DYNAMIC ASPECTS — AMINO ACID POOL TURNOVER 667 
THE FREE NITROGEN COMPOUNDS IN PLANTS CONSIDERED 
IN RELATION TO METABOLISM, GROWTH AND DEVELOPMENT 
B.C. STEWARD. AND R..G. S» BIDWELL* 
Department of Botany, Cornell University, Ithaca, N.Y. (U.S.A.); 
Department of Botany, University of Toronto (Canada) 
SECTION I. DISTINCTIVE FEATURES OF THE FREE OR SOLUBLE NITROGEN COMPOUNDS 
OF PLANTS 
Certain major differences exist between the economy of plants and animals toward 
nitrogenous compounds. This different economy has consequences which are not 
always appreciated; it leads to a very great variety of metabolic reactions and to 
cells and organs with very different nitrogenous metabolism and characteristics, as 
well as to the need to re-cycle and re-use nitrogen compounds in ways that higher 
animals commonly do not do. 
Whereas plants are capable of what might be called “primary” protein synthesis 
from inorganic sources, like nitrate and ammonia, and they derive the necessary 
carbon ultimately from photosynthesis, it is a salient characteristic of animals that 
they are dependent on essential amino acids. This dependence may vary somewhat 
from organism to organism and even within the organism. For example, the mature 
human is said to require eight essential amino acids, but certain cultured tissues re- 
quired 12 (ref. 11). Although whole plants may utilize nitrate or ammonia, their 
individual growing cells may receive the nitrogen for synthesis in already elaborated 
forms, for either roots or leaves may be the prime organs of nitrate reduction. This 
situation creates a consequential demand for movement and subsequent storage of 
what have been called nitrogen-rich forms. Of such nitrogen-rich forms, the amides 
asparagine and glutamine as well as arginine have been historically prominent, 
though such compounds as allantoin and allantoic acid, as stressed by MotHEs*™ and 
by BoLiarp? have also to be considered. Of these “nitrogen-rich storage and mobile 
reserves”, glutamine is now recognized as an equally important metabolite in the 
economy of animals as of plants, but asparagine is still conspicuous as peculiarly a 
plant product and seems only to be used in protein formation by ruminants amongst 
higher animals”?. In fact, much of the asparagine which is ingested by higher animals 
is said to be secreted in the urine. Whereas the body of higher animals is organized 
about the massive ingestion of preformed protein, or of elaborated nitrogen com- 
pounds, and permits the elimination of nitrogenous waste in quantity, this does not 
occur in plants. On the contrary, nitrogenous end products of plant metabolism are 
rarely lost as volatile compounds, nor are they excreted, and certainly not so in 
quantity; hence, higher plants store their nitrogenous end-products of metabolism 
for later use, or they translocate them to other regions in which they are required. 
* Participation of R.G.S.B. in the Symposium was made possible by assistance from the 
National Research Council of Canada. 
References p. 692/693 
