FREE NITROGEN COMPOUNDS IN PLANTS 691 
by the strictly polarized movement of stimuli, like auxin, one may expect that this 
export is directed within the plant body to specific regions. The green leaf is, of course, 
accepted as a prime organ of protein synthesis, though much of that protein may 
be broken down for export to the growing regions. The same, of course, applies to 
starch. Using !CO,, HAusCHILD e¢ al.®@ have re-examined the stage at which a 
tobacco leaf is most effective as an “‘exporter’’ of carbon to the growing regions and 
the stages when it actually makes demands by importing compounds which are 
synthesized elsewhere. They have found that it was only the very young leaves 
which exported any significant amount of newly fixed carbon. Therefore, only the 
young leaves really participate very actively in the metabolism of the whole plant; 
the rest are rather passive appendages. Thus it is mainly during the period of its 
active growth that each leaf is most significant as a nutritional unit. 
Of particular interest are the interactions which have been observed between such 
environmental factors as length of day and night temperature, which interact with 
inorganic nutrition as in the case of potassium and calcium, and determine the balance 
between the soluble or free amino acids and the combined or protein amino acids in 
the plant. But it goes much further than this because these environmental factors in 
some way must make their impact on the enzymatic machinery of the cells to cause 
the great differences in the relative composition of the stored soluble nitrogen com- 
pounds which have been encountered**®. In fact, it seems as though the point of 
contact between these environmental factors and the metabolism lies in that region 
of metabolism where, via keto acids, carbohydrate metabolism impinges upon 
nitrogen metabolism (loc. cit. see Figs. A—D, p. 157). 
The nitrogen metabolism of shoots, however, lacks something essentially attribut- 
able to roots. This was early recognized by CHIBNALL and described as “hormonal 
control”, but the difficulty of culturing minute portions of the shoot apex is also 
significant here. Minute apical segments of the shoot large enough to form angiosperm 
root primordia invariably grow well and will synthesize protein, but the central 
apex of the growing point and the smallest primordia will apparently not do so. 
The facts of rest and dormancy also pose their problem because the quiescent cells, 
despite their frequent richness of soluble nitrogen compounds in both quantity and 
variety, fail to utilize this in protein synthesis and in growth. Perhaps many of the 
recently recognized and unusual soluble nitrogen compounds which have been found 
free in such organs may operate as metabolic inhibitors to suppress some line of 
synthesis. This may well be true of azetidinecarboxylic acid, which, lke hydroxy- 
proline, can act as a competitive proline inhibitor and will, therefore, suppress pro- 
tein synthesis in otherwise growing carrot cells. The marked intervention of factors 
that stimluate cells to grow by reactivating synthesis is significant here. The charac- 
teristic formation in all the rapidly proliferating cells so stimulated which have been 
examined (carrot, potato, crown gall tumor cells, etc.) of a protein moiety unusually 
rich in hydroxyproline is again suggestive that protein metabolism and morphogenetic 
activity may well be correlated. In the climacteric of fruits, after which cells seem 
irreversibly to have lost their capacity to grow again, some irreversible changes in- 
volving protein synthesis and turnover undoubtedly occur. 
From all these points of view, therefore, the problems of amino acid metabolism, 
protein synthesis and turnover are not merely to be regarded as problems of bio- 
chemistry because their full understanding will only come when the biochemistry is 
References p. 692/693 
