AUTOTROPHIC ASSIMILATION 73 



is already high, as after a period in bright light, then there 

 is no restriction on the use of the products of photosynthesis 

 for the synthesis of nitrogenous substances and the rate of 

 nitrate assimilation is high with a corresponding value of 

 the photosynthetic quotient. Since no nitrate reduction 

 occurs when ammonia is available it is evident that nitrate 

 is only reduced in a manner intimately related to the require- 

 ments of subsequent nitrogenous synthesis. ^^ 



TABLE 5 



PHOTOSYNTHETIC QUOTIENTS (Qp= AOg/- ACO2) OF Chlorella 

 pyrenoidosa in the presence of nitrate^^^ 



Warburg and Negelein^^^ considered nitrate assimilation 

 to be a process not directly connected with photosynthesis 

 and explained the increase in nitrate reduction on illumina- 

 tion as due to the effects of light on cell permeability. 

 Nevertheless it is possible that nitrate reduction is closely 

 linked with the photosynthetic mechanism. Thus, nitrate 

 might be reduced by a substance which can be produced 

 by respiratory processes but which is closely related to a 

 primary product of photosynthesis and which is available 

 in greater amounts when photosynthesis is taking place. ^^^ 

 It is also possible that in the light a sensitized photochemical 

 reduction of nitrate by water occurs, i.e: photosynthesis 

 with nitrate substituted for carbon dioxide:^^'' 



HN03+H20->NH3+202 . . (19) 



Evidence that this takes place is provided by the observa- 

 tion that Chlorella can evolve oxygen in the light in the 

 absence of carbon dioxide if nitrate is present.^^* Nitrate 

 cannot, however, act as the hydrogen acceptor in the Hill 



