. Auxins and plant growth. Auxin; Plant growth promoting substances. 178 Fundamentals of Auxin Action hypothetical receptor. The kinetic studies of auxins and anti-auxins described in the preceding chapter further indicate that the affinity of the auxin for the receptor can be competitively inhibited by anti- auxins (see figure 76). Thus the use of K values and maximum growth velocities have permitted a new level of definition of auxin activities. Returning to the concept of two-point auxin attachment, it was found that if the algebraic expressions were modified to fit the assump- tion of two
. Auxins and plant growth. Auxin; Plant growth promoting substances. 178 Fundamentals of Auxin Action hypothetical receptor. The kinetic studies of auxins and anti-auxins described in the preceding chapter further indicate that the affinity of the auxin for the receptor can be competitively inhibited by anti- auxins (see figure 76). Thus the use of K values and maximum growth velocities have permitted a new level of definition of auxin activities. Returning to the concept of two-point auxin attachment, it was found that if the algebraic expressions were modified to fit the assump- tion of two points of attachment between E and S in the given formula, an inhibition of growth by high concentrations of S (auxin) could be predicted. This would result from two auxin molecules be- coming attached to the receptor substance, one at each of the two points of attachment, each preventing the effective functioning of the other. In this way, the inhibition of growth by high auxin con- centrations envisaged earlier by Skoog et al (1942) has found more graphic expression. The two phases of the auxin curve are shown in figure 79. A numerical expression of the inhibitory effects of an auxin can be taken from the slope of the inhibitory section of the growth curve given in figure 78. Two reservations should l)e kept in mind concerning the kinetic £ E u < 01 o a: A A c=o Clf/\CI CI//\C H^C-0\l I H^C-0\ p=o \/ c=o OH A A_ Auxin Inhibition. 10"^ M CONC. OF AUXIN Fig. 79. The promotion and inhibition of growth by 2,4-D as interpreted by the theory of 2-point attachment. At low concentrations, both attachments are completed and growth is promoted. At high concentrations the auxin competes with itself for the two active sites and inhibition results (adapted from Foster et al, 1952).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble
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