PHYSICOCHEMICAL FACTORS DETERMINING THE PHLOEM TRANSLOCATION OF BENZOIC-ACIDS

A series of C-14-labelled benzoic acids. chosen to permit assessment o f the role of pK(a) and lipophilicity in determining movement in plant s of these herbicide analogues, was synthesised and their phloem trans location investigated. Following application of substituted benzoic ac ids to castor bean. Ricinus communis L., by injection into the petiole s. the compounds of intermediate lipophilicity (2-fluoro-, 4-chloro- a nd 3.4-dichlorobenzoic acids) gave highest concentrations in phloem ex udates; 4-methyl-2.3,5,6-tetrafluoro- and pentafluorobenzoic acids wer e less well translocated, perhaps because their pK(a) values are much less than those of the other benzoic acids studied. The polar 4-ureido benzoic acid and the lipophilic 3-(4-methylphenoxy)benzoic acid were m uch less efficiently translocated in phloem. These results are similar to those previously obtained for phenoxyacetic acids. and provide fur ther support for the role of ionisation in the accumulation and retent ion of chemicals in phloem sieve tubes.