Does GA Act Through Auxin-mediated Mechanism? 591 



S4, and greater in S4 than in S7 sections. In summary, the greatest 

 response to GA appears in the more apical sections, while the greatest 

 response lo lAA is in the lower, more mature tissues (8). 



When similar experiments are conducted in the absence of su- 

 crose, the results are different. Under such conditions, the endogenous 

 elongation of SI sections is greatly reduced. The response to GA of 

 all types of sections is so low as to be frequently undetectable; GA 

 and sucrose are in fact synergistic in promoting elongation of SI sec- 

 tions (9). The interaction between sucrose and lAA in the elongation 

 of SI sections is different and more complex; it has been described in 

 detail elsewhere (7). In brief, concentrations of lAA (> 10-« M) which 

 inhibit elongation in the presence of sucrose will promote elongation 

 in its absence. 



Since the object here was to measure the physiological effects of 

 GA and lAA separately and together with a view to judging the de- 

 gree to which these might be biochemically linked, the first note- 

 worthy point is the apparent spatial separation between tissues most 

 responsive to GA and those most responsive to lAA under the con- 

 ditions employed. A number of workers (cf. 2) have noted the rough 

 correlation between endogenous growth capacity and responsiveness 

 to GA. It has been suggested that the more apical sections show a 

 greater response to GA because of their higher endogenous auxin 

 level, a factor which might also occasion their lower auxin response. 

 A partial test of this hypothesis can be made by seeing whether the 

 response of more basal sections to GA is increased in the presence of 

 lAA. Results of all such experiments have been negative. A typical 

 experiment is summarized in Figure 1, where it is evident that the 

 increment of elongation caused by GA in SI sections is greater than 

 that in S5 sections; levels of lAA which increased S5 elongation to that 

 of the endogenous SI elongation failed to increase their GA response. 



Numerous systems have been described in which the GA response 

 is markedly dependent on exogenous lAA and in which synergisms 

 between lAA and GA occur. We have not found any such relation- 

 ships in etiolated pea epicotyl sections. Usually the elongation caused 

 by one substance is approximately additive to that caused by the 

 other under all conditions tried. This is true even if the test sec- 

 tions are starved by incubation in sucrose-free buffer for 24 hrs. be- 

 fore the test proper commences (9). It is also particularly significant 

 that, in SI sections, a marked and only slightly reduced response to 

 GA is obtained even in the presence of highly inhibitory levels of 

 lAA. An experiment showing this is presented in Figure 2. 



Our further attempts to establish some relation between GA and 

 lAA metabolism took the form of treatments which would affect the 



