Effect of citric acid on the transport of cadmium through xylem vessels of excised tomato stem-leaf systems

Abstract

The positive effects of citric acid on the transport of cadmium (Cd) through the xylem vessels of tomato stems was quantitatively studied by measuring the rate constant (k) of the lateral escape from the vessels to surrounding tissues and the amount of Cd adsorbed to the xylem cell walls. These two transport parameters were shown to decrease after addition of citric acid to the Cd solution, probably due to the formation of uncharged and negatively charged Cd–citrate complexes, which show little binding to the mainly negatively charged xylem cell walls. Reduced affinity of metal complexes to the cell walls can explain the long-term positive effects of citric acid on longitudinal transport of Cd and other metals in the xylem.