Does GA Act Through Auxin-mediated Mechanism? 



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DECAPITATED 



| = 3-I0'^M lAA 

 G = lO'^M GA 



Fig. 4. Effects of decapitation of test plants on responses of SI sections to GA and 

 lAA. Decapitation 6 hrs. before taking of sections. 



treatment are contradictory. If indeed they do, it still remains to be 

 shown that such increased levels are causally related to the growth in- 

 duced by gibberellin rather than merely correlated with it. GA pro- 

 motion of elongation in the presence of inhibitory lAA levels (Figure 

 2) is again difficult to reconcile with a mechanism invoking increased 

 auxin production, transport, or binding as the mechanism of GA 

 action, although it can be argued that exogenously applied lAA is not 

 physiologically equivalent to an increase in native pea auxin, whatever 

 its nature. Proponents of such a view would then have to postulate 

 at least partially different modes of action for native and exogenous 

 auxins. 



The two preceding hypotheses are relatively clear, and appear un- 

 satisfactory on the available evidence. In contrast, possible actions of 

 gibberellin on the availability of molecular sites for auxin action are 

 more difficult to formulate clearly, and thus to evaluate. The data 



