No abstract

The effects of ethylene in lowering levels of auxin diffusing through plant tissues are well known (1, 6-8, 12, 16, 20, 22, 32 Corporation, Washington, West Virginia. The canister was constructed of degreased galvanized pipe (10 X 40 cm) with circles of screen wire in drilled pipe caps fitted with laboratory gas outlets. Teflon tape rather than oil was applied to the pipe threads to allow easy disassembly for refilling and to prevent contamination of the Purafil.In the case of plants placed in plastic bags, temperatures inside the bags were maintained at 24 C during the light phase.Unless indicated otherwise, plants were exposed to ethylene at concentrations of 25 or 100 bld/liter for an 18-hr period.The 18-hr period consisted of 6 hr of light, 8 hr of darkness, and light during the remainder of the pretreatment period.Auxin Transport. Following procedures described earlier (31), we measured auxin transport through the uppermost internode of Coleus stems. Stem segments 5 mm in length were removed from the treated and untreated plants, coated with a ring of Vaseline to prevent surface transport, and placed on glass slides. One percent agarose cylinders of a volume of 23 ,ul containing IAA-l-"4C, 5 mg/liter (32,600 cpm average per block), served as auxin donors on the physiological apical end and similar cylinders without IAA served as receivers. During the transport period the sections were in a nearly vertical position with the donor above and the receiver below. After 5 hr in a moist chamber kept in the dark, the donor and receiver cylinders and the stem sections (macerated) were placed in counting vials separately or in groups of five and frozen. The radioactivity was assayed with a Nuclear-Chicago liquid scintillation counting system following procedures described in an earlier paper (33). A second counting medium which proved to be more convenient for counting the radioactivity in aqueous samples was used in later experiments. This medium consisted of Spectrograde toluene containing the fluors PPO and POPOP mixed with Triton X-100 in a ratio of 7:6.

Application of gibberellic acid (GA) to the apical region of the stem enhances 14CO2 release from tryptophan‐l‐14C in cell free preparations of the apical region. Although GA when applied to the apical region markedly accelerates abscission rates of debladed petioles at the 4th node, the enhancement effect on tryptophan metabolism appears to be restricted to the apical bud region. The increased levels of diffusible auxin in Coleus stems, observed earlier by Muir and Valdovinos (1965), appear to be due to the GA effect on auxin precursor conversion rather than to an altered rate of auxin destruction. GA pre‐treatment does not significantly alter destruction rates of auxin in the stem tissue. This is demonstrated by the release of 14CO2 from IAA‐1‐14C by sections of internode tissue. While a multiple deblading pattern retards abscission of debladed petioles considerably, application of GA to debladed petioles at the basal region of the stem restores the normal rates of abscission at debladed distal nodes. No significant change in the abscission rates at treated nodes is observed. The GA effect on abscission at distal nodes is attributed to the effect of the growth substance on auxin precursor conversion in the apical region. In these experiments, as in the case of plants treated in the apical region with GA, auxin destruction rates in the stem are not altered significantly.