GibberelUc Acid and Auxin in Extension Growth 651 



of action of these inhibitors of section growth, but we consider their 

 effect relevant both because they appear to block the response of sec- 

 tions to light, and, in a number of preliminary experiments, to GA. 



Effect of Starvation of Sections 



We have shown (4) that rapid washing of sections in distilled water 

 affects their subsequent growth. This led us to investigate in greater 

 detail the effect of pretreatment of sections in aerated distilled water 

 over longer periods. A 3 hr. treatment has two obvious effects on 

 subsequent section extension. In the first place, it much reduces ex- 

 tension in buffer or buffer plus lAA, but normal extension is restored 

 on addition of sucrose (Table 8A); this is our main justification for 

 interpreting it as a starvation treatment. Secondly, it very strikingly 

 reduces the response to GA even when optimal lAA and sucrose are 

 present (Table 8B). We cannot decide, on the basis of the data at 

 present available, whether this is due to removal of some cofactor 

 necessary for growth, or to removal of an inhibitor normally re- 

 versed by GA. 



CONCLUSIONS 



Our knowledge of the mode of action of gibberellic acid has not 

 been greatly advanced by these experiments, but they have shown 

 that several factors are involved in the response of green sections to 

 GA, and that the response of green sections to light merits further in- 

 vestigation. 



The response to GA is conditional on the presence of an auxin. 

 We previously reported that synergism could be demonstrated not 

 only between GA and lAA but also between GA and synthetic auxin 

 analogues, but we did not then (4) give experimental evidence for 

 the statement. We have since repeated the relevant work several 

 times and it is now quite certain that a response to GA from green 

 pea sections can be induced by supplying either lAA, 2,4-D, or NAA 

 (Table 1). A very tempting explanation of lAA X GA synergism is 

 that GA directly or indirectly inhibits metabolic destruction of lAA, 

 and suggestions have been made that the activity of lAA-oxidase sys- 

 tems may be altered (6, 12), with a good deal of supporting evidence. 

 However, the fact that GA has a synergistic relationship with such 

 substances as NAA and 2,4-D, which are much less susceptible to 

 metabolic breakdown, throws considerable doubt on this theory, 

 unless it be supposed that such substances as 2,4-D and NAA them- 

 selves induce section extension by displacing endogenous lAA from 

 some physiologically inactive complex, as originally envisaged by 

 Skoog (13). There is little evidence that this does occur. 



The response of green sections to auxins, and therefore indirectly 



