Phytochrome is a bluish biliprotein pigment responsible for the perception of light in photo physiological process.
But at (1959) named this pigment and it exists in two interconvertible forms:
(I) Red light absorbing pigment which is designated as Pr and
(II) Far red light absorbing pigment which is designated as Pfr . The Pfr form absorbs red light in 660 nm and changes to Pfr . The Pfr form absorbs far red light in 730 nm and changes to Pr .
.The Pr form is biologically inactive and it is stable whereas Pfr form is biologically active and it is very unstable. In short day plants, Pr promotes flowering and P inhibits the flowering whereas in long day plants flowering is promoted by Pfr and inhibited by Pr form.
Pfr is always associated with hydrophobic area of membrane systems while Pr is found in diffused state in the cytoplasm. The interconversion of the two forms of phytochrome is mainly involved in flower induction and also additionally plays a role in seed germination and changes in membrane conformation.