Alterations in cytosol free calcium in horseradish roots simultaneously exposed to lanthanum(III) and acid rain

• Simultaneous exposure to acid rain and lanthanum affected cytosol free Ca2+.
• The cytosol free Ca2+ regulated the root activity, metal contents and biomass.
• La dose, acid rain strength and treatment mode affected cytosol free Ca2+.
• La contents in roots were positively correlated with cytosol H+.

The extensive use of rare earth elements (REEs) has increased their environmental levels. REE pollution concomitant with acid rain in many agricultural regions can affect crop growth. Cytosol free calcium ions (Ca2+) play an important role in almost all cellular activities. However, no data have been reported regarding the role of cytosol free Ca2+ in plant roots simultaneously exposed to REE and acid rain. In this study, the effects of exposures to lanthanum(III) and acid rain, independently and in combination, on cytosol free Ca2+ levels, root activity, metal contents, biomass, cytosol pH and La contents in horseradish roots were investigated. The simultaneous exposures to La(III) and acid rain increased or decreased the cytosol free Ca2+ levels, depending on the concentration of La(III), and these effects were more evident than independent exposure to La(III) or acid rain. In combined exposures, cytosol free Ca2+ played an important role in the regulation of root activity, metal contents and biomass. These roles were closely related to La(III) dose, acid rain strength and treatment mode (independent exposure or simultaneous exposure). A low concentration of La(III) (20 mg L−1) could alleviate the adverse effects on the roots caused by acid rain, and the combined exposures at higher concentrations of La(III) and acid rain had synergic effects on the roots.