WILLIAM J. ROBBINS 473 



The confused and unsatisfactory status of our information on the 

 relation of amino acids to higher plants may be ascribed in part to a 

 variety of causes among which are the following. 



Sterility. It is probably not necessary to emphasize the importance of main- 

 taining sterile conditions. Any research with amino acids under nonsterile 

 conditions must be viewed with suspicion if the purpose is to determine the 

 effects of the amino acid rather than of its decomposition products. Un- 

 fortunately, a good deal of the earlier work on the influence of amino acids 

 was carried out under nonsterile conditions. 



Purity of Amino Acids. The purity of the sample of acid used is always a 

 matter of concern. Proline, for example, materially influences the action of 

 hydroxyproline on dermatophytes, and many of the so-called pure samples 

 of the latter compound contain sufficient proline to affect the results materially 

 (46). In addition to the purity of the individual amino acid, attention must 

 be given to its optical form. The natural form may act differently from the 

 unnatural or from mixtures of the two. 



Secondary Effects of Amino Acids. In evaluating the action of amino acids 

 per se, consideration must be given also to their buffer action. We found, for 

 example, the favorable effect of glutamic acid on the gametic reproduction of 

 Phycomyces to be due to its buffer effects (49). Amino acids may combine 

 with heavy metals (52) reducing the toxicity of a medium or lowering the 

 amount of a minor essential element below the optimum. Nielsen and Johansen 

 (35) found that asparagine materially reduced the toxicity of copper for 

 Rhizobium radicicola probably by forming a copper complex. Amino acids 

 may react during heat sterilization with other constituents of the medium, 

 especially dextrose (63). Lankford (25) reports that the sterilization of glucose 

 with amino acids produces effects on lactic acid bacteria quite different from 

 those obtained when the amino acids and glucose were separately sterilized. 



Specificity of Action. Another complicating factor is that the effect of an 

 amino acid may vary with the plant species. Hydroxyproline is quite toxic 

 for a number of dermatophytes but is relatively harmless, even beneficial, for 

 other fungi (46). Virtanen and Linkola (57) report that peas and clover use 

 both optical forms of aspartic and glutamic acids well for their nitrogen nutri- 

 tion, but that aspartic and glutamic acids do not function as nitrogen sources 

 for wheat and barley. In fact, aspartic acid appears to interfere with the growth 

 of wheat in media containing nitrate or ammonium salts. Legumes and grasses 

 seem, therefore, to exhibit entirely different behavior toward aspartic or 

 glutamic acid. 



Single Amino Acids versus Mixtures of Amino Acids. The results obtained 

 with microorganisms and the excellent investigation by Sanders and Burk- 

 holder (51) on Datura embryos emphasize that mixtures of amino acids may 

 have substantially greater beneficial effects than any single amino acid. Further 

 work on amino acid requirements of higher plants should include a considera- 

 tion of the balance between amino acids. 



Embryos versus Adult Plants— Parts versus Whole Plants. Attention must be 

 given to the stage of development of a plant and to the results obtained with 

 root or stem tissue as compared to the intact plant. 



Carbon Content of Amino Acid. The possibility that an amino acid may be 



