Background and aims It remains uncertain whether a higher toxicity of either NaCl or Na 2 SO 4 in plants is due to an altered toxicity of sodium or a different toxicity of the anions. The aim of this study was to determine the contributions of sodium and the two anions to the different toxicities of chloride and sulfate salinity. The effects of the different salts on physiological parameters, mineral nutrient composition and expression of genes of sulfate transport and assimilation were studied. Methods Seedlings of Brassica rapa L. have been exposed to NaCl, Na 2 SO 4 , KCl and K 2 SO 4 to assess the potential synergistic effect of the anions with the toxic cation sodium, as well as their separate toxicities if accompanied by the non-toxic cation potassium. Biomass production, stomatal resistance and Fv/ fm were measured to determine differences in ionic and osmotic stress caused by the salts. Anion content (HPLC), mineral nutrient composition (ICP-AES) and gene expression of sulfate transporters and sulfur assimilatory enzymes (real-time qPCR) were analyzed. Results Na 2 SO 4 impeded growth to a higher extent than NaCl and was the only salt to decrease Fv/fm. K 2 SO 4 reduced plant growth more than NaCl. Analysis of mineral nutrient contents of plant tissue revealed that differences in sodium accumulation could not explain the increased toxicity of sulfate over chloride salts. Shoot contents of calcium, manganese and phosphorus were decreased more strongly by exposure to Na 2 SO 4 than by NaCl. The expression levels of genes encoding proteins for sulfate transport and assimilation were differently affected by the different salts. While gene expression of primary sulfate uptake at roots was downregulated upon exposure to sulfate salts, presumably to prevent an excessive uptake, genes encoding for the vacuolar sulfate transporter Sultr4;1 were upregulated. Gene expression of ATP sulfurylase was hardly affected by salinity in shoot and roots, the transcript level of 5′adenylylsulfate reductase (APR) was decreased upon

Brassica juncea seedlings contained a twofold higher glucosinolate content than B. rapa and these secondary sulfur compounds accounted for up to 30% of the organic sulfur fraction. The glucosinolate content was not affected by H2S and SO2 exposure, demonstrating that these sulfur compounds did not form a sink for excessive atmospheric supplied sulfur. Upon sulfate deprivation, the foliarly absorbed H2S and SO2 replaced sulfate as the sulfur source for growth of B. juncea and B. rapa seedlings. The glucosinolate content was decreased in sulfate-deprived plants, though its proportion of organic sulfur fraction was higher than that of sulfate-sufficient plants, both in absence and presence of H2S and SO2. The significance of myrosinase in the in situ turnover in these secondary sulfur compounds needs to be questioned, since there was no direct co-regulation between the content of glucosinolates and the transcript level and activity of myrosinase. Evidently, glucosinolates cannot be considered as sulfur storage compounds upon exposure to excessive atmospheric sulfur and are unlikely to be involved in the re-distribution of sulfur in B. juncea and B. rapa seedlings upon sulfate deprivation.

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Seedlings of Brassica rapa were exposed to increasing concentrations of NaCl, Na2SO4, KCl and K2SO4 to study the effect on glucosinolate content, composition and expression of genes of the glucosinolate biosynthetic pathway and associated transcription factors. Growth was inhibited stronger by sulphate salts and strongest by Na2SO4. Aliphatic, indolic and aromatic glucosinolates were differently affected by the salts in shoot and roots. A decrease in aliphatic glucosinolates in shoots of plants exposed to chloride salts corresponded to a decreased gene expression of a key enzyme for their biosynthesis. Likewise, an increased level of indolic and aromatic glucosinolates by Na2SO4 coincided with an increased gene expression of enzymes responsible for the biosynthesis of these glucosinolates. The results show that changes in glucosinolate content and composition under salt stress depend on the ionic composition of the salts. This has implications for the quality of B. rapa grown under different types of salinity.