Polar Movement of Gibberellic Acid through Young Coleus Petioles

Abstract

For several decades the only type of growth regulator believed to move with strong polarity through plants was the endogenous auxin indole-3-acetic acid (9, 15, 25). After it had been demonstrated that both naphthalene-acetic acid (8, 16) and the weed killer 2,4-D also moved with strong polarity when tested under the same conditions as IAA (17, 18), investigations were started on other types of growth regulators. Several cytokinins have been tested for polar movement through excised sections. The first reports of polarity in the movement of benzyladenine (1, 20) could not be confirmed in more detailed investigations by Fox and Weis (7). If kinetin moved at all through excised sections, it usually moved without polarity (14, 24). Adenine, a weak cytokinin, also moved without polarity through Coleus petiole sections (24). The two authors who have studied gibberellin movement through excised sections concluded that gibberellin moved through pea stems, but without polarity. Kato (13) used primarily ultraviolet absorption to estimate the acid extracted from receiver blocks of agar.3 Presumably forced to do so because of the insensitivity of this method, he used extremely high concentrations of GA34 in the donor blocks (1.4 grams per liter, in contrast to the milligram per liter concentration of GA3 typically used in physiological experiments). Clor (3), purporting to confirm Kato, added tritium-labeled gibberellin of unstated purity, concentration, or even structure and then counted the label extractable from tissue slices. No evidence was presented that any of the label thus counted was still with the gibberellic acid. Basipetally polar movement has been reported for both synthetic and natural abscisic acid isolated from tomato fruits in transport experiments with explants, petiole, and internode segments of Coleus rhenaltianus (6). Milborrow (19) has stated that abscisic acid labeled with radioisotope moves with 3:1 basipetal polarity through petiolar sections. By analogy with the active transport systems studied in cell physiology (22), we could expect more than just two plant hormones to show polar movement. The two negative reports with GA3 did not convince us that this expectation was unfoundedClor's because of its cited inadequacies, and Kato's because the levels of GA3 in the donor were so high. (The polarity of IAA