Effects of Certain Growth Substances on Elongation and Geotropic Curvature of Wheat Roots

Abstract

1. The quantitative relation between elongation and positive geotropic curvature of wheat seedling roots in response to different levels of eight selected growth-regulating compounds has been studied at intervals for 24 hours following initiation of the geotropic stimulus. The seedlings were grown on the surface of an agar medium in vertically placed, covered dishes and were 70 hours old at the time of initiating stimulation. 2. The effects of temperature and pH on these responses paralleled those obtained by other workers, the optimum values for positive curvature being 14⚬ C. at pH 7.0. The chemicals were tested under these conditions. 3. The responses fell roughly into three categories: (a) elongation and curvature inhibited to about the same degree at all levels tested (indoleacetic acid, naphthaleneacetic acid, 2,4-dichlorophenoxyacetic acid, 2,3,6-trichlorobenzoic acid, N-[2,4-dichlorophenyl]maleimide), (b) curvature inhibited more than elongation (2,3,5-triiodobenzoic acid, N-1-naphthylphthalamic acid, 2,3,6-trichlorobenzoic acid), and (c) elongation inhibited more than curvature (isopropyl-N-phenylcarbamate). TCBA gave two types of response, depending on the equilibration time before the initiation of the stimulus. 4. The separation of growth and curvature responses in terms of the differential susceptibility to chemical inhibition may indicate that the perceptive and evocative mechanisms are not identical.