Photoperiodism is the phenomenon of physiological changes that occur in plants in response to relative length of day and night (i.e. photoperiod).
The response of the plants to the photoperiod, expressed in the form of flowering is also called as photoperiodism.
The phenomenon of photoperiodism was first discovered by Garner and Allard (1920).
Depending upon the duration of photoperiod, the plants are classified into three categories.
1. Short day plants (SDP)
2. Long day plants (LDP)
3. Day neutral plants (DNP)
1. Short day plants These plants require a relatively short day light period (usually 8-10 hours) and a continuous dark period of about 14-16 hours for subsequent flowering. These plants are also known as long-night plants
E.g. Rice, coffee, soybean, tobacco and chrysanthemum
2. Long day plants These plants require longer day light period (usually 14-16 hours) in a 24 hours cycle for subsequent flowering. These plants are also called as short night plants.
E.g. Wheat, radish, cabbage, sugar beet and spinach.
3. Day neutral plants These plants flower in all photoperiod ranging from 5 hours to 24 hours continuous exposure.
E.g. Tomato, cotton, sunflower, cucumber, peas and certain varieties of tobacco.
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