The pigment involved in red-far red light interconversion is

The correct option is Phytochrome.

Phytochrome is a specialized photoreceptor pigment found in plants, bacteria, and fungi. It plays a critical role in detecting light and regulating various developmental processes, such as seed germination, seedling elongation, and flowering, in response to environmental light conditions.

The pigment exists in two interconvertible forms based on the wavelength of light it absorbs:
1. P_r (Red-light absorbing form): This form absorbs red light at a wavelength of approximately 660 nm. Upon absorbing red light, P_r is converted into the biologically active P_fr form.
2. P_fr (Far-red light absorbing form): This form absorbs far-red light at a wavelength of approximately 730 nm. Upon absorbing far-red light (or gradually in the dark), P_fr is converted back into the inactive P_r form.

This reversible interconversion between P_r and P_fr acts as a molecular light switch, allowing plants to sense the quality, intensity, and duration of light in their environment.