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).
TYPES OF PHOTOPERIODISM
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 daylight 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 ¢
In short day plants, the dark period is critical and must be continuous.
If this dark period is interrupted with a brief exposure of red light (660-665 nm wavelength), the short day plant will not flower.
2. LONG DAY PLANTS
These plants require a longer daylight 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.
¢ In long-day plants, the light period is critical. ¢ A brief exposure of red light in the dark period or the prolongation of the light period stimulates flowering in long-day plants.
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.
INTERMEDIATE CATEGORIES OF PLANTS
i. Long short day plants
¢ these are short day plants but must be exposed to long days during early periods of growth for subsequent flowering. E.g. Bryophyllum.
ii. Short –long day plants ¢
these are long day plants but must be exposed to short day during early periods of growth for subsequent flowering. E.g. certain varieties of wheat and rye.
PHOTORECEPTORS IN PHOTOPERIOD
all photoreceptors contain an organic non-protein component known as chromophore that serves as the primary site of photon absorption (Möglich et al. 2010). ¢
At present, six classes of photoreceptors are known:
PHYs, cryptochromes (CRYs), light– oxygen–voltage (LOV) sensors, blue light sensors utilizing flavin adenine dinucleotide (FAD), rhodopsins, and xanthopsins (Möglich et al. 2010).
SIGNIFICANCE OF PHOTOPERIODISM
¢hybridization experiments (florigen hormone).
¢Increase yield ¢Plants remain vegetative for a longer period (radish, carrot,etc.)
¢Annuals can be grown twice or thrice a year. ¢Prevention of winter dormancy & autumn leaf fall. ¢Strawberry-Increased stolon formation through long days.