Does GA Act Through Auxin-mediated Mechanism? 



593 



4 - 



E 

 £ 



z 

 o 



< 



z 2 



o 



_i 



UJ 



lO'^M GA 



I = lO-'^M lAA 

 Fig. 2. Effect of inhibitory levels of lAA on response of SI sections to GA. 



test sections are cut from them. As shown in Figure 4, such treatment 

 again markedly changes the lAA response. The control elongation of 

 sections from decapitated plants is reduced, and a level of lAA for- 

 merly inhibitory now promotes elongation. Again, however, the ad- 

 ditional elongation caused by GA is unaffected by such changes. 



DISCUSSION 



Because of space limitations this discussion will have to proceed 

 in general terms; for detailed citations of the relevant literature see 

 (2) and (9), and elsewhere in this volume. 



Evidence Is Against Auxin-mediated Mechanisms 



Some possible mechanisms of auxin-mediated gibberellin action 

 were summarized earlier, and that outline will be followed here. 



The possibility (A) that gibberellin acts by protecting native or 

 exogenous auxin from inactivation in some manner has been raised 

 by a number of workers who have interpreted gibberellin effects on 

 peroxidase or lAA oxidase activities as evidence for this view. How- 

 ever, the literature taken as a whole is somewhat contradictory. Re- 

 ports have been made of decreased, unchanged, or increased peroxi- 

 dase activities in extracts from plants previously treated with gib- 

 berellin; of these, only the first kind would be consistent with an 

 auxin-protecting or auxin-sparing role of gibberellin. lAA oxidase 



