The colour of new leaves of some plants is not green but while growing after few days they become green. Why?
Eral, Tamil Nadu.
A green leaf is green because of the presence of a group of pigments known as chlorophylls. When these pigments are abundant in the leaf cells, as they are during the growing season, the chlorophylls’ green colour dominates and masks out the colours of other pigments namely carotenoids that are present in the plastids along with chlorophylls.
But in the tropical countries, during the rainy season or in the growing season we can notice the appearance of red-coloured young leaves as we see in mango trees. As the leaves age, the red colour disappears and the leaves become green.
The red colouration of these young leaves is due to the development of anthocyanin pigments. These pigments are water soluble pigments and are located in the vacuoles of the long, thin palisade cells, which are stacked upright just under the upper epidermis. These pigments also temporarily colour the edges of some of the very young leaves as they unfold from the buds in early spring.
The development of anthocyanin pigments in young leaves is a kind of adaptation. This we can ensure through the uses rendered by these pigments to the plants. Anthocyanins act as a “sunscreen” by protecting cells from UV or blue-green light and thereby protect the young leaves from too much light. The red colour due to the presence of these pigments may hide leaves from red-colour-blind herbivores (the aphid insects), which would be especially important in young leaves, and the red colour may signal unpalatability to herbivores, since phenols are often produced along with the anthocyanins. The anthocyanins may also deter or kill fungus and thereby can protect the young leaves from fungal attack.
It is true that herbivores are brutal on young, tender leaves. It is also been said that herbivory rates are higher in tropical forests than in temperate ones and that, in contrast to leaves in temperate forests, most of the damage to tropical leaves occurs when they are young and expanding. The protection of young leaves from high intensity of light is mostly needed for tropical plants than the temperate plants. That is why the tropical pants are developing differently coloured young leaves.
Editor, Research Journal of Biological Sciences (Autonomous)
J.J. College of Arts and science
Pudukkottai, Tamil Nadu