**Phytochrome**

Phytochrome is a pigment that plays a crucial role in plant growth and development. It is a photoreceptor protein that absorbs red and far-red light wavelengths. This pigment helps plants sense and respond to changes in light conditions, such as the length of the day and the quality of light.

**Structure and Absorption Spectrum**

Phytochrome exists in two interconvertible forms: the red-light absorbing form (Pr) and the far-red-light absorbing form (Pfr). When Pr absorbs red light, it is converted into Pfr, and when Pfr absorbs far-red light, it is converted back to Pr.

The absorption spectrum of phytochrome shows two peaks: one in the red region (around 660 nm) and another in the far-red region (around 730 nm). These absorption peaks allow phytochrome to efficiently absorb red and far-red light, making it the primary pigment responsible for these light wavelengths in plants.

**Functions of Phytochrome**

1. Seed Germination: Phytochrome controls the germination of seeds by sensing and responding to the presence of light. When seeds are exposed to red light, the conversion of Pr to Pfr triggers the germination process.

2. Photomorphogenesis: Phytochrome regulates various aspects of plant development, including stem elongation, leaf expansion, and branching. In the presence of red light, Pfr promotes growth and development, while in the presence of far-red light, the conversion of Pfr to Pr inhibits growth.

3. Photoperiodism: Phytochrome also plays a crucial role in regulating flowering and flowering time in plants. The ratio of red to far-red light is used by plants to determine the length of the day, which in turn affects the flowering process.

4. Shade Avoidance: Plants can detect changes in light quality and quantity and adjust their growth patterns accordingly. Phytochrome helps plants respond to shade by promoting elongation of stems and leaves, allowing them to reach for more available light.

In conclusion, phytochrome is the pigment that absorbs red and far-red light in plants. This pigment plays a vital role in various physiological processes, including seed germination, photomorphogenesis, photoperiodism, and shade avoidance. By sensing and responding to changes in light conditions, phytochrome helps plants optimize their growth and development.

Phytochrome absorbs red of wavelength 660nm and far red of wavelength 730nm.