ABSTRACTcaps and shaken gently in the dark at 23 C for different time periods. After incubation, roots were blotted, weighed, and measured. A similar protocol was followed for Lens culinaris L. cv. Medica, Phaseolus vulgaris L. cv. Red Kidney, and Zea mays L. cv. Early Sun Glow. In the initial experiments with pea, ethylene produced from the incubating root tips was assayed by gas chromatography using a flame ionization detector (1).Some of the experiments with pea were performed under aseptic conditions. All labware used had been autoclaved. After washing in sterile H20, the tissue sections were placed in incubation medium which had been passed through a sterile 0.2 ,um membrane.ABA is known to inhibit several physiological processes including root growth (3, 7). In taproots of intact pea seedlings treated with ABA, Tietz (1 1) reported the occurrence of ethylene-like responses (1), i.e., inhibition of elongation, swelling of epidermal cells, and induction of root hairs and lateral roots. Ethylene is a potent inhibitor of root growth (1), and ABA promotes ethylene production from a number of different plant tissues (1). We reasoned that root growth inhibition caused by ABA might correlate with its ability to increase ethylene production. We chose to study excised pea root tips because the effects of ethylene on the growth of this tissue have been thoroughly examined (4, 5). This paper reports the anomalous result we found when we treated excised pea root tips with ABA, viz., that low concentrations of ABA stimulated elongation.MATERIALS AND METHODS Pisum sativum L. cv. Alaska seed were imbibed for 6 hr, sown in moist vermiculite, and grown in the dark at 23 C. When the roots were 4 to 5 cm long, 10 terminal 5-mm sections were placed in 125-ml Erlenmeyer flasks containing 6 ml of buffer, pH 5.2. The buffer, with or without ABA (mixed isomers, Sigma Chemical Co.), contained 5 mm dibasic potassium phosphate, 2 mm citric acid, 1 mm Ca(NO3)2 4 H20, and 1 % (w/v) sucrose. The flasks were stoppered with vaccine
