Besides photoperiod certain plants require a low temperature exposure in their earlier stages for flowering. Many species of biennials and perennials are induced to flower by low temperature exposure (0oC to 5oC). This process is called Vernalization. The term Vernalization was first used by T. D. Lysenko (1938).

Two main theories to explain the mechanism of vernalization are:

i. Hypothesis of phasic development ii. Hypothesis of hormonal involvement

According to Lysenko, development of an annual seed plant consists of two phases. First phase is thermostage, which is vegetative phase requiring low temperature and suitable moisture. Next phase is photo stage which requires high temperature for synthesis of florigen (flowering hormone).

According to Purvis (1961), formation of a substance A from its precursor, is converted into B after chilling. The substance B is unstable. At suitable temperature B is converted into stable compound D called Vernalin. Vernalin is converted to F (Florigen). Florigen induces flower formation. At high temperature B is converted to C and devernalization occurs (Figure 15.21).

The seeds are first soaked in water and allowed to germinate at 10o C to 12o C. Then seeds are transferred to low temperature (3oC to 5oC) from few days to 30 days. Germinated seeds after this treatment are allowed to dry and then sown. The plants will show quick flowering when compared to untreated control plants.

Reversal of the effect of vernalization is called devernalization.

1. Vernalization shortens the vegetative period and induces the plant to flower earlier.
2. It increases the cold resistance of the plants.
3. It increases the resistance of plants to fungal disease.
4. Plant breeding can be accelerated.