Manganese (Mn) is an essential plant nutrient, though at elevated levels in plant tissues it may become toxic. The physiological basis for phytotoxicity is largely unclear. Exposure of Brassica rapa to elevated levels of Mn2+ in the nutrient solution resulted in decreased biomass production at 20 μM and chlorosis. The Mn content in the shoot increased with the Mn2+ concentration in the nutrient solution and became toxic when it exceeded a four-fold concentration of the control. In contrast to observations with Cu and Zn, elevated and toxic Mn2+ levels did not affect the water-soluble non-protein thiols in both root and shoot and the expression the sulfate transporters, Sultr1;1 and Sultr1;2, in the root.
