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 NaSO than as NaCl, indicating that SO ions had toxic effects as well. Application of 10 mM calcium in the form of CaCl in the growth medium of plants only slightly ameliorated growth impairment by NaCl and KCl, but almost completely prevented negative effects of NaSO and KSO on plant biomass production. This effect was calcium specific, as MgCl 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 largely alleviated this decrease, however, it did not prevent the higher tissue concentration of sulfate. CaCl 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. Therefore, additional calcium may facilitate an increased vacuolar capacity for sulfate accumulation.