Calcium ameliorates the toxicity of sulfate salinity in Brassica rapa

Salinity stress in Brassica, often only associated with osmotic effects and the toxicity of Na⁺, was more severe when applied as Na₂SO₄ than as NaCl, indicating that SO₄^2- ions had toxic effects as well. Application of 10 mM calcium in the form of CaCl₂ in the growth medium of plants only slightly ameliorated growth impairment by NaCl and KCl, but almost completely prevented negative effects of Na₂SO₄ and K₂SO₄ on plant biomass production. This effect was calcium specific, as MgCl₂ ameliorated sulfate toxicity to a much lower extent. This sulfate toxicity coincided with a strong decrease in the plant content of calcium and manganese upon sulfate salinity. Application of CaCl₂ largely alleviated this decrease, however, it did not prevent the higher tissue concentration of sulfate. CaCl₂ prevented the increase in organic sulfur compounds presumably by reducing of relative gene expression of ATP-sulfurylase (ATPS) and adenosine 5'-phosphosulfate reductase (APR) indicating a possible regulation of sulfate assimilation by calcium. The upregulation of the genes encoding for Group 4 sulfate transporters (Sultr4;1 and 4;2) upon sulfate salinity, was absent in the presence of CaCl₂. Therefore, additional calcium may facilitate an increased vacuolar capacity for sulfate accumulation.