Auxin

Plant Hormone - Auxin

Auxin involves in tropism, apical dominance, and leaf abscission.

In phototropism, auxin in the shoot tips moves away from sunlight. This is triggered by the light receptors in plants causing auxins to redistribute to the dark side of the plant. As a result, the shoot tip of the plant which receives direct sunlight has the least amount of auxin and the opposite part contains the highest amount of auxin. This high concentration of auxin promotes more cell division and elongation, thus the shoot tip with high auxin concentration bends towards light. This is known as positive phototropism.

The same action occurs in geotropism as in phototropism. Auxin is accumulated in the lower side of the root in response to the gravitational force. In roots, a high concentration of auxin inhibits cell division and elongation and as a result, the lower side of the roots becomes shorter and grows downwards. This is known as negative phototropism. 

Auxins are produced in the stems, buds, and root tips. It mainly involves maintaining the epical dominance of plants. The lateral buds remain dormant as long as apical meristem produces a sufficient amount of auxin to the plant. When this apical meristem is removed, the lateral buds break their dormancy and begin to grow. This causes the plant to become bushier.

The presence of auxin in lateral buds inhibits fruits and leaf abscission. Young leaves and fruits produce auxin within them. Therefore, they remain attached to the plant. With time, the amount of auxin production reduces in leaves and fruits and as a result, the auxin level declines. Then an abscisic layer is produced at the base of the petiole or fruit stalk and this causes abscission.