Hoagland — 142 — Plant Nutrition 



The barley root tissues, as investigated in our 

 experiments, do not accumulate appreciable amounts 

 of ammonia, if they remain uninjured. Absorption 

 of ammonia results in the formation of amides, pri- 

 marily of glutamine. This amide can accumulate in 

 the roots or be transported upward and appear in 

 exudates. 



During the absorption of nitrogen compounds, or 

 as a result of subsequent metabolism, a large increase 

 in total respiration has been found to occur in the 

 excised roots of barley high in carbohydrate. In some 

 experiments ammonia nitrogen seems to stimulate the 

 rate of respiration to a greater extent than nitrate, 

 although the latter also has a large effect. This respira- 

 tory response to absorbed nitrogen compounds has as 

 a consequence an accelerated utilization of carbohy- 

 drate. In addition, carbohydrate content is reduced 

 by the building up of soluble organic compounds of 

 nitrogen, as has just been explained. Considering the 

 intact plant, the net physiological result will depend 

 on reactions of this type in various parts of the plant 

 and on the photosynthetic activity of the plant as it 

 is related to climatic complexes. 



Metabolism and Accumulation of Ions : — A current 

 study by Machlis of this laboratory has been devoted 

 in part to testing the hypothesis that an organic acid 

 cycle in a respiratory system may be linked in a chain 

 of processes with ion accumulation by roots. The 

 technique consists of making experiments on young 

 barley roots by micro-respirometer methods and by 

 measurements of ion accumulation through the use 

 of radioactive bromide. It was noted in a previous 

 lecture that a strong inhibitor of ion accumulation is 

 iodoacetate, a respiratory inhibitor. The addition of 

 suitable amounts of organic acids of the cycle to 

 which reference has already been made overcomes this 

 inhibiting effect on salt accumulation. Malonic acid 



