452 RADIATION BIOLOGY 



the high rate of metabohsm in the absence of both carbon dioxide and 

 additional sugars, but it also implies that this represents some slightly 

 abnormal activity of the cells. 



It is obvious, for the rest, that reduction of nitrate requires either 

 added sugar or light and, further, that the rate of reduction can be much 

 increased by light. The question is whether this can be explained entirely 

 by an increase in the supply of carbohydrates accelerating the dark reac- 

 tion, Eq. (8-5a), or whether the light itself takes an active part in the 

 reduction. This may be answered tolerably convincingly in favor of the 

 latter assumption before it is worthwhile to discuss different modes of 

 light actions. Purely theoretical considerations have been made, for 

 example, by Ullrich (1924) and Rabinowitch (1945). 



Warburg and Negelein claim to have disproved the hypothesis of a 

 direct action by the observation that phenylurethane, known to inhibit 

 photosynthesis [Eq. (8-56)], converts the extra oxygen production into 

 an ecjuivalent evolution of extra carbon dioxide. This is certainly an 

 interesting observation, but the artificial experimental conditions men- 

 tioned detract from its value. A more hypothetical objection has been 

 made by Rabinowitch (1945) by pointing to the possibility that nitrate 

 reduction might shift from one photosynthetical or photoreductive sys- 

 tem in the hght to another in the dark when the former is inactivated. 

 A parallel should exist in a shift from photosynthesis to photooxidation 

 (Noack, 1925). 



The assumption of at least partly different dark and light systems for 

 the reduction and assimilation of nitrate was put forth by the author 

 (Burstrom, 1945) and had earlier been suggested by Kostytschew (1926). 

 It has received support from Myers's results (1949) with Chlorella at low 

 light intensity. Myers pictures two systems, one dark and one light, 

 competing for the nitrate, and the way the reduction takes place should 

 depend upon the carbohydrate content of the cells or, expressed more 

 generally, upon the relation between their carbohydrate content and the 

 illumination. Cells developed at a high light intensity, supposedly rich 

 in carbohydrates, when put in the dark showed a higher rate of respiration 

 and a greater production of extra carbon dioxide than cells grown at a 

 low light intensity (Table 8-1). This is in accordance with the expecta- 

 tions [Eqs. (8-2) and (8-3)] if the content of carbohydrates limits the rate 

 of nitrate assimilation. When transferred to light of low intensities, the 

 two sets of cells behaved differently. 



High-carbohydrate cells gave irregular figures for the respiratory quo- 

 tient down to —0.48. At the lowest light intensity of 35 ft-c the rare 

 phenomenon of a positive respiratory (}Uotient was encountered, which 

 means that both oxygen and carbon dioxide were given off by the cells. 

 The immediate conclusion drawn by Myers is that, owing to the high con- 

 tent of carbohydrates, the reduction follows the dark pattern. Nitrate is 



