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A plant pigment that can detect the presence or absence of light and is involved in regulating many processes that are linked to day length (photoperiod), such as seed germination and initiation of flowering. It consists of a light-detecting portion, called a chromophore, linked to a small protein and exists in two interconvertible forms with different physical properties, particularly in the ability to bind to membranes. When exposed to daylight, inactive phytochrome absorbs red light (wavelength 660 nm), causing the protein part of the molecule to alter its folding (conformation) and making it bind more readily to plasma membranes. Also, the absorption spectrum shifts, so that it now absorbs light in the far-red region of the spectrum, at about 730 nm. This physiologically active form of phytochrome is called Pfr (for ‘far red’). In darkness, Pfr reverts over several hours to the inactive Pr (‘red’) form. See also photomorphogenesis; photoperiodism. Compare cryptochrome.

Subjects: Biological Sciences — Chemistry.

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