EFFECT OF REDUCTANTS ON FLUORESCENCE 949 



The question whether the participation of water as reductant in photo- 

 sj'nthesis involves some preUminary transformations similar to those of 

 carbon dioxide remains open. The abundance of water in all cells may be 

 advanced in favor of the convenient assum]:)tion that, even if a transforma- 

 tion of this kind — c. g., hydration of a "water acceptor" — is needed before 

 water can act as a hydrogen donor, the rate of this reaction is high enough 

 to prevent it from playing a limiting role in photosynthesis. However, if 

 the binding of water, which we have sj'mbolized by H2O — > {H2O) in 

 scheme 7.1, and which we may now describe by the equation: 



(27.87) H2O + A' , A'H.O 



in analogy to equation (27.2), is an enzymatic process, the limited amount 

 of the enzyme (particularly in an appropriately inhibited state) may well 

 ])ecome a rate-limiting factor, despite the overabundance of the reactant 

 H2O in the cell. 



2. Effect on Yield of Fluorescence 



Hydration and dehydration of plant cells have a strong influence on the 

 intensity of chlorophyll fluorescence in vivo; but in this case, as in that of 

 gas exchange, it is difficult if not impossible to distinguish between the 

 (undoubtedly possible) direct kinetic effects, and the indirect disturbances 

 caused by changes in the colloidal structure of the pigment-protein-lipide 

 complex. We must therefore refer here to the description of the relevant 

 phenomena in chapter 24. 



The study again becomes much more fruitful when purple bacteria are 

 used. The supply of reductants, such as H2, H2S or H2S2O3, has been found 

 to strongly affect the yield of fluorescence of bacteriochlorophyll in these 

 organisms. The light curves of fluorescence given in figs. 28.31, 28.32 and 

 28.33 (taken from the work of Wassink, Katz and Dorrestein 1942, on 

 Chromatium) illustrate this phenomenon. The plots represent the fluores- 

 cence intensity, F, (not yield, <p) as a function of the incident light intensity 

 /, with the concentration of the reductant. [HR], as parameter. Figure 

 28.31 shows the comparative effect of three different reductants; figure 

 28.32, a set of measurements with different concentrations of thiosulfate; 

 and figure 28.33 a similar set with different hydrogen pressures. In 

 both cases, increased concentration of the reductant causes an extension 

 of the initial, linear part of the fluorescence curve (which corresponds to the 

 "low light" yield of fluorescence, ip\). Figure 28.34 shows the critical 

 intensity," I^ (defined in fig. 28.27) as a function of the thiosulfate concen- 

 tration; the curve has a great similarity nith the corresponding gas ex- 

 change curve (fig. 27.12). It shows that deprivation of the reductant can 



