CHLOROPHYLL FLUORESCENCE AND PHOTOSYNTHESIS 825 



In the case of inhibition of photosynthesis by anaerobic incubation in 

 the dark, the "narcotic" appears to be a fermentation product, an acid, 

 since its effect can be destroj'ed by neutrahzation. The "long" induction 

 period, and the midday depression, maj^ be due to the accumulation of 

 similar acids. The "short" induction period, on the other hand, must be 

 associated more directly with the intrinsic mechanism of photosynthesis. 

 It has been suggested (by Gaffron and Franck, see chapter 33) that, in 

 this case, a "narcotic" is produced as a result of partial oxidation of a 

 metabolite (sugar?) by the first oxidation product of photosynthesis 

 ("photoperoxide" or, more generally, "oxj^gen precursor"); according to 

 Gaffron and Franck, a transient accumulation of this product occurs in the 

 first seconds of illumination, because of inactivation in the dark of the 

 enzjTne (or enzymes, which we called Ec and £J„ in chapter 7), required to 

 convert the oxygen precursor into free oxygen. Franck attributes to 

 similar "internal narcotization" also most, if not all, the fluorescence 

 changes produced by depriving purple bacteria of their specific reductants 

 (H2, H2S2O3, etc.). He sees the reason for the enhancement of fluorescence 

 caused by this treatment, not (or not primarily) in the stoppage of the 

 primar}^ photochemical process by lack of a reactant (and consequent in- 

 crease in both internal dissipation and fluorescence as suggested on p. 823), 

 but in the accumulation of photoperoxides (which in the normal course 

 of bacterial photosynthesis, are destroyed by the specific reductants), and 

 consequent production and deposition on chlorophyll of surface-active 

 (narcotic) substances. In this way, the rates of both the primary photo- 

 chemical process and the internal dissipation are reduced, and the chances 

 of fluorescence are correspondingly increased. 



The effect on the yield of fluorescence of the deprivation of green plants 

 (or purple bacteria) of carbon dioxide (or of cj^anide poisoning — both 

 treatments have the same primary result, namely reduction in the supply 

 of { CO2 1 to the photochemical system) is less pronounced than the effect 

 of the scarcity of the reductants; but, contrary to the belief of Wassink 

 and his co-workers, such an effect undoubtedly occurs (c/. chapter 27, p. 

 940. Here, again, the enhancement of fluorescence could be attributed 

 either to declining rate of the primarj^ photochemical process (and conse- 

 quent increase in the probability of both internal dissipation and fluores- 

 cence) ; or (as suggested by Franck and co-workers) to the formation of 

 an int(nnal "nar(-otic" by pfwloxidalion, which is known to set in when 

 plants deprived of carbon dioxide are exposed to light. In anaerobic 

 purple bacteria, photoxidation cannot occur and CO2 has less effect on 

 fluorescence. 



Franck sees in the formation of a protective layer of an internal narcotic 

 an important safety device the plants have developed to prevent destruc- 

 tive photochemical reactions from being sensitized by chlorophyll, when 



