Abscisic acid is called a stress hormone because

ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone.

Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called respiratory climactic. Ethylene promotes rapid internode/petiole elongation in deep water rice plants.

Cytokinins help overcome the apical dominance.