Movement of Auxin in Coleoptiles of Zea mays L. during Geotropic Stimulation

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Abstract
Sources con-taining Cl4 carboxyl-labeled indoleacetic acid (2.5 x 10-7 to 2.5 x 10-5 [image] IAA) were applied either symmetrically or asymmetrically to the apical cut surface of sections of Zea mays coleoptiles. Between 1 and 4 hours, approximately 10% of the radioactivity in vertical sections was recovered in the half-section opposite an asymmetric source, whereas in horizontal sections about 25% was opposite an upper source but only 4% opposite a lower source. These differences in distribution of radioactivity were not attributable to either differential loss or immobilization of C14 in the 2 halves of differently oriented coleoptiles, and most be the result of an increase in lateral movement of C14 toward the lower half of horizontal sections. The curvature of vertical sections away from an asymmetric source increased to a maximum with concentration; the curvature of horizontal sections away from an upper source was always highly significantly less than that of vertical sections. This difference indicates that lateral movement of radio-activity reflects movement of IAA. Radioactivity moves laterally throughout the length of 15-mm sections of corn coleoptiles and can then again move longitudinally. Lateral movement does not occur by diffusion; it is both independent of the gradient of radioactivity and polarized toward the lower side of the section. Lateral and longitudinal movements of auxin thus have important properties in common, and the orientation of the tissue may simply determine the ratio between basipetal and lateral transport of auxin.