Which hormone promotes seed dormancy and breaks the dormancy in seeds ?

Explanation

The inability of seed to germinate or grow is called dormancy. It may be due to either external factors or internal factors.
Internal factors:
1) Immature embryo : The embryo has not reached morphological, maturity to germinate. Ex : Ranunculus.
2) Hard seed coat: In fabacea members, seeds have hard seed coats which prevent uptake of oxygen or water. It can be broken by scarification in which the hard seed coat is ruptured or weakened.
3) Chemicals : Seeds of some plants (tomato) contain chemical compounds which inhibit their germination.
External factors :
1) Low temperature treatment: Many seeds (polygonum) with not germinate until they have been exposed to low temperatures in moist conditions in the presence of oxygen for weeks to months. The practice of layering thj6 seeds during winter in moist sand- and peat is called stratification or prechilling.
2) Seeds of many domestic plants may be limited only by lack of moisture or warm temperature.