Cellular oxidations and the syntheses of amino-acids and amides in plants 



with yeast by treating the cultures in aerated solutions containing sugars and other 

 mineral nutrients but no nitrogen. In this way a high level of soluble sugars or poly- 

 saccharides is built up in the cells, and their ability to assimilate nitrogen in the 

 absence of external supplies of carbohydrate is greatly increased. A further point, 

 shown by the experiments with seedlings, was that the primary reactions, associated 

 with the assimilation of nitrates or ammonia, occur mainly in the root system. On 

 this account most of the work considered here has been carried out with roots 

 immediately after their excision from the growing seedling. 



Experiments both with yeast and root tissues have shown consistently that the 



Barley Roots 



^ 2 4 



12 Hours 



24 



Figure 2. The effect of ammonium phosphate on carbon 

 dioxide production of excised barley roots. Samples of 40 

 root systems , freshly cut from the seedlings, were treated in 

 aerated culture solutions with ammonium or sodium phos- 

 phate. CO 2 output was measured by the Pettenkofer method. 



rate of oxygen uptake increases rapidly when ammonium salts are supplied under 

 favourable conditions. The rate of carbon dioxide production or oxygen consumption 

 is commonly more than doubled within a short time of supplying the ammonia, as 

 illustrated by typical results in Figures 1 and 2. The highest rates of respiration are 

 maintained for only a short time and it is very probable that depletion of the limited 

 carbohydrate reserves in the cells is an important factor causing the secondary 

 decline in rate. No external supply of sugar was provided in these experiments, and 

 analytical data, which are considered in a later section, show that a rapid breakdown 

 of carbohydrates is associated with intense respiratory activities during the assimila- 

 tion of nitrogen. 



53 



