The Inhibition by Cytokinin of Auxin‐Promoted Elongation in Excised Soybean Hypocotyl

Abstract The experiments characterize the inhibition by kinetin of auxin‐promoted elongation in excised hypocotyl sections of 3‐day soybean seedlings (Glycine max cv. Hawkeye 63). It was found that concentrations of kinetin above 4.2 μM did not further inhibit auxin‐promoted elongation. Kinetin is as potent an inhibitor of elongation as actinomycin D or cycloheximide. Tissue incubated for 3 or 5 h in the absence of auxin or cytokinin would, upon addition of auxin, exhibit a new growth rate similar to that of tissue grown in auxin for the entire incubation period. Similarly, tissue grown for 3 and 5 h in the presence of auxin would revert to the control rate of elongation upon addition of kinetin. A 10 to 30 min preincubation in kinetin yielded the tissue incapable, for the ensuing 6 h, of increasing its rate of elongation in response to auxin. Zeatin and isopentenyladenine were more potent than kinetin and benzyladenine in the inhibition of elongation. Levels of ethylene produced in the presence of auxin plus cytokinin indicated that it was not involved in this auxin‐cytokinin interaction. Kinetin by itself did not promote elongation; nor did it enhance auxin‐promoted elongation at low auxin concentrations.