942 CONCENTRATION FACTORS CHAP. 27 



It was argued, however, in Volume I (page 167) that ACO2 and X- Chi -HZ 

 can be separated by intermediate oxidation-reduction systems, and, 

 nevertheless, an exhaustion of ACO2 may cause an accumulation of the pri- 

 mary oxidant, X, in the form HX, and consequent change in the intensity 

 of fluorescence. (A strike of longshoremen in America can cause tin ore to 

 accumulate at the pit heads in the East Indies.) 



Fluorescence observations on purple bacteria produced new evidence 

 bearing upon the relation of carbon dioxide to the fluorescent pigment 

 complex. 



Wassink, Katz and Dorrestein (1942) were strongly impressed by the 

 above-mentioned observation that changes in the concentration of re- 

 ductants affect the fluorescence of purple bacteria more strongly than 

 changes in the concentration of the oxidant (CO2), and that the former ef- 

 fect persists in the absence of carbon dioxide, while the latter disappears 

 when reductants were absent. They concluded that carbon dioxide does 

 not come into (direct or indirect) energy exchange with excited chlorophyll 

 at all, and that the excitation energy is taken up (directly or indirectly) 

 by the reductants (H2S, H2S2O3, H2 . . . in purple bacteria, H2O in green 

 plants). They thought that the small observed effects of CO2 on fluores- 

 cence are without real significance — an unjustifiable simplification. 



Franck and his co-Avorkers (1947, 1949), on the other hand, interpreted 

 the enhancing effect of the absence of reductants on the chlorophyll 

 fluorescence of purple bacteria as an indirect action caused by accumula- 

 tion of unreduced oxidation intermediates of photosynthesis ("photoper- 

 oxides"), and narcotization of the chlorophyll apparatus by the products 

 of action of these peroxides on cell metabolites. Using this picture, they 

 gave the following interpretation of the effect of [CO2] on yield of fluor- 

 escence, its variability with light intensity, and its disappearance in the 

 absence of reductants (in purple bacteria) : 



Lowering the CO2 concentration has an effect on the yield of fluorescence 

 if it makes the rate of photosynthesis [C02]-limited. When this is the 

 case, the chlorophyll complex becomes free (depleted of the oxidant, 

 ACO2), and light energy absorbed by it cannot be used for the primary 

 photochemical process. This itself should change the yield of fluorescence 

 (p. 941). How^ever, Franck considers this "denudation" effect as of minor 

 importance, as far as fluorescence is concerned, compared to the "self- 

 narcotization" which follows it. He postulates that whenever the photo- 

 sensitizer complex is deprived of carbon dioxide, photoxidation sets in (c/. 

 Vol. I, chapter 19), and produces a "narcotizing" intermediate (probably 

 a carboxylic acid) which settles on chlorophyll. 



This mechanism cannot come into play in purple bacteria, since these 

 are studied under anaerobic conditions; this explains why carbon dioxide 

 deprivation has a comparatively slight effect on fluorescence in these 



