Robert B. Withrow 



405 



photosynthesis in the purple sulfur bacterium, Chromatium, strain D. 

 Irradiation caused a marked uptake of phosphate in an atmosphere of 

 nitrogen and oxygen. When carbon dioxide was present, the uptake was 

 less, indicating the possible utilization of ester phosphate in carbon di- 

 oxide reduction. When the cells were transferred to darkness, there was 

 a small release of inorganic phosphate, apparently associated with meta- 

 bolic energy release. 



LIGHT + KCN 



^-DAIiK +- KCN 



60 120 180 



TIME (MINUTES) 



Figure 2. Effect of light on P uptake by Scenedesmus D 3 . — 

 Gest and Kamen ( 16). 



Data are for trichloroacetic acid-insoluble cellular residue. 



These few rather recent investigations strongly suggest that one of 

 the most important roles of phosphorus in plants may be in phosphory- 

 lation reactions involved in the upgrade flow of energy in photosyn- 

 thesis. 



Nitrogen 



The two principal sources of inorganic nitrogen for the growth 

 of higher plants are ammonium and nitrate. Ammonium nitrogen is 

 already in a reduced form and in the dark is readily assimilated into 

 organic forms of nitrogen. Nitrate nitrogen, on the other hand, must 

 be reduced at least as far as nitrite before it can be assimilated into 

 organic nitrogen. In the roots of most woody plants and legumes, this 

 process takes place as a purely thermal reaction in the dark. Since the 



