In many plants, flowering is influenced also by temperature. In annuals, the flowering is primarily affected by photoperiod. The effect of temperature is secondary to light.
A biennial plant, on the other hand, grows only vegetatively during the first season and will not initiate flowering, until it is exposed to prolonged period of low temperature of the winter. The requirement of low temperature treatment for accelerating flowering is called vernalisation.
The site of vernalisation in the case of biennials and perennials is believed to be the growing point (apical bud). Cold treated shoot apices when grafted on to untreated plants induce the latter to flower. This property is attributed to cold induced production of a stimulus called vernalin. But vernalin remains as a hypothetical substance and has never been isolated from plants.
The plant hormone gibberellic acid can substitute the effect of low temperature treatment for germination, flowering and release of bud dormancy in many plants.
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Vernalisation effect is reversible and the reverse process is called devernalisation. If a vernalized seed or plant is kept under high temperature, the effect of low temperature treatment is completely removed. High temperature reversal can be counteracted if the duration of vernalisation treatment is increased. Devernalized plants can, however, be vernalized under low temperature treatment.