CONCENTRATION OF REDUCTANTS 943 



organisms. The deprivation of rediictants, on the other hand, has a strong 

 effect, because abundant "narcotic" is produced in light by the action of 

 accumulated, unreduced photoperoxides. 



One may ask: why, then, should the fluorescence-stimulating effect 

 of the deficiency of reductants persist also in the absence of carbon di- 

 oxide? No photoperoxides should be formed under these conditions. 

 Franck explains this paradox by questioning the efficiency with which 

 CO2, produced by fermentation, is removed in these experiments. He 

 points out that some hydrogen is taken up by purple bacteria in light even 

 if no extra carbon dioxide is provided in the medium. (Incidentally, fer- 

 mentation could produce "narcotizing" acids also directly — and not via 

 the reduction of the fermentation-produced carbon dioxide in light.) 

 Removal of external carbon dioxide may have no effect on fluorescence in 

 the absence of reductants, because enough CO2 is produced by fermenta- 

 tion to prevent the photochemical process from becoming " [C02]-limited"; 

 instead, it remains "[reductant]-limited" {i. e., its rate and the concentra- 

 tion of the narcotic — and thus also the intensity of fluorescence — remain 

 limited by the rate of the reaction between the photoperoxides and the 

 reductants) . 



It will be noted that Franck's concept differs from the first picture 

 (presented on p. 941) in 3 ways: (1) the ultimate oxidant itself, ACO2, 

 rather than an intermediate, X, is supposed to be coupled with chlorophyll; 

 (3) no ultimate (or intermediate) reductant, ZH, is supposed to be asso- 

 ciated with chlorophyll, in such a way that its depletion, too, can cause an 

 increase in the yield of fluorescence ; and (3) all strong changes in the yield 

 of fluorescence are ascribed to narcotization by "half-oxidized" metabolites, 

 rather than to the depletion of reactants. These three assumptions are in- 

 dependent of each other; the third one, in particular, which is perhaps the 

 most important feature of Franck's interpretation, can be combined, if 

 desired, also with the picture of the primary photochemical process as light- 



induced tautomerization of the complex X- Chi HZ ( > HX.Chl.Z). 



C. Concentration of Reductants* 



1. Efifect on Rate of Carbon Dioxide Reduction in Bacteria 



The ultimate reductant in ordinary photosynthesis of green plants is 

 water. The activity of water in the cells can be changed by direct hydra- 

 tion and dehydration; or by immersion into solutions of different osmotic 

 pressure. Both treatments have a considerable effect on photosynthesis. 

 However, this effect cannot be treated as a kinetic phenomenon obeying 



* Bil)liography, page 963. 



