THE PHOTOREACTION AND ASSOCIATED CHANGES 

 OF PLANT PHOTOMORPHOGENESIS 



STERLING B. HENDRICKS 



Agricultural Research Service, United States Department 

 of Agriculture, Beltsville, Maryland 



THE PHOTOREACTION 



The photoreaction for plant photomorphogenesis can be written sche- 

 matically as 



6500-6600 A max 



(1) Pigment R — Pigment FR 



7200-7400 A max 

 Darkness 



where R and FR refer to absorption of red and far-red radiation, 

 respectively. In writing the reaction in this way, emphasis is placed 

 solely on the regions of absorption of two pigment forms and the ulti- 

 mate reversible interconversion of the forms. With some uncertainty 

 the reaction can be written more specifically as 



6500-6600 A max 



(2) Pigment RH-. + A - Pigment FR + AH-. 



7200-7400 A max 

 Darkness 



which is a photohydrogenation or dehydrogenation, with A as the 

 hydrogen acceptor. 



The term "associated changes" in the title refers to possible varia- 

 tions in the components of reaction (2), particularly A and AH2. It 

 also includes the fact that either one or both forms of the pigment 

 enter into some type of reaction probably as a transferring agent, a 

 more general term than "enzyme," either for electrons or atomic 

 groupings including H. Physiological response is connected solely with 

 this action as a transferring agent, or as a hormone. 



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