Abscisic acid or ABA is a plant hormone or phytohormone which acts as growth inhibitor. It promotes dormancy in seeds and buds,abscission (falling of leaves) and senescence in leaves and thus inhibits growth of plant.

Abscisic acid controls leaf fall and dormancy.

Abscisic acid (ABA) is a plant hormone that plays a crucial role in various physiological processes in plants. One of its main functions is to regulate leaf fall and dormancy.

Leaf Fall:
During the autumn season, plants undergo a process called leaf senescence, in which older leaves deteriorate and eventually fall off. Abscisic acid is involved in this process by promoting leaf abscission, which is the shedding of leaves from a plant.

1. Abscisic acid inhibits cell growth and division: During leaf senescence, the production of auxin, a growth-promoting hormone, decreases. This reduction in auxin levels triggers an increase in abscisic acid concentration. A high concentration of abscisic acid inhibits cell elongation and cell wall formation, leading to the detachment of leaves from the plant.

2. Induces the synthesis of enzymes: Abscisic acid stimulates the synthesis of enzymes such as cellulase and pectinase. These enzymes break down the cell wall components, allowing the cells in the abscission zone to separate and the leaf to fall off.

3. Promotes the closure of stomata: As the autumn season approaches, plants need to conserve water. Abscisic acid triggers the closure of stomata, the small openings on the surface of leaves, to reduce water loss. This closure helps in the process of leaf abscission.

Dormancy:
Dormancy is a period of reduced metabolic activity in plants, usually during unfavorable environmental conditions such as cold temperatures or drought. Abscisic acid is a key player in inducing and maintaining dormancy in plants.

1. Inhibits growth: Abscisic acid inhibits cell elongation and cell division, preventing the growth of shoots and roots. This dormancy state allows plants to conserve energy and survive harsh conditions.

2. Regulates bud dormancy: Abscisic acid controls the dormancy of buds, preventing them from opening and initiating growth during unfavorable seasons. It suppresses the production of gibberellins, another plant hormone that promotes bud growth. By inhibiting gibberellin synthesis, abscisic acid helps maintain bud dormancy.

3. Influences seed dormancy: Abscisic acid also plays a role in seed dormancy. It inhibits seed germination by affecting various physiological processes, such as inhibiting the synthesis of gibberellins and promoting the synthesis of storage proteins. This ensures that seeds remain dormant until favorable conditions for germination are present.

In conclusion, Abscisic acid controls leaf fall by promoting leaf abscission and regulates dormancy in plants by inhibiting growth and influencing bud and seed dormancy.