Geotropic responses in roots 



Another method of approach has involved the external application of 

 auxins to the roots. In this connection, only normal intact roots are of interest. 

 The response of isolated or decapitated roots, in which the auxin level has 

 been reduced artificially, permits no conclusions as to the auxin level in 

 intact roots. 



A mostly slight, but significant acceleration of root elongation following 

 the addition of auxin has been demonstrated in a number of roots {Table 1). 

 These results seem to indicate a sub-optimal auxin concentration in the roots 

 (point B in Figure 1). It would still be possible to apply the classical auxin 

 theory of geotropism to such roots. The change in auxin concentration 

 would only have to be large enough to bring the concentration in the lower 

 half of the root well above the optimum. Theoretically, one would expect 

 that very slight geotropic stimulations would result in negative curvatures, 

 but these might be so small and transitory that they would escape observation. 



Table 1 

 Examples of stimulation of elongation in intact roots after application of lAA 



t After application of lAA. 



J § Auxin treatment 15 and 40 minutes, respectively. 



II Treated with K-salt of lAA. 



It can be questioned, however, whether the reported results really indicate 

 sub-optimal auxin concentrations. As far as can be seen from the papers 

 cited [Table 1), the positive response to added auxin was noted only after 

 several hours, generally 17 hours or more. Only Naundorf (1940) reported 

 a considerable increase in root length (in Helianthus) 5 hours after the addition 

 of auxin. But even 5 hours is a very long time as compared with the time 



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