1598 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



by "dark" reduction or by photochemical reduction not involving water, 

 and (6) on the rate of back reactions, which may lead to the establishment 

 of a photostationary state short of complete reduction. Finally, deactiva- 

 tion of chloroplasts may cause the reaction to stop before all oxidant has 

 been exhausted. In the latter case, addition of new oxidant will not revive 

 the reaction. 



With quinone, the main obstacle to 100% utilization of the oxidant seems 

 to be its decomposition. Warburg and Liittgens (1946) reported yields of 

 the order of 85-90%; Arnon and Whatley (1949i) reported "100%" with 

 added 0.01 M KCl, and 36% without it (chard chloroplasts); Aronoff 

 (1946-) could obtain only 35% of the theoretical oxygen amount from 

 quinone. 



%dye reduced 



10 12 U 

 time in minutes 



Fig. 35.22B. Percentage reduction of 2,6-dichlorophenol indophenol 

 in illuminated chloroplast suspension as function of time (Wessels 1954). 

 Straight ascending segment indicates zero order reaction. 



With Hill's mixture, Holt and French (1946) obtained 93% of theoreti- 

 cal oxygen yield (calculated for the initial amount of ferricyanide) after 1 

 hr. in an atmosphere of nitrogen ; upon longer illumination, oxygen was 

 slowly consumed, probably through photoxidation. For the same reason, 

 not more than 85% of theoretical yield could be reached in air, and the sub- 

 sequent decline of oxygen pressure was much faster. 



With ferricyanide. Anion and Whatley (1949^) reported "100% yield" 

 with 0.01 M KCl, and only 19% without added chloride. 



With phenol indophenol, Arnon and Whatley (1949^ reported a "100% 

 yield" in the presence of KCl, and as much as 90% without added KCl. 

 Earlier, Holt and French (1948) had reported yields of the order of 85% 

 of the stoichiometric amount; however, the calculation was uncertain 

 because of the low content of the dye in the commercial product. 



Clendenning and Gorham (1950^) found that, with both quinone or 



