INTERPRETATION OF LIGHT CURVES OF FLUORESCENCE 1077 



on page 1051 that the curves would be easier to understand if the designa- 

 tions "with carbon dioxide" and "without carbon dioxide" were exchanged!) 



In the case of purple bacteria, several states seem to be needed to ex- 

 plain the light curves of fluorescence. First of all, the low value of if in weak 

 hght (the sigmoid shape) needs interpretation. It is probably associated 

 with the substitution of intercellular hydrogen donors for the external re- 

 ductants, which occurs while photosynthesis is slow. The coincidence of 

 the two curves in figure 28.29 seems to indicate that, when photosynthesis 

 is prevented by the absence of reductants, either chlorophyll accumulates 

 in one and the same form in the presence and in the absence of carbon 

 dioxide, or the two forms (e. g., HX-BChl-Z and X-BChl-Z) accumu- 

 lated under these conditions have a practically identical rate of energy 

 dissipation, h. In the presence of reductants, the two forms accumulated 

 with and without carbon dioxide (perhaps X-BChl-HZ and HX-BChl-- 

 HZ) possess, to the contrary, a very different fluorescence capacity. How- 

 ever, as the light intensity is increased, a further change in the composi- 

 tion of the complex occurs, leading to the crossing of the curves with and 

 without carbon dioxide. Figures 28.31-28.35 confirm that the absence of 

 reductants causes (in the presence as well as in the absence of carbon di- 

 oxide), the accumulation of a form with considerably increased capacity 

 for fluorescence (which may be X-BChl-Z, or AC02-BChl-A'). 



An alternative explanation of the effect of reductants on fluorescence 

 can be given on the basis of Franck's concept of "self-narcotization." 

 Franck assumes that reductants such as hydrogen or thiosulfate intervene 

 in bacterial photosynthesis by reducing the "photoperoxides" formed by 

 the primary photochemical process. If the reductants are deficient, the 

 peroxides accumulate and produce the "narcotic," that blankets the 

 chlorophyll and causes fluorescence to become stronger. The absence of 

 C'Oa has less effect in bacteria because they are studied under anaerobic 

 conditions, permitting no photoxidation. Wassink, Katz, et al. (1938, 

 1942,1949) explained the effect of reductants by assuming that an "energy 

 acceptor," capable of taking light energy over from bacteriochlorophyll, 

 thus quenching its fluorescence, can be formed exclusively by enzymatic 

 transformation of the rc(hictants. They followed that CO2 must have no 

 effect on fluorescence at all — whii-h is not true. 



The effects of cyanide and of loio temperature on fluorescence (and photo- 

 synthesis) can often be explained by assuming that the primary effect 

 of both is the retardation of the carbon dioxide supply processes. However, 

 we have seen, in part A, that not all tlie experimental results on cyanide 



