Hoagland —136— Plant Nutrition 



COOH 

 CHa 



acids ^*^^5^°°^, with malic acid greatly predominat- 



COOH 



ing. When the total organic acids are increased by 

 the absorption of potassium from the bicarbonate, 

 most of the increase is in malic acid, according to the 

 studies of Isaacs in this laboratory. 



Metabolism in Relation to Absorption of Nitrogen: 



— Of special importance to the study of plant nutrition 

 is the relation of the absorption of nitrogen containing 

 components of a culture medium, nitrate or ammonia 

 nitrogen, to the organic acid system. The fate of ab- 

 sorbed nitrogen is concerned with the whole nitrogen 

 metabolism of the plant. The latter is a very large 

 subject already outlined in a monograph by Chibnall 

 (1939) and in the extensive contributions of Vickery 

 and his collaborators (1938-39-40-42), which makes 

 elaborate discussion unnecessary. Several questions, 

 however, warrant brief attention for our immediate 

 purpose. 



Nitrate ions can be stored in plant cells, sometimes 

 in large quantity, but normally their ultimate fate is 

 to be reduced. The nitrogen becomes part of nitrogen 

 containing organic compounds; amides, amino acids, 

 proteins and other nitrogenous products, in part un- 

 known. While nitrate ions can be absorbed by plants 

 in excess of bases absorbed, in considerable measure 

 the nitrate ions enter with cations, commonly potas- 

 sium ions. After the reduction of the nitrate a base 

 remains, which tends to be neutralized by the forma- 

 tion of organic acids in the manner described. Sup- 

 pose, however, that nitrogen is furnished to the plant 

 in the form of ammonia nitrogen. Then the nitrogen 

 is absorbed in cation form, or its equivalent, and the 

 nitrogen is utilized in the plant for synthesis of or- 

 ganic nitrogen. The constitution of the cell sap is 

 greatly modified from that of a plant receiving nitrate 

 nitrogen. The content of organic acids is lower and as 



