Metal-induced cell rupture in elongating roots is associated with metal ion binding strengths

Low concentrations of Al, Cu and La rapidly decrease root elongation and cause transverse ruptures to the rhizodermis and outer cortex, but it is not known if other trace metals have similar effects. Six trace metals, Ga, Gd, Hg, In, Ru, and Sc, decreased cowpea root growth and caused ruptures similar to those caused by Al, Cu and La. Calculated speciation of the metals showed that only Gd was almost exclusively present as the trivalent ion (Gd³⁺), but the other test solutions were dominated by Ga(OH)₂ ⁺, HgCl₂ ⁰, either In³⁺, In(OH)²⁺, In(OH)₂ ⁺, In(OH)₃ ⁰, or InCl²⁺, and Sc³⁺ or ScOH²⁺ (no thermodynamic constants were available for Ru). The results from this and other studies suggest that the ability of these trace metals (plus Al, Cu, and La) to cause ruptures is related to the strength to which the trace metals bind to the cell wall. Therefore, it is proposed that the toxic effects of trace metals results from (1) the strength of binding (either ionically or covalently), and (2) other toxic effects of the metals not dependent on cell wall interactions.