Cadmium — citric acid — xylem cell wall interactions in tomato plants

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Abstract
Mutual interactions between cadmium ions, citric acid and xylem cell walls were examined. Cadmium and citric acid were measured as 115Cd and [1,5‐14C] citric acid, respectively. Xylem cell walls were obtained by bacterial degradation of tomato stem sections (Lycopersicon esculentum Mill, cv. Tiny Tim), and applied as ion‐exchange columns. The xylem column material carried 2·4 dm3 H2O kg−1 dry weight, and was temporarily capable of buffering perfusates at pH 5·7. Sorbed cadmium and citric acid were determined from H2O and HCl rinses after perfusion periods. In all experiments, total cadmium and/or citric acid recoveries were better than 98%, indicating both the effectiveness of the rinses applied and the possibility of full regeneration of the xylem column. The results indicate that the presence of 2·45 mol m−3 citric acid causes an approximately 50% reduction of adsorbed cadmium levels, irrespective of the applied total cadmium concentrations (0·04–0·4 mol m−3 Cd(NO3)2.4H2O). This reduction is probably related to a corresponding reduction to approximately 2% of the control applied free Cd2+ concentration, the latter also independent of the total cadmium concentrations. Furthermore, without inducing positively charged citrate complexes in the applied solution, the presence of cadmium resulted in increased levels of citric acid absorbed in the xylem column. The Donnan Free Space accumulation of citric acid in the presence of Cd(NO3)2.4H2O, observed in the experiments described, could be expressed by its distribution coefficient, as approximately 15 times the control accumulation. These data indicate that the xylem column may operate as a ligand exchanger, suggesting the importance of metal ions for the longitudinal and lateral movement of organic complexing compounds in the xylem.