DYNAMIC ASPECTS — AMINO ACID POOL TURNOVER 657 
DYNAMICS OF AMINO ACIDS IN. PLANTS 
S. ARONOFF 
Department of Biochemistry and Biophysics and the Institute for Atomic Research, 
Towa State University, Ames, Lowa (U.S.A.) 
Amino acid metabolism is among the very oldest of studies in plant biochemistry 
and physiology, going back to the discovery of asparagine in 1806!. The relation of 
the appearance of asparagine to protein catabolism was the object of controversy 
between SCHULZE and PFEFFER as early as the 1870's. The tremendous amount of 
subsequent analytical work required for even meager additional insight continued 
to almost 1940, and is summarized in CHIBNALL’s Classic Szlliman Lectures at Y ale}. 
Progress since then has been sporadic. Almost explosive efforts have been followed 
by periods of relative moderation, as is common in most areas of human endeavor. 
Thus, there was the pioneering work of STEWARD? in the paper-chromatographic 
elucidation of rare, and even novel amino acids in plants. Similarly, the VIRTANEN 
school has been prominent in the identification of heretofore unknown amino and 
imino acids?. 
However, it was only with the advent of isotopic techniques that a study of the 
kinetics of amino acid transformations could be made. Thus, it is only 11 years ago 
since CALVIN’s group first demonstrated? the formation of some of the more common 
amino acids as being among the early products of photosynthesis by algae. And it 
is only this year that a kinetic study has appeared of the rate of formation of these 
amino acids under steady-state conditions®. Similarly, our initial studies on amino 
acid formation in leaves appeared a decade ago® and kinetic experiments are still 
in progress. 
Changes in free amino acids during germination 
The germination of dicotyledonous plants is accompanied by hydrolysis of protein 
in the cotyledon, and the amino acids thereby formed are assumed to be transported 
to regions of growth. This proteolysis was, in fact, much of the basis of the historical 
work noted above. However, more recent chromatographic evidence provides direct 
knowledge of the changes of the individual amino acids of germinating bean seeds’ 
and lupines’. As shown in Table I, initially the seedlings are virtually devoid of 
free amino acids. Indeed, glutathione, notably scarce in mature tissue, is conspicuous 
by its relative prominence. The increase in asparagine was known historically, but 
the prominence of arginine was less understood as was that of the less obvious histidine 
or lysine. Furthermore, as might be expected now, quantitative differences occurred 
even as to direction. Thus, in the case of the bean, germination resulted in a decrease 
of free glycine, whereas in lupines, the same stage produced a decided increase. The 
comparisons are not completely valid, however, as the lupine was germinated in the 
dark to avoid the complications resulting from amino acid formation during photo- 
synthesis. Other complications also occur. For example, in the light, arginine gra- 
References p. 666 
