Toxicity of selenium to Lemna minor in relation to sulfate concentration

The aquatic plant Lemna minor L. was treated with sodium selenite or sodium selenate to test the toxicity of these salts in relation to high or low levels of sulfate in the culture medium. Several morphophysiological aspects, such as multiplication rate (MR), ratio of the number of fronds to number of colonies (Nfr/Ncol), frond size, cell ultrastructure, pigment content and guaiacol peroxidase (EC 1.11.1.7) activity were evaluated. Their variations might be an indirect means of evaluating the degree of susceptibility or tolerance of this plant to selenium (Se). Sodium selenite or sodium selenate treatments at concentrations ranging from 1 to 256 μ M generally decreased the investigated parameters. Moreover, the sulfate concentration influenced the toxicity of both Se salts. In general, with treatments in a medium containing a high sulfate (HS) content, sodium selenite appeared more toxic than sodium selenate, whereas in a low sulfate (LS) medium, sodium selenate seemed more toxic. MR was significantly increased at 1–4 μ M selenite and LS or 8 μ M selenate and HS levels, suggesting that Se may be an essential nutrient for this plant.