Calcium ameliorates the toxicity of sulfate salinity in Brassica rapa

Abstract Salinity stress in Brassica, often only associated with osmotic effects and the toxicity of Na + , was more severe when applied as Na 2 SO 4 than as NaCl, indicating that SO 4 2− ions had toxic effects as well. Application of 10 mM calcium in the form of CaCl 2 in the growth medium of plants only slightly ameliorated growth impairment by NaCl and KCl, but almost completely prevented negative effects of Na 2 SO 4 and K 2 SO 4 on plant biomass production. This effect was calcium specific, as MgCl 2 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 CaCl 2 largely alleviated this decrease, however, it did not prevent the higher tissue concentration of sulfate. CaCl 2 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 CaCl 2 . Therefore, additional calcium may facilitate an increased vacuolar capacity for sulfate accumulation.