Occurrence and Role of Growth Hormones 91 



readily be oxidi/ed may inhibit auxin destruction by competing for 

 the oxygen in the reaction given above. A good example of this type 

 of inhibitor is ascorbic acid (Brauner and Brauner, 1954). Also, 2,4- 

 dichlorophenol (a common contaminant of 2,4-D) stimulates the 

 enzymatic destruction of indoleacetic acid (Goldacre et al, 1953), and 

 it has been found that this compound, like light, increases the forma- 

 tion of peroxides in vivo (Siegel and Galston, 1954). Hence they each 

 apparently facilitate auxin destruction by bringing about peroxide 

 formation. 



It has been found that indoleacetic oxidase, as the auxin destroy- 

 ing enzyme is called, is an adaptive enzyme. That is, tissues which 

 are not active in destroying auxin can become highly active in a matter 

 of 5 to 120 minutes after exposure to auxin (Galston and Dahlberg, 

 1953). This remarkable property is more evident in old, non-growing 

 tissues than in young growing ones, and so it may play a role in aging 

 phenomena as well as in the control of apical dominance. 



Another interesting point concerning the effects of light on auxin 

 has been brought out by Galston and Baker (1951) who demonstrated 

 that, while light inhibits auxin action in etiolated plant parts, it 

 actively promotes auxin-induced growth in green tissues. These work- 

 ers observed, however, that much higher light intensities were effective 

 in this stimulation of growth than were involved in the auxin-destroy- 

 ing system. Furthermore, the stimulatory effect of light could be at 

 least partly substituted for by sugars. They suggested, therefore, that 

 the light-stimulation of growth in green tissues is a fimction of the 

 photosynthetic production of sugars. 



The action of light upon growth appears to be very complicated 

 indeed, involving its role in auxin formation, enzymatic and non-enzy- 

 matic auxin destruction, and the production of substrates for growth. 

 As will be pointed out later, light also reduces the sensitivity of tissues 

 to auxin and through its effect on polarity alters the type of growth 

 obtained. 



The auxin-destroying enzyme has been found to be most abundant 

 in roots (Tang and Bonner, 1947; Wagenknecht and Burris, 1950). 

 This is rather interesting since the polar transport of auxin would 

 otherwise be expected to accumulate large quantities of auxin in the 

 roots. Inasmuch as the roots are the most sensitive to auxin of any 

 plant organ, such an accumulation would be certainly detrimental to 

 the plant. 



It should be pointed out here that auxin destru( tion does not 

 specifically require any given pigment. While riboflavin is very effec- 

 tive in the destruction of auxin, a similar effect can be obtained in 



