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Free radicals, such as the superoxide anion, can damage the photosynthetic apparatus. At the same time they are increasingly formed as by-products of electron transport during stress conditions. For this reason, we studied the kinetics of formation and decay of superoxide anions in relation to changes, induced by increasing water deficit, in the composition of thylakoids. Seedlings of sunflower (Helianthus annuus L. cv. Licia Stella) were subjected to either a mild, a moderate, or a sever water deficit. Upon illumination of thylakoid membranes containing Tiron, we found first order kinetics of formation and decay of the Tiron semiquinone radical. At each stress intensity a higher rate of superoxide formation than in the control was observed. The different decay rates suggested a changed chemical environment around the radical. In comparison with the control, the levels of carotenoids and hydrophobic proteins in the thylakoids decreased, and the lipid/protein ratio increased after a mild water deficit. After moderate drought, the increased levels of hydrophilic proteins and carotenoids seem unsufficient to limit the improved capacity to leak electrons from the thylakoids. After a severe water stress, a decrease in unsaturation and in linoleic acid as well as a loss of lipolytic antioxidants took place, which may have rendered the thylakoids more sensitive to attack by activated forms of oxygen. The results suggest that during water deficit, the interactions among membrane components change in relation to a changed chemical composition. Such changes are probably among the causes for changes in the conformation of proteins and in the position of various molecular species in the lipid bilayer, so that they become more exposed to molecular oxygen. [ABSTRACT FROM AUTHOR]

Plants of garden pea (Pisum sativum L.) were exposed to charcoal‐filtered air with or without addition of 65 ± 5 l−1 ozone. Plants were harvested daily for 9 days and lipids were extracted from the second‐oldest leaf. Visible injury of this leaf was evident from day 5 on, while the differences in lipids between ozone and control treatments were observed earlier. Ozone caused large decreases in the contents of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG), a slower decrease in the content of phosphatidylcholine (PC), but an increase in the content of phosphatidylethanolamine (PE) per leaf area, compared with exposure to charcoal‐filtered air. The content of phosphatidylglycerol (PG) was unaffected by ozone. Compared with charcoal‐filtered air, fumigation with ozone resulted in a decrease in the proportion of linolenic acid (18:3) of the total lipid extract, with a concomitant increase in the proportion of linoleic acid (18:2). For individual lipids, ozone caused a similar pattern of decreased 18:3 and increased 18:2 in MGDG, SQDG, PC and PE, while the fatty acid composition of DGDG was unaffected. In PG, ozone decreased the proportions of 18:3 and trans‐Δ3‐decenoic acid (16:1trans), balanced by increased proportions of palmitic and oleic acids. The contents of chlorophylls and carotenoids were unaffected by ozone. Our results show that moderately elevated levels of ozone cause significant changes in the polar lipid composition of garden pea leaves and in the level of unsaturation of the lipid acyl groups and, furthermore, that ozone has different effects, which could be direct or indirect, on chloroplast lipids (MGDG, DGDG, SQDG, PG acylated with 16:1trans) and cytosolic membrane lipids [ABSTRACT FROM AUTHOR]

The composition and positional distribution of fatty acids in the polar lipids from 4 strains of Chlorella differing in chilling susceptibility and frost hardiness were analyzed by enzymatic hydrolysis and gas‐liquid chromatography. Analysis of the polar lipids from chilling‐sensitive, chilling‐resistant and chilling‐sensitive revertant strains of Chlorella ellipsoidea IAM C‐102 showed that the sum of palmitic and trans‐3‐hexadecenoic acid in phosphatidylglycerol (PG) is about 60% for the sensitive strains and 53% for the resistant strain. The sum of dipalmitoyl and 1‐palmitoyl‐2‐(trans‐3‐hexadecenoyl) PG as estimated from the positional distribution of their fatty acids, is about 10% in the case of each of the three strains. The contents of unsaturated fatty acids in phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were higher in the resistant than in the sensitive strain. This suggests that unsaturation of fatty acids in not only PG but also PC and PE is involved in chilling sensitivity of Chlorella. On the other hand, lipid changes during the development of frost hardiness of C. ellipsoidea IAM C‐27, a frost hardy strain, were examined. The results showed that fatty acids in most lipid classes are unsaturated in the hardening process but their degree of unsaturation is not greatly different from that of the chilling‐resistant strain, suggesting that not only unsaturation of fatty acids in lipids but also other factors are necessary for the development of frost hardiness. [ABSTRACT FROM AUTHOR]

Sunflower seedlings (Helianthus annuus cv. Isabel) subjected to a moderate level of water stress showed a reduced growth and a 0.1 MPa osmotic adjustment came into play. Thylakoid membranes isolated from stressed leaves showed decreased chlorophyll (Chl) and protein contents but the Chl a/Chl b and protein/Chl ratios were unchanged. Water stress caused a preferential hydrolysis in thylakoid proteins: the hydrophilic to hydrophobic protein ratio increased from 0.8 in the control to 4.5 in the stressed plants. However, the degree of unsaturation was unchanged and the electron spin resonance (ESR) measurements did not show an increased level of O2·- radical production by photosynthetic membranes. [ABSTRACT FROM AUTHOR]