What do you understand by photoperiodism and vernalisation? Describe their significance.

The physiological mechanism for flower-ing is controlled by two factors: photoperiod or light period, i.e., photoperiodism and low temperature, i.e., vernalisation. Photoperiodism is defined as the flowering response of a plant to relative lengths of light/ dark period. Significance of photoperiodism is as follows:
1. Photoperiodism determines the season in which a particular plant shall flower. For example, short day plants develop flowers in autumn-spring period (e.g., Dahlia, Xanthium) while long day plants produce flowers in summer (e.g., Amaranthus).
2. Knowledge of photoperiodic effect is useful in keeping some plants in vegetative growth (many vegetables) to obtain higher yield of tubers, rhizomes etc. or keep the plant in reproductive stage to yield more flowers and fruits.
3. A plant can be made to flower throughout the year by providing favourable photoperiod.
4. Helps the plant breeders in effective cross-breeding in plants.
Enable a plant to flower in different seasons.
Vernalisation is promotion or induction of flowering by exposing a plant to low temperature for some time. Significance of vernalisation is as follows :
(i) Crops can be grown earlier.
(ii)Plants can be grown in such regions where normally they do not grow.
(iii)Yield of the plant is increased.
(iv)Resistance to cold and frost is increased.
(v) Resistance to fungal diseases is increased.