Concentrations of metals and potential metal‐binding compounds and speciation of Cd, Zn and Cu in phloem and xylem saps from castor bean plants ( Ricinus communis ) treated with four levels of cadmium

We examined the concentrations of metals (Cd, Zn, Cu, Fe and Mn) and potential metal‐binding compounds [nicotianamine ( NA ), thiol compounds and citrate] in xylem and phloem saps from 4‐week‐old castor bean plants ( Ricinus communis ) treated with 0 (control), 0.1, 1.0, and 10 μ M Cd for 3 weeks. Treatment with 0.1 and 1 μ M Cd produced no visible damage, while 10 μ M Cd retarded growth. Cadmium concentrations in both saps were higher than those in the culture solution at 0.1 μ M , similar at 1.0 μ M and lower at 10 μ M . Cd at 10 μ M reduced Cu and Fe concentrations in both saps. NA concentrations measured by capillary electrophoresis‐mass spectrometry ( MS ) in xylem sap (20 μ M ) were higher than the Cu concentrations, and those in phloem sap (150 μ M ) were higher than those of Zn, Fe and Cu combined. Reduced glutathione concentrations differed in xylem and phloem saps (1–2 and 30–150 μ M , respectively), but oxidized glutathione concentrations were similar. Phloem sap phytochelatin 2 concentration increased from 0.8 μ M in controls to 8 μ M in 10 μ M Cd. Free citrate was 2–4 μ M in xylem sap and 70–100 μ M in phloem sap. Total bound forms of Cd in phloem and xylem saps from 1 μ M Cd‐treated plants were 54 and 8%, respectively. Treatment of phloem sap with proteinaseK reduced high‐molecular compounds while increasing fractions of low‐molecular Cd–thiol complexes. Zinc‐ NA , Fe‐ NA and Cu‐ NA were identified in the phloem sap fraction of control plants by electrospray ionization time‐of‐flight MS , and the xylem sap contained Cu‐ NA .