. 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). Chemical structure and biological activity relatively high concentrations where POA begins to inhibit root growth, non-specific toxic effects are certainly possible, and even if the inhibition should be caused by the residual auxin effect of the POA molecules, the affinity of these molecules for the growth centres must be higher than indicated by the CgQ-value (p. 94). A better measure for this affinity may per
. 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). Chemical structure and biological activity relatively high concentrations where POA begins to inhibit root growth, non-specific toxic effects are certainly possible, and even if the inhibition should be caused by the residual auxin effect of the POA molecules, the affinity of these molecules for the growth centres must be higher than indicated by the CgQ-value (p. 94). A better measure for this affinity may perhaps be obtained from combined experiments with 2:4-D (Aberg, 1952). We will use the concentration of the antagonist which, combined with 10~'^ M 2:4-D, gives a doubled growth rate as compared to that in 2:4-D alone (C = 200), and this concentration will be denoted C^ (negative logarithm = pC^). A calculation from a simple competitive model system shows that the C j- values of pure antagonists may be roughly comparable to the CjQ-values of. Mo/ar concn. —? Figure 2. The effects of phenoxyacetic acid {POA) and some of its derivatives upon the root growth of flax seedlings. The derivatives are indicated by means of the substituents: 2-Cl = 2-chloro-phenoxy- acetic acid {2-ClPOA), and so on. Values presented as in Figure 1. typical auxins as affinity measures, but because of the complexity of the physio- logical situation and the complications caused by the variations in intrinsic activity we prefer to use them in a purely provisional and exploratory manner in order to see if both sets of values may fit the same general picture. Even if we use the pC .^-value for POA (4-0) instead of the pCjo-value (3-4), the comparison gives an increase in affinity of 4-ClPOA (pCgQ = 6-8) of about six hundred times. For 2-chlorophenoxyacetic acid (2-ClPOA) there are clear indications of a weak auxin activity in the Avena cylinder test and in the pea test (see, for example, Muir et al., 1949; Wa
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