Summary. The molecular requirements for ethylene action were investigated using the pea straight growth test. Biological activity requires an unsaturated bond adjacent to a terminal carbon atom, is inversely related to molecular size, and is decreased by substitutions which lower the electron densitv in the unsaturated position. Evidence is presented that ethylene binds to a metal containing receptor site. 002 is a competitive inhibitor of ethylene action, and prevents high concentrations of auxin (which stimulate ethylene formation) from retarding the elongation of etiolated pea stem sections. It is suggested that CO2 delays fruit ripening by displacing the ripening hormone, ethylene, from its receptor site. Binding of ethylene to the receptor site is also impeded when the 02 concentration is lowered, and this may explain why fruit ripening is delayed at low O. tensions.Several gases in addition to ethylene elicit the triple response in etiolated seedlings, and the concentration of each requlired to produce a just discernible effect has been determined (9, 25). Included are propylene, butylene (isomer unknown), acetylene, and CO. Since these vapors also substitute for ethylene in causing epinasty (9, 10), fruit ripening (11,27), and other effects (8,9, 29), it would appear that the molecular requirements for biological action are similar in each case. We have attempted to delineate these requirements using as an assay for ethylene action the effect which the gas has on the growth and tropistic behavior of etiolated pea stem sections. Materials and MethodsPeas (Pisum sativum, var. Alaska) were soaked in water for 5 hours and germinated in moist vermiculite. The seedlings developed in darkness at 230 receiving an occasional exposure to dim red illumination, and were used on the seventh day. Sections 10 mm long were cut from the third internode just below the leaf hook and incubated in a medium containing 1 uM indole-3-acetic acid (IAA), 2 % sucrose (w/v), 0.05 M potassium phosphate buffer (pH 6.8), and 5 ptM CoCl in glass distilled water. Ten ml of media and 10 sections were placed in a 125 ml Erlenmeyer flask, this was sealed with a vaccine cap, the gas phase ad-'This investigation was supported by research grants EF-00214 and EF40078&2 from the United States Public Health Service, Division of Environmental Engineering and Food Protection, and was carried out while S. P. Bukrg was the recipient of Career Research DevelopmentAward 1-K3-GM-6871 from the USPHS. justed as described below, and the tissue slowly shaken at 230 in the dark. After 3 hours the sections were visually scored in red light for curvature and 15 hours later the gas phase was analyzed by gas chromatography before the tissue was removed, blotted dry, weighed and measured. Details of the procedure have been published elsewhere (4). Gas Chromatography and Preparation of GasM&ixtures. All gases were chromatographed on a 3 foot alumina column to determine whether ethylene or other low molecular weight contaminants were present. The instrument, a Perk...

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There have arisen two schools of thought concerning the role of ethylene in fruit maturation: the classic view of Kidd and West (26) and Hansen (22) that ethylene is a ripening hormone, and a recent interpretation by Biale et al. (7,3,4) that it is a by-product of the ripening process. The original presentation of the by-product theory in this journal (7) was tempered with the reminder that 0.1 ppm ethylene may stimulate ripening, so that "in the absence of any information correlating the internal ethylene content with the rate of ethylene production, one can advance the argument that small quantities sufficient to induce ripening are produced prior to the rise of respiration, but measurable amounts are detected only after the onset of the climacteric." The development of highly sensitive gas chromatographic instruments makes it feasible to appraise critically those instances in whiclh fruits have been reported to produce ethylene not at all or only after the climacteric has started, and also to determine the content of ethylene witllin a fruit at the onset of the rise in respiration. Results of such experiments are reported in this communication, and they have a direct bearing on the problem of whetlher or not ethylene is a natural ripening hormone. Materials.Mangoes (Magiiifera indica L., cv. Kent & Haden) were harvested in local orchards. The fruits used for each experiment were picked from the same tree on the same day and were all of about equal size and apparent maturity. Bananas (Miusa acuiiniata cv. Gros Mfichel) harvested at 34 fullness were shipped from Ecuador. Pineapples [Ananas comnosus (L.) AMerr.] at various stages of maturity were flown from Honduras; they arrived in very satisfactory condition w%ithin a day of picking. The Citrus Experiment Station at Tampa, Fla. provided oranges [Citrl(s sinensis (L.) Osbeck] and passion fruits (Passiflora eddlis,Sims); other fruits were purchased in local markets.

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