Osmotic stress induced changes in lipid composition and peroxidation in leaf discs of Brassica napus L.

Leaf discs excised from rape plants (Brassica napus var, oleifera) incubated in vitro under moderate osmotic stress (-1.5 MPa) exhibited a significant reduction in the amount of membrane galactolipids. In addition, the phosphatidylethanolamine (PE) and phosphatidylcholine (PC) levels were also decreased, whereas those of phosphatidylglycerol and phosphatidic acid were enhanced. Fatty acid composition of polar lipids was also affected in response to osmotic stress. The percentage of polyunsaturated fatty acids (PUFAs) showed a moderate decrease (in mono- and digalactosyldiacylglycerol, PE and PC) while that of saturated or of monounsaturated fatty acids was not significantly affected. Relative to the total fatty acids of the membrane lipids, the levels of C18:2 and C18:3 were markedly decreased in response to osmotic stress. Under the same conditions the content of malondialdehyde, one of the cytotoxic products of PUFA peroxidation, was enhanced. These responses were associated with decreases in both the relative water content and the internal osmotic potential. Concurrently, lipoxygenase activity was rapidly enhanced, suggesting that lipoxygenase could be responsible for initiation of the lipid peroxidation process. On the other hand, superoxide dismutase and ascorbate peroxidase activities were found to be enhanced in osmotically stressed leaf discs compared with the control. These results indicate that the biochemical adjustments induced by osmotic stress in rape leaf tissues partly rely on changes in membrane lipid composition, which are in part related to a shift in the balance between the production and scavenging of reactive oxygen species.