162 Fundamentals of Auxin Action 



SUBSTANCES WHICH MODIFY AUXIN EFFECTS 



Auxin Synergists 



Many substances which are not auxins, since they cannot stimu- 

 late growth in auxin-free tissues, can promote growth through a syner- 

 gistic action with auxin. Conversely they can inhibit growth when 

 present in relatively high concentrations. 



10 



or 



I- 

 < 

 > 



O 



LlI 



> 



< 



100 MG/L 



100 MG/L 



CONG. OF TIBA 



Fig. 72. Elfects of TIBA (2,3,5-tiiiodobenzoic acid) upon auxin-induced curvature 

 in the Avena test. A: the inhibition of Avena curvature by high concentrations of 

 TIBA. The indoleacetic acid concentration was held constant at 0.175 mg./l. (from 

 Galston, 1947); B: increases in auxin-induced curvature by applying low concentra- 

 tions of TIBA. The indoleacetic acid concentration was held constant at 0.15 mg./l. 

 (from Thimann and Bonner, 1948). 



