1. Phytochrome pigments receive photoperiodic stimulus and control flowering in plants.
2. Hendricks and Borthwick observed that in SDP flowering is inhibited if continuous dark period is interrupted even by a short duration or flash of red light of wavelength 660 nm.
3. If this interruption is again exposed to flash of far red light of wavelength 730 nm, then these plants flower.
4. From this they concluded that some pigment system in plant receives the photoperiodic stimulus.
5. These pigment proteins are called phytochromes and it exists in two interconvertible forms – P and P .
6. These pigments are located in cell membranes of green cells.
7. P is biologically active form and during daytime it gets accumulated in the plant.
