Phytochrome and the Mechanism of Photoperiodism

Phytochrome : Hendricks and Borthwick (1952) found that flowering in short-day plants is inhibited if the long dark period is interrupted by even a brief flash of red light (660 nm) -- but that if this red-light flash is immediately followed by a flash of far-red light (730 nm), the inhibition is cancelled and the SD plant flowers normally. This observation led them to conclude that a specific pigment system in the leaf receives the photoperiodic light stimulus; that pigment protein is called phytochrome.

Phytochrome exists in two mutually interconvertible forms: a red-light-absorbing form (Pr) and a far-red-light-absorbing form (Pfr). Absorbing red light (660 nm) converts Pr to Pfr; absorbing far-red light (730 nm), or simply standing in darkness, converts Pfr back to Pr.

Pr --(660 nm, red)--> Pfr

Pfr --(730 nm, far red)--> Pr

Pfr --(darkness)--> Pr

During the daytime Pfr accumulates in the plant; it inhibits flowering in SDP but initiates flowering in LDP. During the dark period Pfr changes into Pr, which stimulates flowering in SDP and inhibits flowering in LDP.