Pflra-substitution in regulators with phenyl nuclei 



pea test (Thimann, 1952; Wain and Wightman, 1953; Osborne g/ a/., 1954), 

 which may, however, possibly be of a synergistic nature (Veldstra, 1953) or 

 due to the combined effect of very weak auxin activity and synergistic 

 activity. 



The anti-auxin effects of 2:4:6-T are clearly shown by its inhibiting action 

 on Avena coleoptile section growth induced by simultaneously applied 2:4-D 

 or lAA (McRae and Bonner, 1952) and from its growth-restoring effects on 

 2:4-D-inhibited flax roots (Aberg, 1954; see aho Figure 5). From the experi- 

 ments made by Hoffmann (1953) with tomato plants it seems clear that 

 2:4:6-T not only counteracts externally applied auxins of different types, but 

 also interferes with plant responses controlled by native auxins, such as 

 geotropic curvatures and rooting of cuttings. 



In the wheat root test used in the present investigation, 2:4:6-T gives 

 comparatively slight but highly significant stimulations [Figure 5). For 

 the cell length, however, Hansen (1954) did not find any corresponding 

 increase. As in the case of 2:6-D we must expect the 2:4:6-T curve to be the 

 complex resultant of counterbalancing effects, and it is quite natural that 

 some differences should occur when using different materials and methods. 



On the whole it seems safe to conclude from experiments with wheat roots, 

 with 2:4-D-inhibited flax roots {Figure 5), and with Avena coleoptile sections 

 (McRae and Bonner, 1952), that the introduction of a para chlorine atom 

 in 2:6-D to give 2:4:6-T increases the anti-auxin component of its activity. 

 This increase is probably related to the higher affinity of 2:4:6-T molecules 

 for the growth centres, but may also be partly related to a decreased intrinsic 

 auxin activity of these molecules. 



In the clearly anti-auxinic 3:5-dichlorophenoxyacetic acid (Hansen, 1954), 

 /)flra-chlorination to give 3:4:5-trichlorophenoxyacetic acid induces some 

 slight positive auxin activity (e.g. in Avena cyUnder test and pea test. 

 Wain and Wightman, 1953). Further comparative studies of this highly 

 interesting pair of substances in different types of growth tests, especially 

 those able to show the anti-auxin components of their activity, seem necessary 

 before this phenomenon can be more closely discussed. 



THE EFFECTS OF para SUBSTITUENTS OTHER THAN CHLORINE 



In phenoxyacetic acid the introduction of a para chlorine atom results in 

 maximum auxin activity of the new compound. Fluorine or bromine, on the 

 other hand, gives a compound with considerably lower gross auxin activity 

 than 4-chlorophenoxyacetic acid, but upon closer inspection the two 

 compounds are found to be of rather different character. 



In the flax root test, the growth-inhibiting activity of 4-fluorophenoxy- 

 acetic acid (4-FPOA) is five to six times lower than that of 4-ClPOA {Figure 6) ; 

 further, the inhibition caused by 4-FPOA is strongly counteracted by the 

 auxin antagonist 1-NMSP. In the wheat root test, 4-FPOA gives a rather 

 slowly declining curve {Figure 6), and no stimulation is apparent at low 

 concentrations, whereas in the Avena cylinder test, it causes strong growth 

 stimulations. It seems fairly probable, therefore, that 4-FPOA should be 

 characterized as an auxin with high intrinsic activity but with an affinity to the 

 active sites which is well below that of 4-ClPOA. The action curve of this 

 latter substance in the wheat root test {Figure 6) shows a slight but highly 



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