Phytochromes

Phytochrome is a blue-green photoreceptor pigment found in small amounts in plant cells. It is a protein attached to a light-absorbing group. Phytochrome exists in two forms that can be converted into each other by light:- P_r (also written P₆₆₀) absorbs red light, with a peak at a wavelength of 660 nm.
- P_fr (also written P₇₃₀) absorbs far-red light, with a peak at 730 nm.When P_r absorbs red light it is quickly converted into P_fr. When P_fr absorbs far-red light it is quickly converted back into P_r. In the dark, P_fr slowly changes back into P_r.What happens in day and nightSunlight contains more red light than far-red light, so during the day most phytochrome is converted into P_fr, which accumulates. During the night, P_fr slowly reverts to P_r. The amount of P_fr present therefore tells the plant how long the dark period has lasted.P_fr is the active formP_fr acts as a signal that changes gene expression. It moves into the nucleus and interacts with transcription factors, switching genes on or off. Responses controlled by phytochrome include:- the germination of some small seeds, such as lettuce, which need red light;
- flowering in response to day length;
- shade avoidance: leaves absorb red light but let far-red through, so plants in the shade of others have less P_fr and grow taller to reach the light.