white Bird of Paradise plant have an orange-coloured fuzz on them, which is caused by Bilirubin.

When the leaves age, they turn yellow or red. When a leaf reaches the end days of its life, the chlorophyll in it breaks down allowing the plant to absorb back the nitrogen in it. Obviously, nature does not want the leaf to leave the body of the plant with the saved-up chlorophyll wasted. It is almost like the leaf giving its vital force back to the plant and crossing the rainbow bridge in an ultimate act of self-sacrifice.

The coloured plants are the favourites of any gardener. Science says that the variegated patterns on leaves, when they are just white and green, are caused by plant mutation. These plants are the albinos of the plant kingdom. And many of them are known air purifiers and make them an indoor gardener’s coveted possessions.

The amazing facts about leaf colour remind us that everything in nature has a purpose that will be revealed to a probing eye. Studying nature closely might thus give us an ability to understand better the purpose of our own lives and all the inter-connections involved.