Why is abscisic acid also known as stress hormone?
Abscisic acid (ABA) is known as the stress hormone because it increases the tolerance of plants to various kinds of stresses. ABA stimulates the closure of stomata, which helps plants conserve water during stressful conditions.
Additionally, ABA plays an important role in inducing dormancy in seeds and buds. By inducing dormancy, ABA helps seeds withstand desiccation and other factors unfavourable for growth. It acts as a general plant growth inhibitor and helps plants respond to stresses of both biotic and abiotic origin.
Explanation
The textbook explicitly states that ABA stimulates closure of stomata and increases tolerance to various stresses, which is why it is called the stress hormone. The context also mentions ABA's role in seed dormancy and helping seeds withstand desiccation and unfavourable conditions. These points directly answer why ABA earns this name.