Any of a class of plant pigments that can detect the presence/absence or quality of light, particularly of red and far-red wavelengths. Phytochromes are involved in regulating many processes that are linked to day length (photoperiod), such as seed germination and initiation of flowering, and growth responses to shading. Each consists of a light-detecting portion, called a chromophore, linked to a small protein and exists in two interconvertible forms with different physical properties. In darkness phytochrome is virtually all in the form that absorbs red light (wavelength 660 nm), denoted P_r. When exposed to light, this P_r form is converted to the P_fr form, which absorbs light in the far-red region of the spectrum, at about 730 nm. Exposure to far-red light converts P_fr back to P_r. Because sunlight contains both red and far-red light, so long as the plant is exposed to light, the reversible interconversion between the two forms of phytochrome continues, reaching an equilibrium dependent on the relative amounts of red and far-red light. Conversion to the P_fr form exposes a nuclear localization signal sequence, so that it transfers from the cell cytosol to the nucleus. Here it interacts directly with some transcription factors and phosphorylates other proteins, triggering a signal cascade that changes the activity of multiple genes. For example, P_fr stimulates the transcription of genes responsible for the production of plant hormones such as gibberellins that are involved in seed germination. In darkness, P_fr reverts over several hours to the inactive P_r (‘red’) form. See also photomorphogenesis; photoperiodism. Compare cryptochrome.