Gibherellic Acid and Auxin in Extension Growth 



Table 6. Interaction between light, gibberellic acid, and indole- 

 acetic acid in promotion of extension of 'Meteor' pea stem sections. Basal 

 medium: 1 per cent sucrose in phosphate buffer. 



649 



Significant difference between means: 0.25 (P = 0.05), 0.33 (P = .01), 

 0.43 (P = .001). 



and also the necessity for light if lAA is to have its full effect. 



It is, of course, equally true to say that light has its greatest effect 

 in the presence of an auxin and that its effect is still further enhanced 

 if GA is supplied (Table 6). One possible interpretation of this light 

 effect is that some product of photosynthesis, other than sucrose, is 

 essential for the growth reaction in which auxin intervenes. A num- 

 ber of possible early products of COo-fixation have been tested, in- 

 cluding glucose-6-phosphate, glucose- 1 -phosphate, fructose-l,6-diphos- 

 phoric acid, fructose-6-phosphoric acid, 2-phosphoglyceric acid, and 3- 

 phosphoglyceric acid, but none of these had light-replacing properties 

 greater than sucrose. Nevertheless, some indirect evidence that a prod- 

 uct of photosynthesis may be involved has arisen from work with 

 specific inhibitors. 



Effect of L-Azaserine and 6-Dlazo-5-oxo-L-norleucine 



Two antibiotics, azaserine (AZS) and 6-diazo-5-oxo-L-norleucine 

 (DON), which have marked tumor-inhibiting properties, have been 

 shown by Calvin and his colleagues (1, 11) to have specific inhibitory 

 effects on the COo-fixation pattern of photosynthesizing algae. Both 

 substances are powerful inhibitors of lAA-induced extension of green 

 sections (Table 7). Furthermore, doses which have little effect on ex- 

 tension of sections in darkness cause a marked inhibition of extension 

 in light. Thus these substances appear to inhibit those growth pro- 

 cesses initiated by exposure to light. Specificity, if it exists, is not 

 complete because higher doses will inhibit extension in darkness. 

 Their anti-tumor activity has been attributed (5, 9, 10) to interference 

 with biosynthesis of purines or some amino acids, and their effects 

 can to some extent be reversed by some purines, cyclic amino acids, 



