Effect of 2-Chloroethylphosphonic Acid (CEPA) on the Distribution of Growth Regulators in Geotropically Stimulated Vicia faba Roots

Abstract

Summary In a recent study on Vicia faba roots kept horizontal for 30 min, it was found the concentration of gibberellins to be considerably higher in the upper than in the lower halves of the geotropically stimulated roots, whereas abscisic acid (ABA) attained a higher concentration in the lower than in the upper halves, and thus showed a distribution pattern similar to that usually reported for indoleacetic acid (IAA). In the present investigation the effect of ethylene pretreatment of the roots on the distribution of these growth regulators during subsequent geotropic stimulation was determined. Ethylene treatment was effected by immersing the intact seedling in a solution of 2-chloroethylphosphonic acid (CEPA) at 200 mg/l at pH 6.0 for 1 h prior to stimulation. The contents of both IAA and ABA were determined by wheat coleoptile section assay of purified extracts of the root-halves, whereas gibberellins was determined by lettuce hypocotyl bioassay. As expected the establishment of an asymmetrical distribution of IAA was prevented by the CEPA treatment. Gibberellins and ABA, on the other hand, were distributed as in stimulated roots not pretreated with CEPA. In spite of the presence of an excess of gibberellins in the upper and of ABA in the lower halves, the CEPA-pretreated roots failed to develop geotropic curvatures within 4 h. These results, therefore, provide no evidence for a participation of gibberellins or ABA in the early development of geotropic curvatures in these roots. Some roots of intact seedlings were treated with 100 mg/l (pH 6.0) solution of GA3, ABA, kinetin or IAA for 1 h prior to geotropic stimulation. Both in untreated and GA2,-and ABA-treated samples, 90 to 100% of the roots developed positive geotropic curvatures within 2 h. Since the flooding with GA3 or ABA, assumed to prevent the establishment of an effective transversel gradient by saturating the transport system, did not prevent the development of the curvatures, these results agree with the above conclusion. On the other hand, flooding with kinetin or IAA at 100 mg/l prevented the development of the positive curvatures. Pretreatment with kinetin or IAA at 10 and 50 mgμl did not completely prevent the geotropic reaction but delayed it for several hours.