What is phytochrome? Discuss its two forms and differentiate them?

Phytochrome is a photoreceptor pigment in plants that regulates various physiological processes in response to light. It exists in two interconvertible forms:

1. Pr (phytochrome red):

• Absorbs red light (660 nm)

• Inactive form

• Predominant in dark-grown plants

2. Pfr (phytochrome far-red):

• Absorbs far-red light (730 nm)

• Biologically active form

• Triggers plant responses

Key differences:

• Absorption spectra: Pr absorbs red light, while Pfr absorbs far-red light

• Stability: Pr is more stable than Pfr

• Biological activity: Pfr is the active form, initiating responses

Phytochrome conversion:

Red light → Pr converts to Pfr

Far-red light → Pfr reverts to Pr

This interconversion allows plants to detect light quality and quantity, regulating processes such as:

• Seed germination

• De-etiolation

• Shade avoidance

• Flowering time

Phytochrome's dual forms enable plants to respond dynamically to their light environment, optimizing growth and development.