Growth Regulating Effects of Pheyioxy Compounds 227 



component is still present in 3-IPOA and 3-NiPOA. It is suggested 

 that the meta compounds and, for example, 3,4-CloPOA are closely 

 related to 2-NOA. The ori/io-substituted compounds generally show 

 a fairly weak activity with both auxin and antiauxin components. In 

 2,3-Cl>POA the antiauxin component is fairly strong, and it is sug- 

 gested that this substance is closely related to 1-NOA. The lack of 

 stimulating effects on wheat root growth observed for 2-ClPOA re- 

 appears for 2,6-Cl2POA, which may otherwise be characterized as an 

 intermediate growth regulator. The presence of an auxin component 

 in its activity is further indicated by the positive effects obtained in 

 the pea curvature test (18). In 3,5-CloPOA the antiauxin component 

 is wholly prevailing, but the appearance of a clear auxin component 

 in (-f )3,5-Cl2POP Figure 2 shows that the presence of two meta sub- 

 stituents is not wholly incompatible with auxin activity. The racemic 

 3,5-Cl2POP shows very weak activity in the pea curvature test accord- 

 ing to Toothill et al. (18), but these authors hold that side-chain sub- 

 stitution has a negligible effect on 3,5-derivatives. It seems to be an 

 interesting task to study further how far 3,5-Cl2POA is related to 

 phenoxyacetic acids with large /jrtra-substituents. The induction of 

 auxin activity by further substitution in the ring of 3,5-Cl2POA (18), 

 however, seems to indicate that simple dimensional factors can hardly 

 be at work. 



Rather unexpectedly, 2,4,6-Cl3POA shows a purer antiauxin char- 

 acter than does 2,6-CloPOA, and both a-propionic acids are stronger 

 antiauxins than the acetic acid (Figure 4B). As the racemic 2,4,6- 

 CI3POP is reported to show clear auxin activity in the pea curvature 

 test (12, 18), this is apparently not true for all types of tests. The 

 thymoxyacetic acid (2-iP, 4-Cl, 5-MePOA) exerts only antiauxin effects, 

 presumably due to the large o><//o-substituent. The corresponding 

 (_^) propionic acid, however, is a regulator with quite unusual proper- 

 ties (Figure 4A). Wheat roots are strongly inhibited at 10 'M, 

 but at 10-« to 3 X 10 ''M the inhibition has changed into a con- 

 spicuous stimulation. It is possible that this type of concentration- 

 response curve can be related to that of 2,3,5-triiodobenzoic acid 

 (TIBA) (7), but no stimulation in the oat cylinder test correspond- 

 ing to that obtained with TIBA is apparent. A response of wheat 

 roots similar to that for (+)2-iP, 4-Cl, 5-MePOP has been observed 

 for (+)a-(2-naphthoxy)-;?-valeric acid which belongs to the intermedi- 

 ate growth regulators (unpublished data). 



THE EFFECTS OF OPTICAL ACTIVITY 



The effect of optical isomerism in plant growth regulators is not 

 absolute, but is related to the molecular structure as a whole. The 

 slightest effect is found in the a-(indole-3)propionic acid, and the 



