FREE NITROGEN COMPOUNDS IN PLANTS 6071 
consideration of their interactions with each other and with the source of nitrgen 
supply, 7.e. whether this is nitrate or ammonia. A brief summary of work on this in- 
teresting problem by MARGOLIs?! has already been made*®. 
Another interesting example of the complex factors that affect the relative com- 
position of the soluble nitrogen compounds of plants is that provided by the obser- 
vations of VOSKRESENSKAYA®*: 68, which was also referred to by CAYLE AND EMER- 
son’. The first observations suggested that the balance between the amount of 
the 4CO,, fixed in the light, which enters the protein or the carbohydrate is a func- 
tion of wavelength. On re-examination of this question, CAYLE AND EMERSON® did 
see some effects which were due to wavelength on the specific activity of the carbon 
in certain amino acids. Recent work by HAuscHILD, NELSON AND KRoTKOy!® shows 
that Chlorella cells produced. 25—30°, more aspartic acid and 20%, less serine and 
glycine in red light plus 4° of blue light than in red light alone, even when the 
comparisons were made under conditions that relate to the same amount of carbon 
being fixed. 
Sufficient has, therefore, been said in this introduction to show that the nitrogenous 
metabolism in plants is far more intricate than would be supposed from a mere 
preoccupation with the idea that amino acids are elaborated as the principal soluble 
products and that the chief destiny of these is to combine directly in the form of 
protein. The array of nutritional, environmental, and developmental factors that 
impinge upon nitrogen metabolism to determine the compounds which are stored 
free is not comprehensible on this idea alone. 
In short, the problem of nitrogen metabolism of higher plants should be seen in 
its entirety, and the role of the free or soluble compounds 1s not merely, or immediately, 
in relation to protein synthesis but should be seen in many other contexts. When 
pushed to the point of ultimate explanation, most physiological functions in plants 
make contact with nitrogen metabolism, but this is especially true where the courses 
of nitrogen metabolism and carbohydrate metabolism interlock, prominently at the 
level of the keto acids. The need of plants to synthesize, break down and re-use 
their nitrogen compounds, whether in different parts of the plant body or within a 
single cell, and the interlocking and mutually dependent cyclical processes of nitrogen 
metabolism and carbohydrate metabolism, constitute the main theme of the re- 
mainder of this discussion which will emphasize the distinctive part which these 
events play in plants. 
SECTION II. DIFFERENT BIOSYNTHETIC PATHWAYS OF NITROGEN COMPOUNDS WHICH 
OCCUR IN DIFFERENT METABOLIC SITUATIONS 
In Section I reference has been made to various considerations and circumstances 
which determine the overall composition of the pool of soluble nitrogen compounds 
which may occur in plants. In this section it will be emphasized that the common 
constituents of the soluble nitrogen moiety may arise by entirely different biosyn- 
thetic pathways under different situations. An outstanding example of this concerns 
the principal amides glutamine and asparagine. In many situations the occurrence 
of glutamine is associated with conditions which are favorable to growth and protein 
synthesis, and indeed glutamine is often added to tissue-culture media as a pre- 
ferential source of nitrogen. By contrast, the amide asparagine often seems to be 
References p. 692/693 
