CHAPTER 8 



Nitrate Reduction 



Hans Burstrom 

 Botanical Laboratory, University of Lund, Sweden 



General. Indirect actions of light on the nitrate reduction. Indications of a direct 

 light action. Theories. Addendum. References. 



1. GENERAL 



A reduction of nitrate to nitrogen at lower levels of oxidation occurs in 

 most plants and is probably lacking only in heterotrophically specialized 

 bacteria and fungi. The process is, nevertheless, imperfectly known bio- 

 chemically. There are, apparently, at least two different ways in which 

 nitrate reduction occurs : One is the usually anaerobic reduction of nitrate 

 to nitrite in the bacterial denitrification, in which the nitrate serves as an 

 oxygen donor for the respiration ; another is the reduction that forms an 

 integral part of the assimilation of nitrate in all higher plants and many 

 bacteria and fungi. The bacterial denitrification is undoubtedly a proc- 

 ess that takes place in darkness independently of light, whereas the assimi- 

 lation of nitrate in many instances has been shown to be intimately 

 dependent upon the light climate of the plant. 



The bulk end product of the utilization of nitrate is amino nitrogen, 

 and the process may be simply expressed as HNO3 —* RCH2NH2. Thus 

 it involves the reduction of nitrogen from the N2O5 to the NH3 level and 

 its binding in an organic linkage. The reduction must, of course, take 

 place in several steps, but it is not definitely known at which level of 

 oxidation the nitrogen is assimilated (Burstrom, 1945). It has been 

 shown with fungi (Kostytschew and Tswetkowa, 1920) and bacteria 

 (Burris and Wilson, 1946) that ammonia appears as a direct product of 

 reduction; nevertheless this does not exclude the possibility that the 

 nitrogen is fixed in an organic linkage at a higher level of oxidation. As 

 to higher plants, it is less probable that nitrate is reduced down to the 

 ammonia level before it is assimilated (Burstrom, 1945). This is especi- 

 ally true of chlorophyllous parts of plants (cf. Lemoigne et al., 1936, 1937, 

 1938), in which the utilization of nitrate takes place preferably in the 

 light. 



This uncertainty concerning the connection between reduction and 



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