Effects of cadmium on the sorption of citric acid in isolated xylem cell walls were Investigated. 2.5 nM to 9.5 mM [1.5-14 C]citric acid solutions were perfused through columns of xylem cell wall material, isolated from tomato plants (Lycoperslcon esculentum Mill, cv. Tiny Tim).The anion exchange potential of the column was estimated by amino acid analysis as approximately 46 meq dm" 3 , whereas the apparent anion exchange capacity (A£C) was estimated as 1.65 + 0.18 10 "M (citric acid units). This low AEC was attributed to a 'zipper' effect, a mutual screening of fixed R" and A + charges.Pre-loading with 11s Cd 2+ did not affect citric acid sorption, indicating the absence of Cd-effects on the availability of fixed A + charges, and the absence of the formation of effective R~-Cd 2+ and Donnan free space (DFS) [Cd(cit)HJ + complexes.Simultaneous application of both citric acid and 11B Cd 2+ , ""Ca^, or ^Mg 2 * resulted in increased sorption of citric acid, probably due to capacityimprovement rather than changes in valence-dependent anion sorption; this may be due to the presence of bulk [M(cit)]", held in the column as [M(cit)HJ + after protonation in the DFS. Sorption of citric acid was greatest in the presence of Ca 2+ , which was discussed in the light of the differences between Ca, Cd and Mg in their characteristics as co-ordinative M-complexes of citric acid. The overall results indicate the potential importance of the presence of metal ions for the xylem transport behaviour of organic acids in plants.