a) Stress hormone is a hormone which induces various changes/responses in a plant when subjected to stress. Abscisic acid is a stress hormone and during stress conditions it induces changes like closing of stomata to prevent further water loss during scarcity of water, aids in seed germination when conditions are favourable and dormancy when not and other changes as and when required.

b) This hormone, abscisic acid gets released in the leaves from the mesophyll cells of the plant.