The photoperiodic response to the red (~700 nm λ) and far-red (700-750 nm λ) wavelength of light is mediated by the phytochromes. Phytochromes are a superfamily of photosensory receptors found both in eukaryotes (plant, fungi) and prokaryotes (bacteria). They are present in plants and algae as dimers (monomer molecular weights 120 kDa). They are encoded by phytochrome (PHY) genes in plants and green algae.
Phytochromes absorb the red and far-red wavelength of light in the region of the visible spectrum. They are of two types i.e. Type I that activated by far-red light (700-750 nm λ) and Type II that is activated by red light (~700 nm λ).
