SATURATING LIGHT INTENSITY 



985 



of cyanide on respiration is stronger than on photosynthesis, even in strong 

 Hght; in organisms of this type, addition of cyanide causes a strong down- 

 ward shift of the compensation point (c/. chapter 12). Indications of a 

 peculiar difference between cyanide effects of photosynthesis above and 

 below the compensation point were mentioned in chapter 12 (Vol. I, p. 308). 

 Reduced supply of carbon dioxide decreases photosynthesis without af- 

 fecting respiration. If, in consequence of carbon dioxide deficiency, the 

 light curves begin to bend in very weak light, the compensation point 

 may be shifted to high light intensities (c/. fig. 28.15), or never reached 



o 



(3) (l)(2) 



Fig. 28.15. Shift of compensation point with changing carbon dioxide 

 concentration. (1) — >- (2) decreasing [CO2]; (3) -*- (1) increasing tem- 

 perature. 



at all. This case was mentioned in chapter 26, when we spoke of the ex- 

 periments of Chesnokov and Bazyrina (1932) and Miller and Burr (1935), 

 in which gas balance was observed at light intensities of the order of 20 

 klux. Miller and Burr (1935) noticed that, in this "carbon dioxide-limited" 

 range, the compensating light intensity was independent of temperature. 

 This means that the temperature coefficient of the carbon dioxide supply 

 process (diffusion or carboxylation?) was practically equal to that of respira- 

 tion. 



4. Saturating Light Intensity 



When Reinke discovered tlu^ light saturation of photosynthesis, he 

 found it to occur at an intensity close to that of sunlight at noon (*So = ap- 



