NITRATE REDUCTION 



455 



leaves in darkness (Burstrom, 1943a), but it increases with increased light 

 intensity and attains significant figures already below the point of com- 

 pensation, when there can hardly be any noticeable improved carbohy- 

 drate balance and increase in the respiratory utilization of sugar. The 

 maximal rate of nitrate reduction at saturating light intensity and a 

 normal content of carbon dioxide of the air should, furthermore, corre- 

 spond to a fivefold increase in glycolysis if the reduction were linked with 

 the dark metabolism of carbohydrates. Just as with Chlorella, this seems 

 in itself to exclude the possibility t'hat the rate of the dark assimilation of 



Table 8-2. The Assimilation of Carbon Dioxide and Accumulated 



Nitrate in Leaves at 3250 ft-c 



" Varying initial content of nitrate. 



*" Computed as the equivalent amount of hexose in millimoles. 



nitrate is increased in the light through a more favorable supply of carbo- 

 hydrates. In the case of wheat, however, there are also more positive 

 evidences of a connection between the nitrate assimilation and the photo- 

 synthetic mechanism. 



The products of the simultaneous assimilation of carbon dioxide and 

 nitrate are of special interest. In the absence of nitrate (Table 8-2), 

 soluble sugars are formed quantitatively from the assimilated carbon 

 dioxide, but with nitrate present the formation of sugar decreases. Other 

 assimilates must be produced, and this is caused by the simultaneous 

 assimilation of nitrate. The amount of carbon disposed of in this way 

 stands in a fairly fixed ratio to the amount of assimilated nitrate, which 

 must be assumed to be converted into some organic compounds. The 

 ratio between carbon and nitrogen deposited in such non-sugar assimi- 

 lates depends upon the specific properties of the leaf material in question 

 but is independent of the light intensity. In Table 8-2 the carbon/ 

 nitrogen ratio amounts to 6-7, but both higher and lower figures are 

 obtained if leaves of different age are separated (Table 8-3). In mature, 

 nongrowing leaves the carbon/nitrogen ratio of these assimilates is 

 approximately 4, or very near that of amino acids, but in rapidly grow- 

 ing leaves it attains values as high as 15-18. This corresponds, as a 

 matter of fact, to the average composition of the cell matter and appar- 

 ently means that the simultaneous assimilation of nitrate and carbon 



