98 Fundamentals of Auxin Action 



tiopisni of the Avenn coleoptile. The action spectrum of phototropism 

 is very similar to the absorption spectrum of /^-carotene and they found 

 this pigment to be present in the coleoptile. More recently Galston 

 (1949) has demonstrated that riboflavin can mediate the photodestruc- 

 tion of auxin, that it is present in the coleoptile, and that it too shows 

 aj>proximately the same absorption spectrum. The debate as to 

 whether /^-carotene or riboflavin is the phototropic pigment is not yet 

 settled, ^-carotene lacks the ability to mediate the photodestruction of 

 auxin /// xiitro (Reinert, 1953), and by masking the riboflavin absorp- 

 tion wavelengths it can reduce the photodestruction in solutions of 

 riboflavin as well as reduce phototropic sensitivity of the mold, Phy- 

 comyces (Reinert, 1952). The largest factor in phototropic responses 

 to weak light, however, is the induction of lateral transport of auxin, 

 and the relative merits of yS-carotene and riboflavin in this function 

 have never been explored. It seems possible that phototropism may 

 not be mediated by a single specific pigment, but more than one pig- 

 ment may absorb the light which activates the response just as more 

 than one pigment may activate the photodestruction of auxin. 



An interesting question related to phototropism has been raised 

 in the literature. What is the identity of the auxin responsible for 

 phototropism? Several workers have found that the exposiue of de- 

 capitated coleoptile sections to luiilateral light will result in curvature 

 if preparations of auxin diffused from plants are added, but if in- 

 doleacetic acid is added instead no curvature will be obtained (van 

 Overbeek, 19.S6). On this basis it has been proposed that auxin a may 

 be the auxin responsible for phototropism. Carrying this a step fur- 

 ther it has even been proposed that the effect of indoleacetic acid upon 

 growth in general is obtained simply by causing the release of auxin 

 (I (von Guttenberg, 1942). The evidence on this point with reference 

 to phototropism is carefully reviewed by Galston (1950) who raises 

 the logical question of the purity of auxin a preparation in the photo- 

 tropism studies, and points out that conclusions from these experi- 

 ments must be tentative until pure auxin a preparations can be com- 

 pared with indoleacetic acid in this respect. There is considerable 

 doubt of course as to whether auxin a even occurs naturally in plants 

 and whether it would be an active auxin if it did occur there (see 

 chapter III). 



Geotropism 



1 he phenomenon of geotropism was clarified almost simultane- 

 ously with phototropism. This tropism appears to be dependent upon 



