Occurrence and Role of Growth Hormones 89 



in figure 44 where the enzymatic destruction of auxin is being followed 

 in relation to time. It can be seen that the destruction proceeds very 

 slowly in the dark control solutions containing the crude enzyme taken 

 from peas. If the solutions are exposed to light, the destruction occurs 

 very rapidly. Moreover, if the solutions are dialyzed, some water-soluble 

 inhibitor to the reaction is lost, and the destruction proceeds even in 

 the dark. It is deduced that the effect of light is in part a prevention 

 of the effect of the inhibitor of enzymatic destruction of auxin. Both 

 light and the inhibitor are believed to act on step 1 of figure 43. 



The nature of the enzyme which destroys auxin is a subject of 

 some disagreement. It was first thought to be an iron enzyme (Tang 

 and Bonner, 1947), and probably identical with peroxidase associated 

 with a flavin material (Galston, Bonner and Baker, 1953). Other 

 workers, using another means of following the reaction, have found 

 evidence that the enzyme may be a copper enzyme instead (Wagen- 

 knecht and Binris, 1950). An auxin-destroying enzyme from the para- 

 sitic fungus, Omphalia flavlda, appears to be neither a peroxidase nor 

 a polyphenol oxidase (Sequeira and Steeves, 1954). 



The products of the reactions in which auxin is destroyed have 

 been clarified only in part. Tang and Bonner (1947) observed that 

 carbon dioxide was formed at the same rates as oxygen was consumed 

 in the reaction. It was later suggested (Wagenknecht and Burris, 1950) 

 that the reaction might proceed as follows: 



CH2COOH + O2 light f \ ^CHO + COz 



indoleacetic acid indolealdehyde 



Von Denffer and Fischer (1952) have shown that indolealdehyde is in 

 fact a product of the non-enzymatic destruction, and it seems entirely 

 likely that this same product would be produced from the enzymatic 

 destruction as well. There must be further products of the reaction, how- 

 ever, even in the non-enzymatic system, for Brauner (1953) has shown 

 clearly that the indole ring of the auxin is destroyed in light, though 

 at a somewhat slower rate than the destruction of the carboxyl group. 

 He followed the disappearance of the acidic part of indoleacetic acid 

 by measuring pH changes, and simultaneously followed the destruc- 

 tion of the indole ring by use of the Hopkins-Cole reaction. The auxin 

 was added to a dilute solution of riboflavin and exposed to ultraviolet 

 light for different intervals of time, as shown in figure 45. One sees that 

 the acid group was destroyed most rapidly, but that a simultaneous de- 



