228 



B. A berg 



MOLES 



LITER 



Fig. 4. Growth effects on wheat roots of: A. (-i-)Chlorothymoxypropionic acid 

 [(-|-)2 IP,4-Cl,5-MePOP] and triiodobenzoic acid (TIBA); B. Of 2,4,6-trichloro- 

 plienox) acetic acid (2,4,6-Cl3POA) and the corresponding propionic acids, i.e., dex- 

 trorotatory propionic acid [(+)?], and laevorotatory [(_)P]. Data presented as in 



Figure 1. 



Strongest effects among the phenoxy- and naphthoxy-propionic acids 

 (see 8), where the change from d to l form does often result in a 

 complete change from auxin to antiauxin activity. It is of interest 

 that such effects are also found among the structurally unique thio- 

 carbamic acid derivatives (unpublished data). 



The usually strong effect of optical isomerism among the phenoxy 

 derivatives is strongly weakened or almost completely abolished either 

 by the insertion of a large substituent in the a-position in the side 

 chain (e.g., POC) or by some types of nuclear stibstitution (e.g., 2,4,6- 

 CI3POP, 2,4,5,6-Cl4POP). For 2-iP, 4-Cl, and 5-MePOP the situation is 

 complicated by the occurrence of an unustial, possibly synergistic, 

 component in the activity of the (-|-)-form. 



In the case of the 2,4,6-Cl3-phenoxy compounds (Figure 4), both 

 propionic acids show a strongly increased stimulating effect on w'heat 

 root growth in comparison with the acetic acid, which might indicate 

 that the introduction of an a-methyl group in the side chain compen- 

 sates for an otherwise weak affinity to the receptor sites of this ma- 

 terial. 



The D-propionic acids related to acetic acids of intermediate char- 

 acter often show stronger and purer atixin activity than the corre- 

 sponding acetic acids [e.g., (+)POP, (-f)4-IPOP, (+)2,3-CLPOP]. 

 Weak auxin activity may appear even if the acetic acid is purely anti- 



