Differential Effects of 2,3,6-Trichlorobenzoic Acid on Growth and Geotropic Curvature of Avena Coleoptiles

Abstract

The effects of 1 x 16-6 [image] to 1 x 10-3 [image] TCBA on the growth of 5 mm floating Avena sections, on the elongation of 15 mm apical segments, and on the geotropism and phototropism of apical segments were measured. Growth of floating sections in the absence of exogenous IAA, was increased by TCBA concentrations from 1 x 10-6 to 1 x 10-4 [image]. In the presence of 0.1 mg/1 of IAA, these concentrations of TCBA have no effect on floating section elongation. 1 x 10-3 [image] TCBA suppressed section elongation in the presence and absence of exogenous IAA. Elongation of 15 mm apical segments was increased by 1 x 10-5 [image] and 1 x 10-4 [image] TCBA for a 20-hour period. Concentrations as high as 2 x 10-3 [image] accelerated elongation for 4-6 hours. Geotropic curvature of 15 mm apical coleoptile segments, stimulated in the horizontal position, was extensively inhibited by 1 x 10-3 [image] and 1 x 10-4 [image] TCBA. Lower concentration had no effect on negative geotropism. A stimulation angle of 135[degree] (the obtuse angle between a vertical line and a line of the longitudinal axis of the downward pointing coleoptile) increased geotropic curvature. Geotropic curvature in response to this angle of stimulation was inhibited by 1 x 10-4 [image] TCBA, but the extent of inhibition is not as large as observed with 90[degree] stimulation. TCBA (1 x 10-4 [image]) had no effect on elongation of 15 mm apical segments for a period of 24 hours at 4[degree]C. Geotropic curvature was inhibited by this concentration of TCBA at 4[degree]C. Positive phototropic curvature was not affected by concentrations of TCBA from 1 x 10-6 to 1 x 10-3 [image]. These results were interpreted to indicate that TCBA in some way inhibits the geotropic perception mechanism.