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The effect of salinity on plant growth, chlorophyll content, photosynthetic parameters, photochemical efficiency of PSII, membrane lipid content and fatty-acid composition of halophyte Thellungiella halophila and glycophyte Arabidopsis thaliana was investigated to examine the possible role of unsaturated fatty acids in photosynthesis under saline conditions. The growth of Arabidopsis was significantly decreased by the 100 and 200 mM NaCl treatments; however, there was no significant difference in the fresh and dry weight of Thellungiella at different concentrations of NaCl. Exposure of Arabidopsis to salt resulted in a progressive decline in chlorophyll content, while there was no significant change in that of Thellungiella. The net photosynthetic rate, maximal photochemical efficiency of PSII (Fv/Fm) and actual PSII efficiency were significantly reduced in Arabidopsis but remained unaffected in Thellungiella. Arabidopsis under NaCl treatment showed decreased PG levels and decreased values for the unsaturated fatty acid content and the double bond index (DBI) of monogalactosyldiacylglycerols and phosphatidylglycerols; these values significantly increased in Thellungiella under NaCl treatment, as did the DBI values of digalactosyldiacylglycerols and sulphoquinovosyldiacylglycerols. These results suggest that Thellungiella under salt stress displays high resistance to photoinhibition and that increased concentrations of unsaturated fatty acids in membrane lipids enhances the tolerance of photosystem II to salt stress.
Suaeda salsa L., a C3 euhalophytic herb, is native to saline soils, demonstrates high resistance to salinity stress. The effect of chilling stress on S. salsa under high salinity, particularly the change in unsaturated fatty acid content within membrane lipids, has not been investigated. After a 12 h chilling treatment (4°C) performed under low irradiance (100 μmol m⁻² s⁻¹), the chlorophyll contents, maximal photochemical efficiency of photosystem II (Fv/Fm) and actual PSII efficiency (ΦPSII) were determined. These measurements were significantly decreased in S. salsa leaves in the absence of salt treatment yet there were no significant changes with a 200 mM NaCl treatment. Chlorophyll contents, Fv/Fm and ΦPSII in S. salsa under 200 mM NaCl were higher than those without salt treatment. The unsaturated fatty acid content and the double bond index (DBI) of major membrane lipids of monogalactosyldiacylglycerols, digalactosyldiacylglycerols (DGDG), sulphoquinovosyldiacylglycerols and phosphatidylglycerols (PG) significantly increased following the chilling treatment (4°C) (with 12 h of low irradiance and 200 mM of NaCl). The DBI of DGDG and PG was decreased in the absence of the salt treatment. These results suggest that in the euhalophyte S. salsa, a 200 mM NaCl treatment increases chilling tolerance under conditions of low irradiance (100 μmol m⁻² s⁻¹).
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