Which phytohormone is often referred to as the 'stress hormone' due to its role in stimulating stomatal closure?

The correct option is Abscisic acid.

Explanation

Phytohormones are chemical messengers that regulate plant growth, development, and responses to environmental stimuli. They are broadly classified into growth promoters and growth inhibitors based on their physiological effects.

Option Analysis:

• Abscisic acid Abscisic acid (Correct): Abscisic acid (ABA) is widely referred to as the 'stress hormone'. Its production increases significantly during adverse environmental conditions, such as drought or water stress. ABA triggers the closure of stomata to reduce water loss through transpiration and increases the plant's tolerance to various stresses. It also plays a key role in inducing seed dormancy.
• Auxin Auxin (Incorrect): Auxins are primarily associated with cell elongation, apical dominance, and root initiation. They are growth-promoting hormones.
• Cytokinin Cytokinin (Incorrect): Cytokinins promote cell division and differentiation. They help in delaying leaf senescence and overcoming apical dominance, often acting antagonistically to ABA.
• Gibberellin Gibberellin (Incorrect): Gibberellins are responsible for stem elongation, breaking seed dormancy, and promoting fruit development.

Key Takeaway: Abscisic acid (ABA) acts as a plant growth inhibitor and stress hormone by stimulating stomatal closure during water stress and inducing dormancy in seeds and buds.