PHYTOCHROME SYSTEM

=Phytochrome System= [image:http://i.imgur.com/vrM2iXi.png?1] '''Plants recognise the changing day length using a specialised type of pigment known as a Phytochrome'''. This pigment has two interchangeable forms. One form '''P~665''' (also called '''PR''') is '''inactive''' while the other form '''P~725~''' (also called '''PFR''') is '''active''' and can either promote or inhibit flowering. White light or red light, rapidly converts P~665~ to its active form P~725~. However, this occurs so rapidly that all of the phytochrome is converted into its active state regardless of day length. However, the conversion of the active form P~725~ back into the inactive form P~665~ only occurs during a prolonged, uninterrupted period of darkness. Because this is a slow process, the inactive form can only accumulate if the plant experiences a long night. Alternatively, far-red light causes the rapid conversion of the active form P~725~ back into the inactive P~665~, thus having the same effect as an extended period of uninterrupted darkness. ===Phytochrome in Long Day Plants:=== •Short nights mean that the active P~725~ persists and it '''promotes''' flowering. •Long nights mean that all the active P~725~ is converted into inactive P~665~, thereby preventing flowering. ===Phytochrome in Short Day Plants:=== •Short nights mean that the active P~725~ persists and it '''inhibits''' flowering. •Long nights mean that all the active form P~725~ is converted to the inactive P~665~, thereby allowing the plant to flower. ''Neither form of the Phytochrome directly stimulates flowering, the two forms act as a switch stimulating the release of a hormone that induces flowering. Phytochrome pigments are found in the leaves of plants and if a single leaf is exposed to the correct photoperiod, this can stimulate the release of hormones, causing the whole plant to flower.''