. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Para-substitution in regulators with phenyl nuclei most pronounced with 4-EtPOA, and correspondingly we find stronger stimulations in the wheat root test with this substance than with 4-MePOA {Figure 7). The intermediate character of 4-MePOA is also apparent from the type of action curve obtained in the Avena cylinder test {Figure 1). In the flax root test with externally applied 2:4-D the antagonistic effect o
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Para-substitution in regulators with phenyl nuclei most pronounced with 4-EtPOA, and correspondingly we find stronger stimulations in the wheat root test with this substance than with 4-MePOA {Figure 7). The intermediate character of 4-MePOA is also apparent from the type of action curve obtained in the Avena cylinder test {Figure 1). In the flax root test with externally applied 2:4-D the antagonistic effect of the 4-alkyl-phenoxyacetic acids culminates with 4-?.fopropyl-phenoxyacetic acid (4-/P-POA), which also gives a significant stimulation of flax root growth in the range 10~'^ to 10~^M. With 4-?^ acid (4-/B-POA) both this stimulation and the restorative effect upon 2:4-D- inhibited flax roots are slightly weakened (Aberg, 1954). For wheat roots, on the other hand, growth stimulations have already Figure 7. The effects of some pzn-a- alkyl-phenoxyacetic acids on the growth of flax and wheat roots. Me = 4- methjlphenoxyacetic acid {4-MePOA), and so on. Et = ethyl, iP = isopropyl, tB = , Growth values (G) in per cent of control, presented as in Figure 1. The iP and XB effects on wheat roots do not differ significantly, and the values for both substances have therefore been jointly 10 ' 10" 10 ^ 10 10 ^ 10'^ 10 "' Molar concn. —- reached their maximum with 4-EtPOA {Figure 7). This is probably partly related to the lower auxin sensitivity of this material as compared with flax roots. To some extent it may perhaps also be caused by a decreased 'uptake' of the substances with 'bulky' alkyl-substituents (cf. p. 95). Summarizing, it may be said that the highest intrinsic auxin activity is possibly attained in the 4-FPOA molecules which, however, show a low affinity to the active sites. In 4-ClPOA this affinity is much increased, and
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