EFFECTS OF WATER AND INORGANIC IONS 1919 



dration under the heading of "Inhibitions and Stimulations," rather than 

 "Concentration of the Reactants." The implication that these effects 

 illustrate the general fallacy of approaching photosynthesis from the point 

 of view of physical chemistry rather than that of physiology, is an example 

 of ideological "self-inhibition" which has weakened Russian research in 

 photosynthesis ever since Kostychev had enunciated his "physiological 

 concept" of photosynthesis in 1931 {cf. p. 872). 



Effect of D2O. In chapter 11 (section 5) the influence of heavy water 

 on the rate of photosynthesis was described as similar to that of a catalytic 

 poison (decrease in steady rate in saturating light, no effect in limiting 

 light; decrease in yield per flash with brief dark intervals, disappearing 

 as the intervals are prolonged, cf. figs. 25 and 26). These conclusions were 

 confirmed by Horvitz (1954) who found, however, that with quinone as 

 hydrogen acceptor (instead of carbon dioxide) the rate of oxygen produc- 

 tion by Chlorella cells was affected by the substitution of D2O for H2O, at 

 all light intensities — i. e., in this case heavy water acted like a "narcotic" 

 rather than like a "catalytic" poison. To find out whether the effect of 

 D2O has anything to do with changes in the ionization of weak acids, the 

 effect of 7>H changes on the rate of the quinone reaction in Chlorella also 

 was compared in H2O and DoO. In this case the effect of heavy water was 

 restricted to high light intensity. Consequently, the D2O effect on the Hill 

 reaction in weak light cannot be attributed to changes in ionization; its 

 origin — and the reason why it does not occur in photosynthesis — remain 

 to be explained. 



(d) Inorganic Ions 



pH Effect. The effect of [H+]-ion concentration on the rate of photo- 

 synthesis (cf. chapter 13, section 2(a)) was again studied by Steemann- 

 Nielsen (1952) on higher aquatic plants and by Thomas (1950) on purple 

 bacteria. Steemann-Nielsen found that certain higher aquatic plants, 

 such as Fontinalis dalecarlica, can survive, and photosynthesize at the nor- 

 mal rate, for several hours, at pH values between 3.0 and 10.5. Thomas, 

 on the other hand, found a sharp maximum of bacterial photosynthesis (in 

 saturating light) at pH 7.3 (in Rhodospirillum rubrum, in 0.015 M sodium 

 butyrate). The effect of pH on the rate of the Hill reaction was described 

 in chapter 35 (cf. p. 1600). 



Ionic Inhibition Effects. Frenkel (1947) noted that endogenous respira- 

 tion of Chlorella was only slightly affected by uranijl chloride, while respira- 

 tion of added glucose was 80% inhibited by 10"^ mole/1, of this salt. No 

 inhibition of photosynthesis could be noted up to 10 ~^ mole/1. UO2CI2 — a 

 result that can be considered remarkable in view of the generally high 

 sensitivity of photosynthesis to heavy metal ions. 



