LIGHT AND THE FLOWERING PROCESS: THE NATURE OF THE PHYTOCHROME SYSTEM

The relative lengths of alternating periods of lightness and darkness can affect the change from the vegetative to reproductive development in plants. This phenomenon is called photoperiodism and, as in any light controlled process, it is possible only if a substance is present that absorbs the light. This substance has been isolated and is a blue chromoprotein, the phytochrome (P). The aim of this note is to give some information about the mature of P, the localisation in the cell, and the biochemical rôle of P.

P differs from all others common plant pigments by existing in two light-absorbing forms, each of wich can be converted reversibly into the other as a result of light absorption:

Formula available in full text only

The first question is:how are possible these changes at a molecular level? The chromophore of P has been separated from the protein and appears to be an open chain tetrapyrrole, as are the phycobilins of blue-green and red algae.² Light energy can cause molecular changes known as photoisomerizations: cis-trams isomerization about a double bond; changes in carbon-carbon bonds, for example ring cleavage or formation; or a double bond shift, like that implicated in the interconversion between the two forms of phytochrome, which are both higly conjugated. The Pr form has 7 double bonds in conjugation, but not all are in the conjugated system, since only those alternating with single bonds along the molecule are part of the main conjugation and only slight affects the wavelengths position for maximum absorption? The Pfr form has a total of 10 double bonds in the conjugated system, including 2 double bonds in the carbonyl groups on either side of the chromophore. On the basis of the extensive conjugation, we