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chapter three t Photoperiodism: 



Attempts at Analysis 



Faced with the various phenomena of the previous chapter, 

 many investigators of photoperiodism have naturally tried to dis- 

 cover characteristics common to the various response classes, and 

 particularly to look for indications of whatever cellular and bio- 

 chemical mechanisms might be involved. Two major lines of such 

 research, by no means completely separate, are the subject of this 

 chapter. 



A. PHOTOPERIODISM AND LIGHT QUALITY 



So far, photoperiodism has been considered simply in terms 

 of white light versus darkness, but experiments with light quality— 

 different colors or wavelengths of light— have proved very valuable. 

 They have opened up photoperiodism itself to further manipula- 

 tion and linked it to a biochemical system, still incompletely 

 known, that is probably universal among plants except perhaps for 

 the bacteria and fungi. The main point of departure for this work 

 was the effectiveness of relatively brief, low-energy "light-breaks" 

 in opposing the flower-promoting or flower-inhibiting (for LDP) 

 effects of appropriate dark periods. 



ACTION SPECTRA FOR LIGHT-BREAKS 



In order to act on any process, light must first be absorbed. 

 Compounds, called pigments, that absorb visible light are generally 



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