Developmental Effects of Auxins 97 



of phototropic sensitivity could be produced. After the tinfoil barrier 

 was removed, phototropic sensitivity was restored. 



Differences in electrical potential between the lighted and un- 

 lighted side were proposed as early as 1927 by Brauner as a possible 

 explanation of the lateral transport of auxin. There are many types 

 of evidence which suggest that electrical phenomena may be involved 

 in auxin transport, and the evidence for and against such a mechanism 

 has been ably reviewed by Schrank (1951). 



A second mechanism which may participate in the phototropic 

 response is the light reaction of growing cells. There is good reason 

 to believe that light causes a desensitization of growing cells to a given 

 amount of auxin (van Overbeek, 1933; Galston and Baker, 1953). 

 Accurate experimental distinction between the reduction in sensitivity 

 to auxin and the capacity of the cells to destroy auxin remains to be 

 drawn. It has been estimated, however, that as much as 50 per cent 

 of the curvature of Avena coleoptiles toward light may be a result of 

 the reduced growth associated with the light reaction (van Overbeek, 

 1933). 



A third factor which may participate in phototropism is the for- 

 mation of auxin in response to light. Van Overbeek noted as early as 

 1932 that light can bring about auxin formation, and the action is 

 discussed in some detail in chapter III. As a factor in phototropism, 

 auxin formation serves mainly to reduce the tropic bending of green 

 plants and to affect the orientation of green leaves and stems in light 

 (Laibach and Fischnick, 1936). 



A fourth factor which may participate in phototropism is auxin 

 destruction by light. The action spectrum for phototropism, which 

 compares the relative effectiveness of different wavelengths of light in 

 stimulating the reaction, has been worked out in detail by Galston and 

 Baker (1949) as shown in figure 41 (p. 85). It can be seen that blue 

 light of wavelengths from 400 to 500 m^ is effective, and peak efEective- 

 ness is obtained with 440 m^. There is a striking similarity between the 

 wavelengths which are effective in causing phototropism and those 

 which are effective in causing auxin destruction (cf. figures 41 and 

 42). Phototropism as a response to weak light does not apparently 

 involve a reduction in total auxin present in a coleoptile, but it is 

 obvious that under strong light intensities auxin destruction will cer- 

 tainly occur on the lighted side, and will undoubtedly play a part in 

 phototropism. 



Early workers (Wald and DuBois, 1936) suggested that /3-carotene 

 may be the pigment responsible for the perception of light in photo- 



