Hordeum vulgare L. (cv. Barberousse) seedlings were continuosly exposed to SO 2 (40±5 and 117±20 ppb) for 48 days and compared to control plants exposed to charcoal‐filtered air. Glyco‐and phospolipid amounts were strongly affected by exposure to SO 2: , and decreased with increasing SO 2 , fumigation. The chloroplast membranes of fumigated leaves were characterized by lower quantities of rnonogalacto‐syldiacylglycerol (MGDG) as well as of sulphoquinovosyldiacylglycerol (SQDG). Among the phospholipids a decrease in the phosphatidylglycerol (PG) content was observed, indicating that the chloroplast membranes are affected even by such low concentrations of SO 2 as used here. As for the other phospholipids, phosphatidylcholine (PC) and phosphatidylinositol (PI) suffered a reduction and phosphatidylethano‐lairrine (PE) almost disappeared at the highest SO 2 , concentration, whereas phosphatidic acid (PA) followed the opposite trend. The fatty acids of both glyco‐ and phospholipids fell sharply upon fumigation without any decline in the degree of unsataration. In particuSar, neither linoleic nor linolenic acid showed any evident change in level, suggesting that no lipid peroxidation of polytmsaturated fatty acid tails occurred. Furthermore, the electron spin resonance (ESR) showed no differences in the free radical levels between control and fumigated leaves. The level of malondialdehyde (MDA) decreased, while lipid‐soluble antioxidants, as well as superoxide dismutase (SOD; EC 1.15.1.1) and catalase (EC 1.11.1.6) activities increased with the increase in SO 2 concentration, providing evidence that defence mechanisms against oxy‐radical action are activated by the plants at low levels of SO 2 ,. Even if SO 2 , at low concentrations may not directly oxidize polyunsaturated fatty acids, it may act as a deesterifying factor on each component of both glyco‐ and phospholipids.
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Navari‐Izzo et al. (1992) studied this question.