The interaction between auxin and ethylene and its role in plant growth.

Auxin induces ethylene formation in several plants1' 2 and in segments cut from roots, stems, and fruits.3 If ethylene alters the metabolism of auxin, as has been suggested,2' 4, 5 then this auxin-induced ethylene formation might interfere with studies of the uptake, transport, and destruction of the applied growth hormone. Moreover, the newly synthesized ethylene would mask or modify a response to applied gas and thus complicate any investigation of the effect of ethylene on auxin metabolism. It will be shown that all of these problems arise under the conditions commonly used to study auxin metabolism, and the significance of this finding will be discussed with special reference to a postulated mechanism of ethylene action. Materials. -Pea seeds (Pisum sativum var. Alaska) were soaked for 5 hr and planted in vermiculite if the seedlings were to be etiolated, or in potting soil if light-grown. The plants were kept at 23 4 30C either in darkness with occasional dim red light for 7 days, or on a 16-hr light cycle under Gro-lights for 14 days. Sections were then cut from the apical part of the uppermost internode. Other seedlings were planted in the same manner as etiolated pea plants, except that corn (Zea mays var. Barbecue) was sterilized in 10% Clorox for 10 min and rinsed for 3 hr before it was sown. Oat (Avena sativa L. var. Victory) and corn coleoptiles were harvested at 80 hr, sunflower (Helianthus annus L.) after 5 days, and sections were cut 2-3 mm below the cotyledons or tip. Methods. -Straight growth test: Groups of ten 10-mm sections were placed in 10 ml of solution contained in 125-ml flasks. The medium typically consisted of 10-6 M indole-3-acetate (IAA), 2% sucrose, 5 X 10-2 M potassium phosphate buffer (pH 6.8), and 5 X 10-6 M CoCl2 in glassdistilled water. To add ethylene, the flasks were sealed with vaccine caps and a known quantity of a calibrated ethylene-air mixture was injected. The flasks were gently shaken in the dark at 230C, and at various times the fresh weight and length of the sections were determined. In some studies the tissue was shadowgraphed with green light, and stem curvature measured from the film with a protractor. Ethylene production was estimated by removing samples of air from the flasks and gas-chromatographing them on an alumina column.6 Transport and destruction of C14-IAA: Polar transport through 7-mm stem sections was determined by applying apical donor blocks containing carboxyl-labeled C'4-IAA (8mC/mM) in 3% agar, and 1.5% agar receiver blocks. After 2 hr the receiver blocks were remelted, dried on a grooved planchette, and counted with a 30% efficient automatic gas-flow counter; the radioactivity remaining in the stem segments was extracted by crushing them on a planchette with 1 ml of 50% ethanol. Plating a homogeneous suspension of duplicate tissue at less than 2 mg/cm2 resulted in the same count, so it was concluded that self-absorption was not a complication. All transport experiments were carried out in sealed desiccators at 100% relative humidity, and enough ethylene was added to bring the concentration to 100 ppm. Polar transport was also studied indirectly by incubating 15-mm pea stem sections in media containing 10-6 M C'4-IAA (8mC/mM). At various time intervals the sections were rinsed, cut into 5 equal lengths, and the radioactivity per mg dry weight was determined for each. The destruction of auxin which occurred during the incubation was appraised by counting the C1402 released. Air samples removed from the flasks were passed through a dry-ice freezing trap into an evacuated 250-ml CaryTolbert ionization chamber, and radioactivity was determined by the rate of charge method using a Cary electrometer. When tissue was omitted from the flasks, 98% less volatile radioactivity was produced in 24 hr, and when samples of air from around the tissue were pretreated with 5% KOH, all counts were removed, indicating that they were due solely to C'402 evolved by the stem sections. For lateral transport studies ten 15-mm corn coleoptile sections were mounted horizontally with their vascular bundles oriented in a vertical plane, a donor block containing C14-IAA