594 



IF. S. HiUmnn and \V. K. Piirves 



I*I0"'^M lAA 

 G « lO'^M GA 



Fig. 3. Effect of a-(p-clilorophenoxy)isobutyric acid (PCIB) on responses of SI 

 sections to lAA and GA. 



activity has been reported as decreased or as unalTected by gibberellin 

 treatment, and gibberellin has also been found to inhibit the activity 

 of lAA oxidase preparations in vitro. Whether or not such results 

 are meaningful for the question at hand depends on whether lAA 

 oxidase or other peroxidase-based systems known to destroy lAA in 

 vitro are believed to do so in vivo. There is no compelling evidence 

 for such a view at present (cf. 4); while there are frequent correla- 

 tions between certain developmental phenomena and lAA oxidase 

 or peroxidase activity, the body of data is not consistent. 



The fact that GA can still promote pea section elongation in the 

 presence of inhibitory auxin levels (Figure 2) argues strongly against 

 any auxin-protecting mechanism of GA action. In addition, there 

 are in the literature growth systems in which auxin and gibberellin 

 appear to have ojjposite effects, and these are also inconsistent with 

 such a mechanism. 



The growth phenomena just mentioned are almost equally valid 

 objections to the suggestion (B) that gibberellin acts by increasing 

 the production or translocation of native auxin. Here again, reports 

 on whether or not native auxin levels increase following gibberellin 



