450 RADIATION BIOLOGY 



rials a reductase that reduces nitrate to nitrite, with hydrogen ions and 

 reduced triphosphopyridine nucleotide (TPNH + H+) serving as the 

 hydrogen-donating system [see Eq. (8-9)]. They point out that this 

 explains the connection with the carbohydrate metabolism, which is only 

 partly true. Physiologically it is scarcely possible to separate the initial 

 nitrite formation from the bulk process of reduction and assimilation of 

 nitrate, and it is even likely that nitrite in the free state cannot appear 

 in vivo in higher plants as a free intermediate product (Burstrom, 1945), 

 although in some instances it has been detected in minute amounts (e.g., 

 Nance, 1948). Nitrite is not attacked by the reductase system of Evans 

 and Nason. However, carbohydrates or derivatives thereof must be con- 

 sumed in the whole sequence of reactions leading to amino acids. It is 

 therefore pertinent to present the physiological data that have a bearing 

 on a direct light action on the nitrate reduction and assimilation before 

 returning to the enzyme chemical results, which deal only with the initial 

 reduction to nitrite. 



3. INDICATIONS OF A DIRECT LIGHT ACTION 



There are only two green plants in which the light actions have been 

 studied closely, namely, Chlorella and wheat. Since both the materials 

 and the methods have been rather different, it seems appropriate to dis- 

 cuss these two cases separately. 



3-1. Experiments on Chlorella. The formation of extra carbon dioxide 

 must, of course, cease in the light. Warburg and Negelein (1920) studied 

 the gas exchange of Chlorella in light in the presence of nitrate and in 

 the absence of carbon dioxide, finding a respiratory quotient of less than 1. 

 This means that oxygen was produced in excess of the assimilated carbon 

 dioxide. This "extra oxygen " in the light should correspond to the extra 

 carbon dioxide produced in darkness and originate in a normal photo- 

 chemical assimilation of the extra carbon dioxide. The dark reaction is 

 expressed by Warburg and Negelein by 



HNO3 + H2O + 2C = NH3 + 2CO2, (8-5a) 



and the ensuing light reaction by 



2CO2 = 2C 4- 2O2. (8-56) 



The net result in light thus becomes 



HNO3 + H2O = NH3 + 2O2. (8-5c) 



This formation of extra oxygen in the light in Chlorella has been 

 confirmed by Myers (1949), Myers and Johnston (1949), Pirson and 

 Wilhelmi (1950), and Davis (1950). The theoretical oxygen/ammonia 

 ratio, according to Eq. (8-5c), should be 2/1. Warburg and Negelein, 



