Changes in chloroplast lipid contents and chloroplast ultrastructure in Sulla carnosa and Sulla coronaria leaves under salt stress

The possible involvement of chloroplast lipids in the mechanisms of NaCl tolerance was studied in leaves of two varieties of Fabaceae: Sulla carnosa and Sulla coronaria, which were subjected to 200 mM NaCl over 20 days. Changes in membrane lipid peroxidation, chloroplast lipids content, fatty acids (FA) composition and the ultrastructure of chloroplasts under salt stress were investigated. Chloroplast lipids were separated and quantified by high performance liquid chromatography coupled to evaporative light scattering detection (HPLC/ELSD). The results showed that salinity induced a significant decrease in digalactosyldiacylglycerol (DGDG), phosphatidylglycerol (PG) and sulfoquinovosylglycerol (SQDG) content in both S. carnosa and S. coronaria leaves, whereas monogalactosyldiacylglycerol (MGDG) content did not change significantly in S. carnosa leaves. The MGDG/DGDG ratio remained stable in S. coronaria leaves but increased in those of S. carnosa. In addition, the unsaturated-to-saturated fatty acids ratio (UFAs:SFAs) did not change under salt stress in S. coronaria leaves, while it decreased significantly in S. carnosa leaves. Moreover, salinity did not induce significant changes in MGDG and DGDG unsaturation level in S. carnosa leaves, in contrast to S. coronaria, in which salinity seems to enhance the unsaturation level in MGDG, DGDG and PG. Furthermore, the level of membrane lipid peroxidation, as expressed by malondialdehyde (MDA) levels, increased at 200 mM in S. carnosa leaves, while it did not change significantly in those of S. coronaria. With respect to the ultrastructure of chloroplasts at 200 mM NaCl, investigated by transmission electron microscopy (TEM), salt-stress caused the swelling of thylakoids in S. carnosa mesophyll. These ultrastructural changes were observed especially in the spongy tissue in S. coronaria. Taken together, these findings suggest that the stability of MGDG/DGDG ratio, the unchanged unsaturation level, and increasing unsaturation level in MGDG, DGDG and PG may be effective to some degree in suppressing the ultrastructural damage caused by salinity effects and may contribute to protect the chloroplast membrane integrity against salt stress.