96 Fundamentals of Auxin Action 



TROPISMS 



The first recognized developmental effect of auxin was its partici- 

 pation in tropisms and plant movements. Indeed, the very discovery 

 of auxins resulted from studies of the tropisms and it is through the 

 effects of auxins on the curvature of Avena coleoptiles that the prin- 

 cipal bioassay for auxin is carried out. 



Phototropism 



If we consider first phototropism, it will be recalled that Cholodny 

 (1927), and Went (1928) each concluded independently that unequal 

 distribution of growth hormone was responsible for the turning of 

 plants toward light. Perhaps one of the most graphic demonstrations 

 of the differences in auxin distribution following unilateral exposure 

 to light is one in which auxin was diffused simultaneously from the 

 lighted and from the unlighted sides of coleoptile tips. Not only was 

 it found that more auxin was obtained from the unlighted side than 

 from the lighted, but also the unlighted side yielded more than similar 

 halves of non-stimulated control tips. Thus it was clear that a lateral 

 redistribution of auxin was involved (Went, 1928). The unequal dis- 

 tribution of auxin in the coleoptile results, of course, in unequal 

 growth on the lighted versus the unlighted side and a consequent 

 curvature toward the light. 



The unequal distribution of auxin is apparently largely a result 

 of light-induced lateral translocation of auxin. The work of van Over- 

 beek (1933) indicates that the total amount of auxin in coleoptiles 

 which have been exposed to weak light is essentially the same as in 

 coleoptiles kept in the dark, but that 85 per cent of this auxin present 

 exists on the shaded side and only 15 per cent on the lighted side. The 

 fact that almost the same lateral distribution resulted if the coleoptile 

 tip were replaced with an auxin block indicated that the irregular 

 distribution is not attributable solely to lateral differences in produc- 

 tion of auxin by the tip. The auxin block, of course, furnished auxin 

 uniformly to all sides of the coleoptile and the resultant uneven dis- 

 tribution must necessarily have been a result of lateral transport of 

 the auxin. 



A clear demonstration of the lateral redistribution of auxin in 

 response to light can be found in the studies of Boysen-Jensen and 

 Nielsen in 1926. These workers showed that by making a vertical slit 

 through the coleoptile and inserting a tinfoil barrier in such a way 

 that the lighted side was not in contact with the unlighted side, a loss 



