GibberelUc Acid and Auxin in Extension Groxvth 



647 



Table 2. Interaction between light and sucrose in promotion of extension of pea 

 stem sections, using two pea varieties. Mean final lengths (mm.) of sections. Basal 

 medium: 10 /ig/ml indoleacetic acid in phosphate buflfer. 



Lighting Conditions . 



0.5 



Sucrose, Per Cent 



1.0 2.0 4.0 



Significant differences (P = .01) between means: 'Meteor,' 0.42; 'Improved 

 Pilot,' 0.44. 



tween GA and such auxin analogues as 1-naphthaleneacetic acid 

 (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) (Table 1). This re- 

 sult is relevant to discussions of the mode of action of GA and is 

 mentioned again later. 



Effect of Light 



Galston and Baker (7) showed that light enhanced extension of 

 green sections; they showed that it could be largely replaced by su- 

 crose, though their data also showed that sucrose could not com- 

 pletely substitute for light. In our experiments (Table 2) different 

 varieties of pea differed in their response to light. With sections 

 from the tall variety 'Improved Pilot' the effects of light and sucrose 

 were less than additive (light X sucrose interaction significant at P 

 = 0.001), indicating partial replacement of light by sucrose. Sections 

 from the tall variety 'Alaska' behaved similarly; this was the variety 

 used by Galston and Baker. But with sections from the dwarf pea 

 'Meteor' the effects of light and sucrose were approximately additive 

 (light X sucrose interaction not significant at P =r 0.05). Thus, par- 

 ticularly in 'Meteor,' light has some effect of importance which cannot 

 be replaced by sucrose. In the presence of optimal sucrose light has 

 the effect of enhancing the response to lAA (Table 3), 2,4-D and 

 NAA (Table 4). In experiments not reported here in detail we have 

 found that this is a high intensity light effect and that the response 

 is proportional to the length of exposure to light. 



The response to GA also is dependent on light (Table 5). Factorial 

 experiments in which light, lAA, and GA are supplied separately and 

 in combination (Table 6) suggest that the dependence of the GA re- 

 sponse on light is a consequence of the known GA X lAA interaction 



