672 F. C. STEWARD AND R. G. S. BIDWELL 
associated with relatively adverse conditions for growth of the organ in which it is 
formed or with conditions where protein synthesis has ceased, where protein break- 
down occurs and where the storage of soluble nitrogen is in excess. Similarly, as 
mentioned for the mint plant*®, the occurrence of arginine in quantity in leaves is 
often associated with quite marked mineral deficiency conditions as observed by 
MILLAR (cf. ref. 50). When recovery from sulphur deficiency occurs in the light the 
amide present is glutamine; when recovery takes place in the dark asparagine is 
formed from arginine (cf. also MILLAR in ref. 46). 
These considerations became apparent when plants which normally contain as- 
paragine as a principal storage product, as in the case of both potato tuber tissue and 
carrot root tissue, were brought into a state of active growth; the asparagine dis- 
appeared almost entirely, whereas some glutamine tended to persist®®. 
But even though glutamine may be present in both a resting organ and in the cor- 
responding growing cells, the method by which it originates may be quite different 
in the two types of system. In the storage organ, as for example in the beet root, the 
general idea is that the glutamine originates from glutamic acid by amidation after 
the manner worked out originally by SpEcK*®. In fact, this was virtually proven by 
Hoop, LyNAm AND Tatum!8, who showed that in the formation of glutamine in the 
beet root the amide nitrogen derived from exogenous ammonia, but the amino group 
came from endogenous sources, 7.e. glutamic acid. However, in tissue cultures derived 
from storage organs, for example the carrot root, it has been shown that glutamine 
may arise readily from exogenous y-aminobutyric acid (y-AB)*. In other words, 
the carbon framework which appeared in the glutamine of the storage organ derives 
prominently from a-ketoglutaric acid originating in the Krebs’ cycle, whereas the 
carbon framework in the glutamine of the actively growing cell may originate in a 
somewhat different way via y-AB, perhaps from succinic semialdehyde. 
A similar contrast applies to glutamine as it occurs in leaves. In normally photo- 
synthesizing and full nutrient leaves it has been shown that glutamine may be 
derived directly from CO, in photosynthesis*. In nutritionally deficient leaves, 
notably mint leaves suffering from sulfur deficiency, glutamine is known to accumu- 
late in great quantity. However, in the attempt to use the latter system to label the 
glutamine so formed directly with carbon (by supplying the sulfur-deficient leaves 
with CO,) it was found that the glutamine simply did not become labeled (R.G.S.B., 
unpublished result). Hence the carbon which entered the glutamine in the sulfur- 
deficient mint plants must have come from some endogenous source, presumably 
via protein breakdown. Similar attempts to label the glutamine of beet roots by 
supplying the @CO, to their leaves also failed, because the immediate fate of the 
carbon was to enter other compounds. Hence the glutamine which was formed in the 
roots under these circumstances was able to get its carbon much more readily from 
some other source than immediately from the metabolism of the sugar being pro- 
duced in photosynthesis?. 
It is, therefore, quite apparent that glutamine may originate in different ways in 
different metabolic situations, and different compounds may be the source of its 
carbon framework. It has, of course, been well known for a long time that excised 
and senescent leaves, notably of barley, form glutamine readily when their protein 
is undergoing breakdown”. 
Although the carbon of glutamine may be labeled in different ways, albeit in 
References p. 692/693 
