NITRATE REDUCTION 457 



tory carbon dioxide or to a utilization of intermediate products of the 

 respiratory breakdown of sugar. In any case this resynthesis of carbon 

 compounds parallels fairly well the assimilation of nitrate, resulting in a 

 synthesis of compounds containing carbon and nitrogen in a ratio of 

 between 15/1 and 19/1. It seems to have been possible in this instance 

 to separate respiration from photosynthesis and show that the assimi- 

 lation of nitrate follows the latter. This is rather good evidence of a 

 light-induced reduction and assimilation of nitrate, and, although not 

 conclusive, the results point to a connection between the assimilation of 

 nitrate and carbon dioxide. 



4. THEORIES 



If we start from the reduction of nitrate expressed as in Eq. (8-1), 

 HNO3 + 8H ^ NHii + 3H2O, and try to connect it with the photo- 

 synthetic mechanism, this is easily performed theoretically by adding 

 the efjuation of the photolysis, 



4H2O -> 8H -}- 2O2, (8-6) 



to the sum 



HNO3 + 4H2O -> NH3 + 2O2 + 3H2O. (8-7) 



Photolytically formed hydrogen should reduce nitrate to a lower level of 

 oxidation, presumably to ammonia. The arguments in favor of some 

 connection with the photosynthesis have been presented in the foregoing 

 and are rather strong, but there are no arguments in favor of some simple 

 mechanism as expressed in Eq. (8-7). 



First, it must be emphasized that we do not know of any photo- 

 chemical mechanism, other than that of chlorophyll, which can be made 

 responsible for the nitrate reduction. It is true that Tottingham et al. 

 (1934) found a special action of visible light of short wave lengths on the 

 utilization of nitrate, but they measured only the nitrate absorption, not 

 the consumption, it is possible that migration effects may have been 

 involved by means of the stomatal mechanism (cf. Sierp, 1933) or in 

 other ways. In any case, the results of Tottingham et at., although 

 interesting and suggestive, form a weak basis for speculations about 

 special photochemical systems for the nitrate utilization. 



Second, there are several possibilities even if we assume a connection 

 between nitrate reduction and the general photosynthetic mechanism. 

 Equation (8-7) implies a formation of extra oxygen independently of the 

 transformation of carbon dioxide. The extra oxygen should be added to 

 the one normally produced, and the reduction of carbon dioxide should 

 proceed undisturbedly. This means that the utilization of hght energy, 

 as shown by the evolution of oxygen, is increased by a reduction of nitrate 

 together with that of carbon dioxide. Another possibility is that, if the 



