Citric acid in tomato plant roots and its effect on cadmium uptake and distribution

Experiments were carried out to investigate the effects of root citric acid on uptake and initial distribution of cadmium (Cd) in tomato plants (Lycopersicon esculentum, cv. Tiny Tim). Cd was measured by γ-spectrometry, using ¹¹⁵Cd spikes. Citric acid was measured by UV-detection, and, after spiking with ¹⁴C-citric acid, by β-spectrometry. Cd was applied for 48 h periods, in control experiments, in the presence of citric acid, and after 24 h plant pre-incubation with citric acid. Pre-incubation resulted in two-fold increases in fast-exchangeable amounts of root citric acid, as measured by the presence of citric acid in xylem exudates of decapitated and pressurized roots. Simultaneous application of Cd and citric acid did not change Cd accumulation in total plants and in the roots, nor did any significant change occur with respect to Cd root-to-shoot transport, and Cd concentrations in shoot tissues and xylem fluid. After citric acid pre-incubation, total plant uptake of Cd increased twofold, without any significant change in Cd accumulation in the roots. Cd root-to-shoot transport was increased 5–6 fold, and Cd concentrations in shoot tissues and xylem fluid were increased 6–8 fold. Speciation calculations indicated that, under the conditions applied, xylem Cd may be, at least partly, complexed in citric acid. A C Borstlap Section editor