416 A. C. Leopold and S. L. Lam 



benzoic acid (TIBA), 2-naphthoxyacetic acid (NOA), and indole-3- 

 propionic acid (IPA). Each of these except the MOPA and NOA has 

 been reported as being an inhibitor of lAA transport, and these last 

 two had been found to be inhibitors in experiments in this laboratory. 

 Results of typical inhibition experiments are presented in Table 

 1. The tests were conducted with each transported auxin in one test, 

 so that the relative effectiveness of the various inhibitors as they in- 

 fluence one auxin is a more reliable comparison than as they influ- 

 ence the several auxins. The data show clearly that each of the in- 

 hibitors is effective against the transport of any of the three auxins. 

 Within this array of transport inhibitors, then, there is no evident 

 distinction between the auxins being transported. It is clear that 

 TIBA is by far the most potent inhibitor of transport of each of the 

 three auxins, causing more than 80 per cent inhibition of transport 

 in each instance at a concentration of only \{)-^M. The inhibitions 

 obtained with the other inhibitors appear to be comparable to one 

 another. It might be mentioned that the naphthalene containing 

 inhibitor NOA was not apparently more effective against the trans- 

 port of NAA than the other inhibitors. Similarly, IPA was not more 

 effective against the transport of the indole auxins than the other in- 

 hibitors tested. 



DISCUSSION 



The experiments reported here provide a comparison of the trans- 

 port characteristics of three different auxins: lAA, NAA, and IBA. It 

 is clear that all three are polar in their movement, and they vary 

 somewhat in velocities. They are all sensitive to the same transport 

 inhibitors, and there does not appear to be a selectivity by transport 

 inhibitors for the different auxins. The transport of the three auxins 

 therefore appears to be very similar indeed, except for the diilerences 

 in velocity. 



While lAA and NAA appear to have very closely similar transport 

 velocities, IBA is markedly slower. It has been established by Fawcett 

 et al. (1) that IBA is metabolized in plant tissues giving rise to lAA, 

 and while it is not certain that this conversion must occin- before 

 growth activity is obtained, it is reasonable to assinne that there 

 would be a gradual production of lAA in the tissues during transport 

 tests. The delay in conmiencement of cmvatine responses in the time 

 curves with IB.\ may well be due to the time required for the con- 

 version of the IBA into lAA, which then would, of course, have the 

 transport characteristics of that auxin. 



A comparison of the transport velocities of these three auxins plus 

 anthraceneacetic acid was made by Went and ^Vhile (10) with some- 



