Question

Which hormone in the plant is commonly called "stress hormone"

Answer 1

Abscisic acid is also known as stress hormones 

Answer 2

Abscisic acid is also known as stress hormone. It is called so because it stimulates responses during stress conditions.  

For example, when the temperature is hot and the rate of evaporation is fast, it stimulates the closure of stomata cells to avoid further evaporation. Similarly, it also helps seed from being drying out in summers as well.