Differential Uptake of 2,4-D Acid and Its Octyl Ester by Seedling Corn Roots and Coleoptile Sections

Although much research lhas been (lirected toward the uptake of inorganic ions by roots, the entry of organic compounds has received little attention. The present study concerns a basic difference in permeability between seedling corn roots and coleoptile sections as indicated by a differential uptake of ionized 2,4-D acid and the apolar octyl ester of 2,4-D. It is known that the addition of an alcohol through an ester linkage drastically changes the physical properties of the 2,4-D molecule (5). In any study of the effect of molecular changes on toxicity, it is important to determine wlhether the changes, if any, affect the inherent toxicity or merely affect the ability of the molecule to reach the site of activity. It seemed possible that the transformed dosage-response curve, as applied to spore toxicity by Rich and Horsfall (6), could provide this information. The inhibition of primary root growth was determined for corn seedlings growni in buffere(d solutions which contained gradled conicentrations (on an aci(d equivalent basis) of either 2,4-D acid or its octyl ester. The experiments were coln(lucte(l at pH 7, with a 24 hour incubation period. The (lifference in slopes of the resultant lines established by linear regression was not statistically significant, implying that the inherent toxicity of the 2,4-D mlolectule is not changed by esterification. However, a large difference was found for the ED50 (concentration necessary for 50 I% inhibition of root growth) i.e., 4.5 mg/l for 2,4-D