Photoperiodism is the effect of light on flowering.
- Garner & Allard 1st reported the effect of light on flowering in Maryland Mammoth (variety of tobacco) & Glycine max.
On the basis of photoperiodic requirements, plants are divided into 3 types:
- Short day plants: in these plants, the dark period is very important for flowering. The dark period should exceed a critical length. Light has no effect on flowering. E.g., rice, sugarcane, potato, tobacco, soybean, Chrysanthemum.
- Long day plants: in these plants, the light period is very important for flowering. The light period should exceed a critical length. Darkness has an inhibitory effect on flowering. E.g., wheat, oat, spinach, radish, barley, lettuce.
- Day-neutral plants: these plants blossom throughout the year. Length of light or dark period has no effect. E.g., tomato, cucumber, sunflower, cotton, maize, pepper.
- Leaves receive the photoperiodic stimulus. The minimum number of appropriate photoperiods required for induction is 1 to 25.
- Effect of light on flowering is due to the presence of a photoreceptive pigment called Phytochrome. Discovered by Hedrick & Borthwick.
Phytochrome occurs in 2 forms that are inter-convertible. PR & PFR form.
- PR form absorbs red light (during the day) & changes into PFR,
- While the PFR absorbs far-red light (during night) & changes back to PR.
- PFR is the physiologically active form.
- In short day plant, PR form causes flowering.
- In long day plant, PFR form causes flowering.
- A flowering hormone called Florigen (hypothetical) is produced in the leaves that are transferred to the shoot apices which causes flowering.