THE ENERGETICS OF PHOTOSYNTHESIS 67 



It is only possible to determine the strength of respiration in the dark, so 

 that 



gain — photosynthesis — dark respiration 



The assumption that dark respiration is equal to light respiration is not quite 

 correct. This is only permissible when illumination is so strong that respira- 

 tion will be small in comparison with photosynthesis. Then 



gain = photosynthesis 



It is possible to select such a low intensity that photosynthesis just compensates 

 respiration. If the quantum yield of the gain is <£>' and that of photosynthesis 

 is ^, a relation between both magnitudes and the degree of compensation n 

 can be expressed as follows 



_ O2 production in light 

 O2 removal in dark 



or 



photosynthesis 



a. = -. -. 



respiration 



However 



respiration = photosynthesis — gain 



so that 



photosynthesis 



a 



photosynthesis — gain 

 If we introduce cp and tp', we find that 



(28) 



a 



(f — (f' 



or 



1^' = V^^4 (29) 



a — 1 



At complete compensation of respiration (sufficiently weak illumination) 

 the degree of compensation is 1 . It follows from equation 28 that when a = \ 

 the gain is nil, and from equation 29 that the quantum requirement of the 

 gain \/(p' is infinite. 



If the quantum requirement of photosynthesis is kept constant at 1/^ = 3, 

 it is easy to calculate the quantum requirement of the gain for various values 

 of a. The degree of compensation increases with the intensity. Table 10 

 shows the quantum requirement of the gain at various degrees of compensa- 

 tion. At 10-fold compensation, 1/^' = 3.3, i.e. the quantum requirement of 

 the gain is 90% of the quantum requirement of photosynthesis. At 40-fold 



