380 K. V. Thimann and N. Takahashi 



However, it is not necessary to use a chelation mechanism to interpret 

 these absorption spectra. 



By measuring the pH, Cohen et al. also confirmed that cupric 

 nitrate reacted differently from calcium and magnesium nitrates when 

 added to solutions of lAA and NAA. From a study of several closely 

 related acids comprising active and inactive growth regulators, I 

 found that the pH changes, observed upon addition of cupric ions, 

 do not correlate with growth regulating activity and, moreover, are 

 explicable by mechanisms not involving chelation (Nature. 194: 796. 

 1959). 



We have recently compared a range of chelating agents with 

 lAA, 2,4-D, and NAA in four different biological tests. The lAA, 

 2,4-D, and NAA were highly active in all tests. Of the forty chelating 

 agents tested most were inactive, though weak activity was shown 

 by nine of them, namely, ethylenediaminetetraacetic, diethylene- 

 triaminepentaacetic, l,2-diaminoc);c/ohexanetetraacetic, citric, tar- 

 taric, gluconic, nicotinic, succinic, and nitrilotriacetic acids. This 

 slight but significant activity was shown only in the wheat coleoptile 

 test. The remaining chelating agents were inactive in the wheat 

 coleoptile, the pea segment, the slit pea curvature, and tomato epinasty 

 tests. Thus, the results obtained in these experiments provide strong 

 evidence against the idea that plant growth regulating activity is 

 explicable in terms of chelation. 



