Question

Apical dominance is caused by
(a) Abscisic acid in lateral bud
(b) Cytokinin in the leaf tip
(c) Gibberellin in lateral buds
(d) Auxin in shoot tip

Answer 1

Hint: Apical dominance is a phenomenon that occurs in plants because of the improper secretion of plant growth regulators. The main axis of the shoot is affected in this situation as the main axis grows more than the rest of the plant.

Complete Step by Step Answer:
Apical dominance occurs when the apical buds dominate and inhibit the growth of the lateral axillary buds. This results in the plant becoming long and leggy. Such a plant does not have a firm stem to support its length and thus ends up bending. This phenomenon is caused by auxin which is released from the shoot tip.
Apical dominance can be prevented by removing the shoot tips of such plants i.e. decapitation. This principle is used in tea plantations and hedge- making as the growth of lateral buds will result in more leaves.
So, the correct answer is ‘Auxin in the shoot tip’.

Additional information: Let us look at the other plant growth regulators mentioned in the options:
Abscisic acid: Abscisic acid is mainly responsible for causing abscission and dormancy in the plant. In case of any stress, it protects the plant by inhibiting all activities and hence is known as the stress hormone.
Cytokinin: Cytokinin as the name suggests induces cytokinesis in plants. It was first isolated in the form of kinetin from autoclaved herring sperm DNA.
Gibberellin: It was isolated from a fungus named Gibberella fujikuroi hence the name gibberellin. They cause elongation of internodes and thus are used in sugarcane plants.

Note: Auxin refers to the compound indole-3-acetic acid (IAA) and other natural and synthetic compounds which are similar to it.
Auxin was first isolated from human urine as it is produced in our liver.
Cytokinin is used to break the effects of apical dominance because the relative ratio of auxin and cytokinin determines the extent of apical dominance.