Q.Define vernalization.

Vernalization is the requirement, in many biennial and perennial plants, for a period of low temperature exposure (roughly 0-5 degrees C) early in development before the plant will flower - a term ('spring-like') first used by T. D. Lysenko in 1938. Two theories explain the underlying mechanism. Lysenko's hypothesis of phasic development holds that development runs through a cold-and-moisture-requiring thermostage followed by a high-temperature-requiring photostage that synthesises florigen. Purvis's hypothesis of hormonal involvement (1961) proposes a defined biochemical chain instead: a precursor is converted by chilling into an unstable intermediate, which becomes the stable compound Vernalin at a suitable temperature, and Vernalin is then converted into Florigen, which induces flowering; high temperature at the intermediate stage instead diverts …