Q.Explain why abscisic acid is often called the 'stress hormone' of plants, with particular reference to its effect on stomatal guard cells during water stress.

[!TLDR]

ABA is the 'stress hormone' because water stress raises its level, and it directly closes stomata to reduce water loss. …

Abscisic acid is called the plant's 'stress hormone' chiefly because its internal concentration rises sharply in response to adverse environmental conditions -- water shortage being the single most thoroughly studied trigger -- and because its resulting physiological effects genuinely help the plant cope with that specific stress rather than being an incidental side-effect. When a plant experiences water stress, rising ABA levels act directly on the guard cells flanking each stomatal pore, triggering the pore to close; because stomata are the plant's main route of water loss through transpiration, this closure substantially reduces further water loss at exactly the moment the plant can least afford it, buying the plant time to survive a period of limited water availability. …

Link the two halves of the explanation directly: (1) stress conditions raise ABA level, and (2) ABA's direct action on guard cells closes stomata, reducin …

Describing ABA only as an 'inhibitor' in general terms without specifically naming its direct action on guard cells and stomatal closure, whic …