Studies on Foliar Penetration

Abstract

A comparative study has been made of the penetration into leaf disks of Phaseolus vulgaris of (a) phenoxyacetic acid and its 2-, 4-, 2,4-, 2,6-, 3-5, 2,4,5-, and 2,4,6chloro derivatives, and (b) benzoic acid and its 2-, 2,4-, 2,5-, 2,3,6 chloro and 3,6-dichloro-2-methoxy derivatives. The methods of synthesis of each compound with 14C incorporated in the carboxyl group are described. In the series of substituted phenoxyacetic acids it was established that only for 2,4,5-T was there an appreciable loss of radioactivity from the system either in the light or darkness. In contrast, with the exception of 2,4-DCBA, 14C is lost from each member of the series of substituted benzoic acids. The level and pattern of penetration in the two series is differentially affected by chlorination. In general progressive chlorination of phenoxyacetic acid leads to an increase in the rate of penetration in both light and darkness. There are, however, exceptions; for illuminated disks the rate of entry of 2,4,5-T is exceeded by 2,4,6-T, 2,4-D, and 2,6-D whereas in the dark 2,4,5-T has by far the highest rate of penetration. Progressive chlorination of benzoic acid, however, depresses entry in both light and darkness. Possible relationships between these findings and changes in selected physical properties of each series of compounds have been examined. Between members of the series of substituted phenoxyacetic acids there is little variation in the dis sociation constant whereas for the substituted benzoic acids there is a marked lowering of the pK as the degree of chlorination increases. The rate of elution with chloroform of each compound from a buffered Silocel column gives a measure of the degree of partition into a lipid medium at a given pH. At pH 5-2 chlorination of phenoxyacetic acid results in more rapid elution whereas chlorina tion of benzoic acid causes a longer hold up. An apparatus was designed to enable measurements to be made of the rate at which a compound moves from one aqueous phase to another through a layer of lipid. Chlorination promotes the rate of transfer of phenoxyacetic acid through octanol but retards the transfer of benzoic acid. The extent to which chlorination depresses the rate of transfer of benzoic acid matches the diminution in the rate of leaf penetration. For the series of substituted phenoxyacetic acids transfer rates and penetration rates follow similar trends in the dark but in the light agree ment is less good. These findings are discussed in relation to prior studies on the rate of uptake and the metabolic fate of the individual compounds in a number of plant tissues. Journal of Experimental Botany, Vol. 20, No. 63, pp. 426-50, May 1969. This content downloaded from 207.46.13.163 on Fri, 08 Jul 2016 06:06:55 UTC All use subject to http://about.jstor.org/terms Studies on Foliar Penetration, III 427