The mechanisms of aquatic plant defense against salinity were studied by long-term exposure of Spirodela polyrhiza (greater duckweed) to NaCl. In this study, the effects of 200 mM NaCl on greater duckweed were evaluated after 6 and 12 days of treatment, while plant growth was measured every day. High concentration of NaCl caused an inhibition of plant growth, reduced in the content of photosynthetic pigments, increased lipid peroxidation, and enhanced the entire antioxidant defense. The responses of five antioxidant enzymes showed that ascorbate peroxidase, guaiacol peroxidase, and superoxide dismutase activities were the most enhanced after NaCl exposure, catalase moderately, and glutathione reductase least. The content of soluble proteins was decreased, while ascorbic acid was drastically increased. In NaCl-treated fronds, the appearance of two NaCl-induced polypeptides with apparent molecular weight of 16 and 21 kDa, as well as the accumulation of two polypeptides with molecular weights 18 and 27 kDa, were observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). NaCl also led to accumulation of the heat shock protein 70 (HSP70) and induced an isoform of the glutamine synthetase (GS1) expression. Our results suggest that in S. polyrhiza, different adaptive mechanisms are involved in counter balancing high doses of a particular toxicant (sodium chloride). The possible application of the examined biomarkers in ecotoxicological research is discussed.
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