Plant Growth Regulators: Abscisic Acid

Abscisic acid (ABA) is a growth inhibitor, often called the stress hormone.
It induces and maintains dormancy in seeds and buds, so that seeds do not germinate during unfavourable conditions such as drought or winter.
Gibberellins break seed dormancy by acting against ABA. Ethylene and IAA are not the main causes of seed dormancy.
So seed dormancy is due to abscisic acid.

Abscisic acid (ABA) is a growth inhibitor. It promotes senescence (ageing) of leaves and other organs, and abscission (shedding) of leaves, flowers and fruits.
Cytokinins have the opposite effect: they delay senescence (Richmond–Lang effect). Auxins and gibberellins also generally delay it.
So the hormone responsible for senescence is ABA.