<P> Phytochromes are characterised by a red / far - red photochromicity . Photochromic pigments change their "colour" (spectral absorbance properties) upon light absorption . In the case of phytochrome the ground state is P, the indicating that it absorbs red light particularly strongly . The absorbance maximum is a sharp peak 650--670 nm, so concentrated phytochrome solutions look turquoise - blue to the human eye . But once a red photon has been absorbed, the pigment undergoes a rapid conformational change to form the P state . Here indicates that now not red but far - red (also called "near infra - red"; 705--740 nm) is preferentially absorbed . This shift in absorbance is apparent to the human eye as a slightly more greenish colour . When P absorbs far - red light it is converted back to P. Hence, red light makes P, far - red light makes P. In plants at least P is the physiologically active or "signalling" state . </P> <P> Chemically, phytochrome consists of a chromophore, a single bilin molecule consisting of an open chain of four pyrrole rings, bonded to the protein moiety . It is the chromophore that absorbs light, and as a result changes the conformation of bilin and subsequently that of the attached protein, changing it from one state or isoform to the other . </P> <P> The phytochrome chromophore is usually phytochromobilin, and is closely related to phycocyanobilin (the chromophore of the phycobiliproteins used by cyanobacteria and red algae to capture light for photosynthesis) and to the bile pigment bilirubin (whose structure is also affected by light exposure, a fact exploited in the phototherapy of jaundiced newborns). The term "bili" in all these names refers to bile . Bilins are derived from the closed tetrapyrrole ring of haem by an oxidative reaction catalysed by haem oxygenase to yield their characteristic open chain . Chlorophyll too is derived from haem (Heme). In contrast to bilins, haem and chlorophyll carry a metal atom in the center of the ring, iron or magnesium, respectively . </P> <P> The P state passes on a signal to other biological systems in the cell, such as the mechanisms responsible for gene expression . Although this mechanism is almost certainly a biochemical process, it is still the subject of much debate . It is known that although phytochromes are synthesized in the cytosol and the P form is localized there, the P form, when generated by light illumination, is translocated to the cell nucleus . This implies a role of phytochrome in controlling gene expression, and many genes are known to be regulated by phytochrome, but the exact mechanism has still to be fully discovered . It has been proposed that phytochrome, in the P form, may act as a kinase, and it has been demonstrated that phytochrome in the P form can interact directly with transcription factors . </P>

What wavelengths of light are absorbed by phytochromes