CARBON-14 PREILLUMINATION EXPERIMENTS 221 



e. Under ordinary experimental conditions (i.e., in the concentra- 

 tion range occurring in ordinary experiments), the reactivity of re- 

 agents acting upon R increases progressively in the order: 02<C02 

 <H202, quinone. 



/. As corollaries of these assumptions we postulate: that the 

 "spontaneous" decay of R (occurring in N2 atmosphere) is a result of 

 reaction (IV) caused by the endogenous oxygen formed in reaction 

 (I),* and 



g. That the Hill reactions occurring in the presence of exogenous 

 O2 or quinone are results of reactions (I) and (II) and (III) or (I) 

 and (V).t 



On the basis of these assumptions we can explain the following 

 assemblage of experimental facts: 



1. The decay (in the dark) of R occurs even in N2 atmosphere, 

 but is accelerated markedly in the presence of exogenous oxygen or 

 quinone. 



2. Cyanide accelerates the decay of R, because it inhibits reaction 

 (III) by inactivating catalase, so that R is consumed by reactions 

 (II) and (VI) (instead of by reaction (IV) as is the case in the ab- 

 sence of cyanide) . 



3. When R reacts with C^^02, the C^* fixed is almost entirely in- 

 corporated in the carboxyl group of phosphoglyceric acid (and partly 

 in that of pyruvic acid), as was observed by Gaffron et al. (5). 



4. The rate of photosynthesis, which is initiated by reaction (I) 

 followed by reaction (X) , is decreased in the presence of cyanide and 

 oxygen (7,8), and the depressing effect of these substances is stronger 

 when the concentration of CO2 applied is low (because of the com- 

 petition between CO2, O2, and H2O2 for R). 



5. The Hill reaction caused by quinone is not inhibited by cyanide 



* In reaction (I) oxygen is supposed to be formed in direct proximity to R, 

 so that it reacts readily with R without being removed by such procedures as 

 bubbling of algal suspension with helium or N2. 



t It may be argued that this statement is contradictor}^ to the observation of 

 Mehler (6), who, using isolated chloroplasts, could not demonstrate the after- 

 effect of preillumination to provoke the reduction of Hill reagent (2,6-dichloro- 

 phenol-indophenol). It should, however, be pointed out that in his experiment the 

 preillumination lasted only V2 to 2 minutes, which, according to our observation, is 

 such a short time as to produce only V20 to Vb of the stationary R level to be 

 established in the hght. Conceivably, the insufficiency of the length of preillumina- 

 tion was the cause of the negative result obtained in Mehler's experiment. 



