296 



LIBERATION OF OXYGEN 



CHAP. 11 



mained reversible, even after 15 hours. Craig and Trelease (1937) made 

 measurements in various water mixtures, and over a wide range of 

 temperatures, light intensities and carbon dioxide concentrations. The 

 yield was found to decline smoothly with increasing concentration of 

 deuterium oxide: analysis of this function showed that the three kinds 

 of water (H2O, HDO, D2O) participate in photosynthesis independently 

 of each other. The effect of deuterium oxide is strongest at the high 

 light intensities, and disappears in weak light (Fig. 25). The temperature 

 coefficient is unaffected by heavy water below 30° C, but is changed in 

 the region between 30° and 46°; the maximum rate is reached at 35° in 

 ordinary water and at 39° in heavy water. 



-►-0.4 



oa 



- 04 



0.4 



30 



3.5 4.0 



log light intentity 



4.5 



Fig. 25. — The rate of photosynthesis of 

 Chlorella as a function of light intensity in ordi- 

 nary water and heavy water (after Craig and 

 Trelease 1937). 



Pratt, Craig, and Trelease (1937), and Pratt and Trelease (1938) used 

 flashing light to distinguish between the effects of deuterium on the 

 photochemical reaction and on the dark reactions in photosynthesis 

 (c/. Vol. II, Chapter 34). Flashes of 0.005-second duration were inter- 

 rupted by dark intervals of 0.012-0.086 seconds. With the shorter dark 

 periods, the yield per flash in heavy water was 40% of that in ordinary 

 water (t. e., the ratio was the same as in continuous light) ; but with dark 

 periods of 0.028 second, the ratio rose to 0.57; and with 0.062 and 0.086 

 seconds, it became practically equal to 1 (Fig. 26). 



