408 Mineral Nutrition of Plants 



dioxide concentrations show that about 15 mols of carbon dioxide are 

 used in the reduction of one mol of nitrate. Thus it appears that in the 

 process of reduction of nitrate and carbon dioxide, some nitrogenous 

 organic compound is formed. 



Myers (jj, 38) and Cramer and Myers (8) have studied nitrate 

 reduction in Chlorella. This alga behaves like both the root and the 

 leaf of wheat in that it is capable of both thermal and photochemical 

 reduction of nitrate. The data in Table III show that the assimilatory 



TABLE III 



C0 2 /0 2 Quotient for Chlorella Cells Grown at 40 Foot-candles and Studied 

 at 40 Foot-candles (from Jack Myers, ref. jS) 



C0 2 /0 2 by C0 2 /0, 



Nitrogen Manometric Calculated from 



Source Measurement Cell Analysis 



NO s ~ -0.68 -0.69 



NH 4 —0.94 —0.91 



N0 3 "+NH 4 + -0.94 



quotient (C0 2 /0 2 ) is considerably less than one (0.68) in the presence 

 of nitrate. In the presence of a mixture of nitrate and ammonium or 

 ammonium alone, the assimilatory quotient is nearly one (0.94). When 

 ammonium is present, photochemical nitrate reduction ceases. 



Some have objected to any concept which requires two such diverse 

 pathways for nitrate reduction. This does not, however, appear to be 

 a serious factor considering that in carbon dioxide reduction, two 

 pathways have already been shown to exist in some of the lower 

 organisms, one thermal and one photochemical. In the autotrophic 

 chemosynthetic organisms, energy is derived from the oxidation of 

 sulfur or hydrogen, which in turn is used for the fixation and reduction 

 of carbon dioxide. In the presence of light, however, some of these 

 organisms carry out a photosynthetic reduction instead of a thermal 

 reduction. 



REFERENCES 



1. Arnon, D., Plant Physiol., 24:1 (1949). 



2. Aronoff, S., and Calvin, M., Plant Physiol., 23:351 (1948). 



