Inhibition and Retardation of the Oxidation of lAA 165 



Rb + 2ROH > Rb.2H + 2RO- 



Rb-2H + O. — > Rb + HoOo 



H.,Oo + 2ROH P!I^^^!£^> 2RO- + 2HoO 

 " " catalase 



In this illuminated system riboflavin would be maintained in the 

 oxidized state by oxygen rather than by Mn+-^; thus it would not 

 interfere in the reaction sequence of lAA oxidation and cause a 

 transfer of the chain reaction. Also oxygen consumption would con- 

 tinue well past the theoretical for lAA oxidation since it would re- 

 vert to a slower cyclical reaction in which Mn+4 (MnOo) may be the 

 eventual end product accompanied by the uptake of oxygen as dis- 

 cussed earlier. 



Implicit in the interpretation of the mechanism of action of in- 

 hibitors and retarders in this paper is the essential correctness of 

 the scheme proposed by the present authors to explain the catalytic 

 action of wheat leaf enzymes, horseradish peroxidase, and beef liver 

 catalase on the oxidation of lAA in vitro. The fact that all com- 

 ponents of the system exist or are readily available in vitro justifies 

 consideration of the physiological significance of the system in the 

 control of plant growth, in which lAA apparently plays a central 



role. 



From a biochemical viewpoint, the enzymically catalyzed free 

 radical mechanism proposed for the oxidation of lAA represents a 

 departure from classical interpretations of reaction mechanisms in 

 plant physiology. Such phenomena as the photoreversible inhibition 

 of lAA oxidation by riboflavin, as well as providing a useful system 

 to study the kinetics and mechanism of photochemical reactions, 

 may also prove, in their interpretation, to strengthen our knowl- 

 edge of probable biochemical mechanisms underlying the photo- 

 control of plant growth. 



ACKNOWLEDGMENTS 



The authors gratefully acknowledge the financial support of 

 the Canada Department of Agriculture under Contract EMR-14 

 and also the able technical assistance of Miss Sally Dangerfield. 



LITERATURE CITED 



1. Andreae, W. A. Effect of scopoletin on indoleacetic acid metabolism. Nature. 

 170: 83, 84. 1952. 



2. . The photoinduced oxidation of manganous ions. Arch. Biochem. 



Biophys. 55: 584-586. 1955. 



