954 CONCENTRATION FACTORS CHAP. 27 



with pH in the case of thiosulfate, and its increase in the case of hydrogen, 

 could be interpreted as evidence of different pH dependence of the enzy- 

 matic processes by which the two reductants are supphed to the photo- 

 complex. 



The influence of pH on the fluorescence of bacteriochlorophyll also was 

 studied by Wassink and co-workers. With thiosulfate, the fluorescence 

 was about 30% more intense at pH 7.6 than at pH 6.0 (fig. 27.15A), a re- 

 sult that may be attributed to a lower supply of effective reductant (un- 

 dissociated H2S2O3). As expected, the trend is reversed with hydrogen 

 (fig. 27.15B), where the enzymatic supply of the reductant appears to be 

 better in the more alkaline solution. 



The effects on the rate of photosynthesis of various other inorganic ions 

 also were described in chapter 13. The only systematic data, showing the 

 rate in relation to the concentration of the inhibiting ion, were obtained by 

 Greenfield (1941, 1942); some of his curves were reproduced in figure 34 

 (Vol. I, page 341). Among new results pertinent to this field we may quote 

 the observations of Warburg and Liittgens (1946) that the reduction of 

 quinones by broken chloroplasts in vitro requires the presence of chloride 

 ions (c/. Chapter 35). 



2. Poisons and Narcotics 



In chapter 12 (Volume I) we discussed the influence of various inhibitors 

 on the rate of photosynthesis in a qualitative way, although some quantita- 

 tive data also were given, such as a curve showing the rate of oxygen libera- 

 tion by Chlorella as a function of the concentration of phenylurethan (fig. 

 30, page 323). Systematic kinetic study should include also rate measure- 

 ments with varying amounts of poisons, and some such measurements 

 have recently been reported, particularly by Wassink, Katz and Dorrestein 

 (1942). These investigators found that the photoreduction of carbon di- 

 oxide by purple bacteria {Chromatium D) is as sensitive to cyanide as is 

 the photosynthesis of Chlorella. With thiosulfate as reductant, some in- 

 hibition was observed even in weak light, such as 2 kerg/(sec. cm.^), while 

 in reduction with molecular hydrogen (as in normal photosynthesis), the 

 cyanide effect disappeared completely in weak light, / < 4 kerg/(sec. cm.^). 

 Wassink and Kersten (1945) found a considerable effect of hydrogen cya- 

 nide also on the photosynthesis of diatoms in weak light (~2 kerg/sec. 

 cm.-), a result which differs from many observations on green algae and 

 higher plants. 



Figure 27.16 shows the relation between P and [KCN], in Chromatium 

 (with hydrogen or thiosulfate as reductant) and in Nitzschia. Photo- 

 synthesis of Chromatium is half-inhibited by about 2.5 X 10 ~' per cent 

 KCN (with either hydrogen or thiosulfate). The pH (6.3 to 7.6) has an ef- 

 fect on inhibition in thiosulfate, but none in hydrogen, which makes the at- 



