A wide range of cellular responses occur when plants are exposed to heavy metal stress, including changes in the membrane fatty acid composition. In this study, we examined the effect of excess copper (100 lm) on the degree of unsaturation [double bond index (DBI) of 18-C fatty acids] and the polar lipid profiles in maize (Zea mays L.). The level of unsaturation of total fatty acids decreased from 1.42 to 1.28 in maize roots. However, copper-induced compositional changes of polar fatty acid composition show a general trend of increasing DBI. These responses suggest that individual 18-C fatty acid classes may have specific roles in maintaining optimal membrane function, enabling plant growth under copper stress. The results also reveal key changes including marked decrease in total root phospholipid levels and consistent increase in the steryl lipid/phospholipid (SL/ PL) ratio, as well as a decrease in monogalactosyldiacylglycerol/digalactosyldiacylglycerol (MGDG/DGDG) ratio, reflecting alteration of membrane permeability and fluidity. In shoots, the DBI obtained from phosphatidylcholine (PC), sulphoquinovosyldiacylglycerol (SQDG) and monogalactosyldiacylglycerol (MGDG) was increased. In contrast, these indices were decreased in phosphatidylinositol (PI) and phosphatidylglycerol (PG). However, the PI, PC, PG, MGDG and DGDG contents in shoots were significantly decreased, suggesting alterations in the photosynthetic membranes.

A comparative analysis of fatty acid composition was conducted in maize (Zea mays L.) under copper and cadmium stress. The unsaturation level (double-bond index) of phosphatidylethanolamine (PE) and digalactosyldiacylglycerol (DGDG) was increased in response to both metal treatments, whereas the phosphatidylinositol (PI), the phosphatidylcholine (PC) showed no significant changes. The Cu-treated roots showed a marked increase (about 2-fold) in the phospholipid (PL) content, while the Cd-treated roots showed a slight but insignificant increase. The steryl lipid SL/PL ratio was markedly decreased in response to Cu stress, and therefore, may indicate an activated phospholipid biosynthesis and turnover, in response to damage caused by Cu stress. The double bond indices of chloroplastic lipids: phosphatidylglycerol (PG), monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) revealed a similar but not identical pattern of change. The PG and MGDG contents in shoots were markedly decreased under Cu (by 53 and 48%) and Cd (by 78 and 65%) stress. The increase in the malondialdehyde (MDA) content in roots induced by both metals indicates lipid peroxidation. Generally, in the presence of Cu fatty acid composition was markedly modulated but to lesser extent under Cd stress. These results suggest that changes in the fatty acid composition under Cu and Cd stress conditions are metal-specific and may therefore result in differential metal tolerance.

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