460 RADIATION BIOLOGY 



followed by 



nitrate 

 rpfii 1 o tfisf* 



NOT + TPNH + H+ > NO7 + TPN + H2O. (8-9) 



This important result fits in well with the conclusions drawn from the 

 physiological experiments. It implies a photolytic system adding itself 

 to that of the carbon dioxide assimilation, but it can account for only 

 one-fourth the extra gas expected according to Eq. (8-7) or, in the case of 

 a dark production of TPNH, Eq. (8-5), because it describes only the 

 reduction of nitrate to the nitrite level. The fate of the nitrite nitrogen 

 is still unknown, and so is the role of light in its reduction. 



However, the results have led Evans and Nason to assume that the 

 difference between the light and dark reactions may lie only in the mode 

 of formation of TPNH. From the nitrite stage the assimilation of nitro- 

 gen could follow the same course. It must then obviously be postulated 

 that the carbohydrate derivatives or reducing agents necessary for the 

 following steps of the assimilation are either of the same origin in light 

 and darkness — formed in the respiration — or that the principle of Evans 

 and Nason is repeated. This means that the symbol X in Eq. (8-4), 

 which stands for the whole sequence of reactants necessary for the com- 

 plete reduction and assimilation of nitrate, can arise either in connection 

 with the photosynthesis or in the respiratory metabolism. The picture 

 outlined by Calvin and Benson (1948), to the effect that carbon dioxide 

 assimilation is at least partly the reversal of the respiration with phospho- 

 glyceric acid in the center, is interesting in this connection. 



It is also worth mentioning that Kamen and Gest (1949) have demon- 

 strated the presence of a light-stimulated nitrogen-fixing system in Rhodo- 

 spirillum by means of N^^. The connection may seem remote, but there 

 are, on the other hand, unmistakable similarities between the nitrogen 

 fixation and the reduction of nitrate, for instance, in regard to hypotheti- 

 cal intermediates (Virtanen, 1950) and the dependence upon molybde- 

 num, but the particulars of the processes are too little known to warrant 

 more than mention without comments. 



ADDENDUM 



This manuscript was completed in 1951 and a few additions were made in 1953. 

 The author regrets that he has not had an opportunity to pay regard to the important 

 contributions by the following: 



Kessler, E. (1953) Flora, 140: 1-38, and Arch. Mikrobiol., 19: 438-457. 

 Van Niel, C. B., M. B. Allen, and B. E. Wright (1953) Biochim. et Biophys. Acta, 

 12: 67-74. 



They give answers to some of the main points discussed in this chapter and make this 

 treatise, on the whole, obsolete. 



