NITRATE REDUCTION 459 



More confusing are the low-carbohydrate plants in Myers's experi- 

 ments. They should illustrate a condition with photochemical nitrate 

 reduction with a hmited supply of light. Thus there ought to be, just in 

 this instance, a competition between nitrogen and carbon dioxide for light 

 energy if such a phenomenon occurs and, consequently, a reduced carbon 

 dioxide assimilation together with a constant photolytical formation of 

 oxygen. On the contrary, the figures definitely indicate an increased 

 formation of oxygen, which must be interpreted as a reduction of nitrate 

 independent of the assimilation of carbon dioxide. It is thus impossible, 

 from the gas-exchange data with Chlorella, to arrive at a clear picture of 

 the relation between the assimilation of carbon dioxide and nitrate and 

 the utilization of light energy. 



In wheat plants the conditions are much simpler owing to the lack of 

 dark reduction, and there is no doubt that the assimilation of carbon 

 dioxide proceeds independently of the assimilation of nitrate, as shown in 

 Table 8-2. 



Thus there seem to be no experimental data necessitating the assump- 

 tion of a competition between nitrate and carbon dioxide for a limited 

 amount of available light energy, but several findings can most likely be 

 interpreted as a nitrate reduction adding itself to that of carbon dioxide 

 according to Eq. (8-7). There are, however, no real proofs of the strict 

 validity of the equation. 



Furthermore such a simple picture is perhaps chemically less plausible 

 because the reduction of nitrate involves a successive reduction over inter- 

 mediates. In green leaves no such intermediary inorganic compounds 

 are definitely known to appear. It was therefore assumed (Burstrom, 

 1945) that especially the light-induced assimilation of nitrate involves an 

 early fixation of nitrate in an organic linkage and subsequent reduction. 

 That the assimilation of carbon dioxide is known to follow this pattern 

 increases the probability of the picture. Another point of interest in this 

 connection is that, in both Chlorella and wheat, organic nitrogen com- 

 pounds seem to appear as direct assimilates and that they have a definite 

 composition characteristic of the cells but independent of the fight inten- 

 sity. In nongrowing leaves they are of the approximate elementary com- 

 position of amino acids. This might imply that carbon dioxide and 

 nitrate are reduced simultaneously in one system, leading directly to 

 amino compounds. A fixation of nitrate or eventually some simple 

 inorganic product of reduction to some intermediate in the carbon dioxide 

 assimilation would afford a simple basis for such a synthesis. 



The enzyme chemical studies of Evans and Nason (1953) have pro- 

 vided strong evidences of a photochemical reduction located in the grana 

 fraction in leaves and based on the reaction 



lisht 



H2O -{- TPN -> 3^02 + TPNH + H+, (8-8) 



