86 Fundamentals of Auxin Action 



of the wavelengths of light which are effective in causing this destruc- 

 tion have demonstrated that the action spectrum for auxin destruction 

 in both such systems (figure 41) is strikingly parallel to the absorption 

 spectrum of riboflavin shown in figure 42 (Galston and Baker, 1949). 

 The light-inactivation of auxin can also be mediated by pigments 

 other than riboflavin and so it would seem unnecessary to assume that 

 riboflavin is the only pigment in plants which serves this function. 

 y8-carotene has an absorption spectrum very similar to riboflavin 



O 



UjCQ 

 UjO 



Li_> 

 LUiZ 



< 

 LJ-I 

 >liJ 



100 



80 



60 



40 



20 



400 



420 



440 



460 



480 



500 mu 



WAVELENGTH 



Fig. 42. A comparison of the action spectrum for phototropism in the Avena 

 coleoptile with absorption spectra of riboflavin and ^-carotene (Galston and Baker, 

 1949). 



(figure 42) and it has been pointed out that it is difficult to distinguish 

 between the two possible pigments on the basis of the action spectrum 

 information (Galston, 1950). 



Jn vivo studies have clearly shown that the effectiveness of a given 

 amount of auxin is strongly reduced by exposure of plant material 

 to light, as in the pea straight-growth test (Galston and Hand, 1949). 

 How much of this reduced growth response is owing to the enzymatic 

 destruction and how much is caused by non-enzymatic destruction is 

 not yet clear. Some preliminary evidence suggests that the enzymatic 

 process may be responsible for destruction in weak light, and the non- 



