3. Reduction of Various Oxidants 



Contributions to the Problem of Photochemical 



Nitrate Reduction 



ERICH KESSLER,* Research Institutes (Pels Fund), University of Chicago, 



Chicago, Illinois 



It has been known for a long time that light has a strong accelerat- 

 ing influence upon nitrate reduction in green plants (1-3). In spite 

 of many studies, however, the reason for this light effect is still 

 obscure. Several explanations have been suggested: {!) The mech- 

 anism of nitrate reduction might be essentially the same in the 

 light and in the dark, the higher rate observed in the light being due 

 to a larger supply of carbohydrates produced by photosynthesis. 

 (2) The influence of light might be an indirect one, due to special 

 products of photosynthesis reacting with the nitrate (4). (3) A chloro- 

 phyll-sensitized oxidation by nitrate of organic compounds (5). 

 (4-) A photochemical reduction of pyridine nucleotides with subse- 

 quent transfer of the hydrogen to nitrate (6). (5) A photochemical 

 reduction of nitrate with nitrate acting instead of CO2 as the hy- 

 drogen acceptor in photosynthesis (7-9). 



A truly photochemical reduction of nitrate, if it exists at all, should 

 easily be demonstrated with algae in the light in the absence of CO2. 

 Experiments of this kind, carried out under pure nitrogen in order 

 to prevent respiration and photooxidations, would be an obvious way 

 to exclude the first two possibilities mentioned above, namely, the 

 occurrence of a nitrate reduction bound to respiration or indirectly 

 accelerated by carbon intermediates of photosynthesis in the light. 

 All experiments performed thus far that have been interpreted as 

 supporting a "photochemical reduction of nitrate," however, have 

 been made in the presence of CO2. On the other hand, all attempts to 



* This work was performed during the tenure of a fellowship awarded by the 

 National Academy of Sciences and International Cooperation Administration. 

 Permanent address: Botanisches Institut, Universitat Marburg, Marburg/Lahn, 

 Germany. 



250 



