Application of 1-naphthaleneacetic acid (NAA) or 1-aminocyclopropane-1-carboxilic acid (ACC) to maize roots growing in hydroponic solution inhibited root elongation, and increased radial growth, but the responses to those treatments differed in degree. Auxin was more effective than ACC as an elongation inhibitor and root swelling promoter. Whereas NAA fully inhibited elongation and maintained swelling over 48 h, ACC inhibited elongation partially (50%) and only promoted swelling for 24 h. It is well-known that auxin, like ACC, promotes ethylene production, but similar levels of ethylene production reached by means of NAA or ACC treatments did not elicit the same response, the response being always stronger to NAA than to ACC. These results suggest that the effect of auxin on root growth is not mediated by ethylene. Elongation and swelling of roots appear to be inversely related: usually a reduction in elongation was accompanied by corresponding swelling. However, these two processes showed different sensitivities to growth regulators. After 24 h treatment with 0.5 μM NAA or 5 μM ACC, root elongation was inhibited by 90% and 53% respectively, but the same treatments promoted swelling by 187% and 140% respectively. Furthermore, 1 μM ACC was shown to promote inhibition of root elongation without affecting swelling. The ethylene antagonist STS (silver thiosulfate) did not affect elongation in control or NAAtreated roots, but increased ethylene production and swelling. These results indicate that longitudinal and radial expansion could be independently controlled.
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