Mechanism of Photoperiodism

For over a century, biologists have been searching for a flowering hormone, also known as "florigen," which is responsible for the growth changes leading to flower development. While evidence has been accumulated since 1936, when M.H. Chaila-khyan and his colleagues in Russia experimented with photoperiodic induction of flowering, the flowering hormone has yet to be isolated.

However, researchers have made significant progress in understanding the biological response of plants to light, which is crucial to the process of flower development. The photoreceptor responsible for this response is called phytochrome. Despite being present in small amounts, phytochrome has been successfully isolated from plant tissues.

Phytochrome is a protein attached to a non-protein part that is a blue-green pigment. It has two forms, PR and PFR. PR strongly absorbs red light at 660 nm, which causes the phytochrome molecule to change shape to the PFR form. PFR, on the other hand, strongly absorbs far-red light at 730 nm and reverts back to PR in the dark.

The PFR form of phytochrome, designated as P730, is less stable than PR, and it triggers physiological responses such as flowering. Since sunlight has more red light than far-red light, the phytochrome in a plant exposed to sunlight is a mixture of both PR and PFR, with PFR predominating during the day. During the night, PFR slowly reverts back to PR.

While the flowering hormone is yet to be isolated, the discovery of phytochrome has helped shed light on the intricate process of flower development and the biological response of plants to light.