The hormone which occurs naturally in plant tissues and is commonly known as stress hormone is:

Abscisic acid (ABA) is called stress hormone
It is called a stress hormone because its synthesis is stimulated by environmental stresses like drought, water logging, etc.

• It induces various responses in plants against stress conditions:
increases the tolerance of plants toward various stresses
induces the closure of the stomata during water stress
promotes seed dormancy 
helps seeds withstand desiccation
important role in the growth and modification of root during nitrogen deficiency and drought
involved in the synthesis of dehydrins, osmoprotectants and protective proteins
inducing dormancy in plants at the end of the growing season
promotes abscission of leaves, fruits, and flowers
plant growth inhibitor and regulates abscission and dormancy

🔴Additional Information:
Phytohormones – 
Plant hormones (also known as phytohormones) are signal molecules
They are chemical compounds that regulate the various physiological processes in plants
Produced within the plant and can be transferred to other parts
They are produced in very small quantity
Hormones, along with extrinsic influences, play an important role in processes such as , seed germination, phototropism, dormancy, etc

• Other Plant Hormones and Their Functions:
Auxin – 
Auxin was the first plant hormone to be discovered.
They are produced in growing apices of roots and stems, young leaves, developing fruits and seeds.
Functions are Cell elongation , Apical dominance, Rooting (stem cuts and grafting), Induces parthenocarpy, Prevents prematurely dropping, etc. 
Gibberellin – 
There exist more than a hundred (100) known Gibberellins (GA1, GA2, GA3, etc.).
They are synthesized in apical tissues and youngleaves.
Functions are Stem elongation (bolting), Delays senescence, helps in Fruit setting, Seed germination, etc.  
Cytokinins – 
Cytokinins are naturally synthesized in root apices, shoot buds, young fruits, etc.
Functions are Cell division, Morphogenesis, assists in resolving auxin-induced apical dominance, slows leaf senescence, etc.
Ethylene – 
Ethylene acts as both inhibitor as well as a growth promoter
It occurs in gaseous form
The synthesis of ethylene occurs in the maturing fruit and tissues which are undergoing senescence.
Functions are: Ripening, Flowering, Epinasty, breaks the bud and seed dormancy and Abscission, etc.