74 Fundamentals of Auxin Action 



reappearance was roughly proportional to the light intensity applied 

 (figure 36). Some evidence was brought forth to suggest that conditions 

 favoring auxin formation were the same as those favoring photo- 

 synthesis. 



The role of light in the formation of auxin presents something 

 of a paradox, for not only does light cause the formation of auxin, 

 but simultaneously it causes the destruction of auxin as was deduced 

 from some of the early experiments on phototropism. The destruction 

 of auxin by light is more striking, however, in etiolated plant mate- 



20' 



LiJ 

 (T 



Z) 



< 

 > 



O 



§ 



15- 



10- 



5- 



o 



o 

 / 



o 



3000 



6000 ERGS/CM^/SEC. 



LIGHT INTENSITY 



Fig. 36. The effectiveness of light of various intensities upon auxin formation in 

 tobacco plants (Avery et al, 1937). 



rial than in green plants. Moderate amounts of light can cause de- 

 struction of a large proportion of the auxin in etiolated seedlings, 

 contributing to the tropic responses, whereas green plants can main- 

 tain high auxin levels even under rather high light intensities. 



Evidence that auxins gradually disappear in darkened plants has 

 been accumulated by several workers. A good example of this was the 

 demonstration by Zimmerman and Hitchcock (1937) that darkened 

 tomatoes lose their sensitivity to geotropism and that the sensitivity 

 can be restored by the application of auxins to the plants. 



The formation of auxins in the light and their disappearance in 



