Effects of cadmium on the behaviour of citric acid in isolated tomato xylem cell walls

Abstract

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]crtic 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 whereas the apparent anion exchange capacity (AEC) was estimated as 1.65±0.18 10−4(citric acId units). This low AEC was attributed to a ‘zipper’ effect, a mutual screening of fixed R− and A+ charges. Pre-loading with 115Cd2+ 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−-Cd2+ and Donnan tree space (DFS) (Cd(cit)H2]+ complexes. Simultaneous application of both citric acid and 115Cd2+,45Ca2+ or 28Mg2+ resufted in increased sorption of citric acid, probably due to capacity improvement rather than changes in valence-dependent anion sorption; this may be due to the presence of bulk (M(cit)H2]+, held in the column as [M(cit)H2]+ after protonation in the DFS. Sorption of citric acid was greatest in the presence of Ca2+ which was discussed in the light of the differences between Ca, Cd and Mg in their characteristics as co-ordinative M-com plexes 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.