Lecture 6 — 137 — Biochemical Problems 



Clark (1936), in Vickery's laboratory, has demon- 

 strated in experiments on tomato plants, the qualita- 

 tive composition of the organic acid mixture may be 

 markedly altered. The proportion of unknown organic 

 acids was much greater for the plants absorbing am- 

 monia nitrogen than for those absorbing nitrate nitro- 

 gen. 



ViCKERY, PucHER, Wakeman and Leavenworth 

 (1940), who have contributed so many results to 

 research on the nitrogen and organic acid metabolism 

 of higher plants, have pursued these studies with 

 reference to the tobacco plant. The effect of increasing 

 the proportion of ammonia to nitrate nitrogen in the 

 culture solution produced a profound decrease in the 

 concentrations of organic acids in all parts of the 

 plant. Malic and citric acid were most sensitive to 

 change in cultural conditions. It is noted that in 

 certain treatments the plants were of the same size 

 and appearance, yet were chemically very different as 

 regards the organic acid constitution of the sap. The 

 flexibility of plant metabolism is illustrated. 



Plants growing under ordinary soil conditions are 

 usually supplied nitrate as their chief source of nitro- 

 gen and consequently would have the organic acid 

 metabolism characteristic of the absorption of bases 

 in association with nitrate ions, followed by the reduc- 

 tion of the latter within the plant. Nevertheless, the 

 capacity of the plant to respond metabolically to an- 

 other source of nitrogen is of great interest to the 

 effort to elucidate the kinds of reactions that can take 

 place. Furthermore, there are some soil conditions 

 under which ammonia nitrogen may become important, 

 as in the growth of rice plants in a flooded soil. 



In nutrient solution investigations the ammonium 

 ion ordinarily can be completely substituted for the 

 nitrate ion, but generally more precautions must be 

 taken in controlling the solution when nitrogen is sup- 

 plied as ammonia than when it is supplied as nitrate. 

 The point has been made in a previous lecture that 



