668 F. C. STEWARD AND R. G. S. BIDWELL 
Thus, even in a single cell the large aqueous plant vacuole may function as a reservoir 
for stored, soluble, free amino acids and other nitrogenous compounds; older and 
mature leaves may function somewhat in the same way; and fleshy storage organs 
such as tubers, bulbs, corms, rhizomes, etc., are proverbially rich in soluble nitrogen 
compounds. Seeds and fruits also deposit rich stores of nitrogen, often in the form of 
proteins, especially in fleshy cotyledons. All this makes for an overall pattern of re- 
cycling nitrogen and re-using it with appropriate mechanisms of movement, storage, 
and re-entry of the stored forms of nitrogen into the main stream of metabolism. 
Thus, once nitrogen is reduced from nitrate or is incorporated into organic com- 
pounds from ammonia, the organic nitrogen is largely retained by plants. Further- 
more, the actual formative or growing regions in the plant body are strictly confined 
to very limited areas. To nourish and furnish these regions with their essential require- 
ments, specific regions of the shoot and root may be physiologically active in the de 
novo synthesis of the nitrogenous and carbohydrate compounds which are required, 
but their period of active function in this way may be relatively brief; senescence of 
cells and organs may set in earlier than is often supposed. Such mature or senescent 
organs (e.g. older leaves), though often available for storage, may themselves make 
demands on more active centers of metabolism for their maintenance. 
In rapidly growing plant cells, as for example in tissue cultures, the ratio of their 
alcohol-soluble to alcohol-insoluble nitrogen (largely protein) tends to be small, 
whereas their quiescent counterparts in storage organs may deposit a large part of 
their nitrogen in the form of the free amino acids which have been described in an 
earlier paper. Whereas in the potato tuber some two thirds of the total nitrogen and 
in the carrot root approximately half is in the free or soluble form, the proportion 
may be very much less in the corresponding cultured or growing tissue®?,®8. Indeed, 
one often finds that the unusual nitrogen compounds that have recently been dis- 
covered in plants have been first recognized in such storage organs as seeds, fruits, 
tubers, rhizomes, etc. On examination, the actual growing region of the shoot of an 
angiosperm, e.g. Lupinus® did yield a reasonably complete range of amino acids 
which are needed to make protein, although these were in somewhat low concen- 
tration. By contrast, examination of the growing apex of a fern, Adiantum®, disclosed 
a surprising amount of soluble nitrogen which was in the form of an unexpected 
metabolite which proved to be y-hydroxy-y-methylglutamic acid!*, and this re- 
presented approx. 90% of the total soluble nitrogen of this growing region! 
Therefore, before reaching the later theme of this discussion, which is to be the 
interactions of nitrogen metabolism with other physiological functions, mainly res- 
piration, it is well to summarize some of the factors which are known to determine 
the composition of the soluble nitrogen pool in plants. 
Even though a given organ may be in approximate nitrogen balance with respect 
to its total content of protein, the composition of the soluble nitrogen fraction may, 
nevertheless, be subject to change. The source of these changes is often obscure, 
except that they may be events in a sequential process of development. Such an 
example appeared in the fruit of the edible banana‘’. Early in its development the 
very young fruit contains soluble nitrogen compounds. In the phase of most rapid 
growth of the banana fruit, its soluble nitrogen content falls to a very low level and, 
as the organ subsequently grows by cell enlargement, it again deposits storage 
material and its content of amides increases, although whether this amide is glutamine 
References p. 692/693 
