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Antagonism of Some Gibberellin Actions by a Substituted Pyrimidine ¹

^a Horticulture Department, Purdue University, Lafayette, Indiana 47907

From a comparison of the effects of seven growth retardants and abscisic acid (ABA) on various growth systems, it was found that the gibberellin-regulated growth of lettuce hypocotyls was uniquely inhibited by the growth retardant, -cyclopropyl--(4-methoxyphenyl)-5-pyrimidine methanol (EL-531). Auxin-regulated growth of coleoptile sections was inhibited by Phosfon and only slightly by EL-531 and Alar. Cytokinin-regulated growth of Xanthium cotyledons showed little or no inhibition by any of the retardants. ABA was inhibitory in all three types of tests. The distinctive effects of EL-531 against gibberellin-stimulated growth and the general ability of gibberellic acid to relieve EL-531 inhibition suggest that this retardant acts in part against the gibberellin-stimulated growth system, but at a locus which discriminates between growth and nongrowth functions of gibberellic acid. It shows little or no antagonism of gibberellin actions which do not involve growth: the barley endosperm test and the Rumex leaf senescence test.