Effects of cadmium on the longitudinal and lateral xylem movement of citric acid through tomato stem internodes

Short-term experiments (2-5 h) were performed to gain insight in the effects of cadmium (Cd) on the longitudinal and lateral xylem movement of 14C-citric acid through tomato stem internodes (Lycopersicon esculentum, Mill, cv. Tiny Tim). The behaviour of citric acid was expressed by the concentrations of citric acid sorbed to the xylem walls (apparent anion exchange capacity AEC), and by the rate constant k for citric acid lateral escape. For 0 05 mM citric acid applications, the presence of 1 0 him cadmium resulted in approximately 2-fold increases in values for apparent AEC and k; for the 9 5 mM citric acid concentrations, cadmium resulted in less than 2-fold increase in AEC, but here k did not change. In the presence of Cd, the stem-length profile of absorbed citric acid was shown to be in accordance with stem-length profiles of Cd xylem concentrations, rather than with the xylem concentrations of citric acid itself. These results were discussed in the light of the expected presence of Cd-citric acid complexes in the xylem sap; effects on k values were discussed with consideration for the possible processes of escape rate limitation in the stem.