1466 



PHOTOSYNTHESIS IN INTERMITTENT LIGHT 



CHAP. 34 



This result can be interpreted in several ways. Pratt and Trelease,. 

 considered two possibilities: deuterium oxide acting as a poison, and deu- 

 terium oxide acting as a partner in the Emerson- Arnold reaction (with a 

 velocity smaller than that of ordinary water). They considered the sec- 

 ond alternative the more probable. Weller and Franck agreed, and said 

 that the retarding influence of deuterium oxide may indicate that the 

 reaction catalyzed by Eb involves the transfer of hydrogen atoms. (We 

 have repeatedly suggested that it may be a dismutation.) If it should be 

 confirmed that the shape of the P = f{td) curve in heavy water is similar 

 to that observed in the presence of cyanide, this explanation will have to be 



0.03 0.05 0.07 



DARK PERIOD, sec. 



0.09 



Fig. 34.20. Yield in flashing light as function of dark period in 

 H2O and D2O (after Pratt and Trelease 1938). 



changed. It is not necessary to revert to the other alternative of Pratt and 

 Trelease and to assume that heavy water acts as a poison; the more plau- 

 sible explanation is that the participation of deuterium oxide retards an- 

 other dark reaction — not the usually limiting Emerson-Arnold reaction — 

 to such an extent that it becomes rate-limiting. This reaction cannot be 

 the carboxylation (as in the case of cyanide poisoning) since the latter in- 

 volves no hydrogen, but it may be, e.g., a preparatory reaction on the "oxida- 

 tion side" of the primary photochemical process. We will see (section 8) 

 that a reaction of the latter kind actually is rate-limiting in so-called 

 "adapted algae," where hydrogen is supplied by a substitute donor, in- 

 stead of by water. 



