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Importance of Time after Excision and of pH on the Kinetics of Response of Wheat Coleoptile Segments to Added Indoleacetic Acid ¹

^a Chemistry and Biology Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6

Segments of coleoptiles of 3-day-old wheat (Triticum x Aestivum L. cv. Kharkov M.C. 22) grown at 24 C were strung on a glass rod and the kinetics of their elongation in 0.01 M K-phosphate buffer was examined photometrically. Measured rates of elongation in response to treatments were corrected by subtraction of endogenous rates. The customary practice of testing the effects of growth regulators added between the two endogenous surges of growth, that is, up to 3 hours after segments were excised from coleoptiles, gave erroneous kinetic data. Rates of response were then limited by the passive penetration of added auxin and the second endogenous surge interfered with late responses. It was necessary to wait for a phase of more rapid but more steady elongation after the second endogenous surge was over, about 4 hours for wheat at 25 C, to attain the active uptake required for nearly synchronous response through the segment. The more active uptake in this steady phase was confirmed with -[2-¹⁴C]indoleacetic acid and it was greater at pH 5 than at pH 7. The degree of dissociation of indoleacetic acid added at pH 7 was an impediment to penetration that could be compensated for by removal of intercellular air. The pH did not influence the endogenous rate of elongation. The dependence of the rate of elongation on the concentration of indoleacetic acid added at pH 5 was bell-shaped with maximum rate at 10 µM indoleacetic acid, in confirmation of previous measurements made over long intervals of time. The relation between the response and suboptimal concentrations was not sigmoid but was indicative of greater binding affinity than previously reported.