106 THE FLOWERING PROCESS 



phytochrome. The reactions are summarized in the following 

 formula : 



Orange-red light 

 600 to 680 m/x 



R-phytochrome (Pgeo) ^ ^ F-phytochrome (P730) 



Far-red light 

 700 to 760 m/x 



Metabolically 



in darkness 



When the pigment is in the R-phytochrome form, the plant's 

 biochemistry, at least as it relates to flowering, is adjusted in some 

 way so that it is typical of a plant in the dark. Changing the pigment 

 to F-phytochrome induces biochemical changes which inhibit flower- 

 ing of short-day plants and promote flowering of long-day plants. An 

 essential part of the formula is the shift from F-phytochrome to 

 R-phytochrome in the dark (with no illumination by far-red light). 

 This shift fails to occur with extracted phytochrome in the test tube, 

 so it must depend upon the plant's metabolism. 



5. Extraction of Phytochrome (10, 19) 



Information about the reversible nature of the pigment should be 

 of considerable value in searching for it. One would look not only 

 for a greenish-blue pigment with a certain absorption spectrum, but 

 for a pigment which changes color (absorption spectrum) when it is 

 illuminated with red light and then changes back again when it is 

 illuminated with far-red light. With this information, researchers 

 initiated an intensive program aimed at isolation of the pigment from 

 plant tissue. It was ultimately the Beltsville group that succeeded. 

 At least 7 years were spent before any degree of success was obtained, 

 and the pigment has yet to be completely isolated and characterized, 

 although its presence in a test tube can now be clearly demonstrated. 



An immediate problem concerned the source of the pigment. If 

 one were to use cocklebur leaves, for example, the isolation of one 

 greenish-blue pigment present in very small quantities would be 

 extremely difficult in the presence of chlorophyll, a bluish-green 

 pigment present in very large quantities. So the approach was to use 



