STUDIES ON THE RELATION BETWEEN 



MOLECULAR STRUCTURE AND PENETRATION 



OF GROWTH REGULATORS INTO PLANTS 



J. VAN OVERBEEK 

 Agricultural Research Division, Shell Development Company. Modesto. California 



INTRODUCTION 



The penetration of auxins into plants is a property which so far has not been 

 given much attention. One generally accepted reason for this is that it has 

 not been possible to locate a specific site in the molecule which can be held 

 responsible for auxin activity. It is therefore impossible to distinguish 

 between those properties of the auxin molecule which cause it to penetrate 

 and those which cause it to react inside the cell as an auxin. 



It thus becomes necessary to reason by analogy, that is to study the relation 

 between physiological activity and molecular structure in compounds which 

 possess something that resembles an active grouping in the molecule, even 

 though this activity is not auxin activity. 



An example of a compound of this type is maleimide. There is actually a 

 whole group of maleimides (all having the maleimide ring in common), 

 differing from one another by the substituent on the nitrogen atom. It is 

 reasonably certain that the anti-auxin properties of the maleimides reside 

 in the maleimide ring (more precisely in the sulphydryl reactivity of its 

 double bond) and that the substituents on the nitrogen atom merely function 

 as a 'lipophilic tail' to carry the molecule into the cell. 



THE PROBLEM 



In Figure 1 has been presented a number of comparisons in which the increased 

 auxin activity was caused by a modification of the molecular structure. In 

 the writer's opinion the resulting increases in physiological activity of these 

 compounds can be satisfactorily explained by increased penetration into the 

 cells. One can distinguish three types : ( 1 ) alpha substitution of phenyl- and 

 phenoxyacetic acids, (2) phenyl- versus phenoxyacetic acids, and (3) chlorina- 

 tion of the benzene ring. The relatively more active member of each pair of 

 compounds appears on the right, and is indicated by -(-. We will now scruti- 

 nize the evidence for this contention on the basis of the study of the 

 maleimides and later also on another pertinent example, that of the amino 

 phosphine oxides. 



The maleimides 



The maleimides are anti-auxins in the sense that they inhibit the growth 

 rate of coleoptile sections growing under the influence of applied indoleacetic 

 acid. The inhibition of the growth rate is restored by increasing the con- 

 centration of the auxin. These relations are shown on maize coleoptile 

 sections in Fimre 2. 



'to' 



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