LIPID PEROXIDATION:- Lipid peroxidation refers to the oxidative degradation of lipids. It is the process whereby free radicals "steal" electrons from the lipids in cell membranes, resulting in cell damage. This process proceeds by a free radical chain reaction mechanism. It most often affects polyunsaturated fatty acids, because they contain multiple double bonds in between which lie methylene -CH2- groups that possess especially reactive hydrogens. As with any radical reaction the reaction consists of three major steps: initiation, propagation and termination. Effect of aluminium on lipid peroxidation, superoxide dismutase, catalase, and peroxidase activities in root tips of soybean (Glycine max) Inhibition of root elongation and modification of membrane properties are sensitive responses of plants to aluminium. The present paper reports on the effect of AI on lipid peroxidation and activities of enzymes related to production of activated oxygen species. Soybean seedlings (Glycine max L. cv. Sito) were precultured in solution culture for 3–5 days and then treated for 1–72 h with Al (AICI3) concentrations ranging from 10 to 75 μM at a constant pH of 4.1. In response to Al supply, lipid peroxidation in the root tips (< 2 cm) was enhanced only after longer durations of treatment. Aluminium-dependent increase in lipid peroxidation was intensified by Fe2+ (FeSO4). A close relationship existed between lipid peroxidation and inhibition of root- elongation rate induced by Al and/or Fe toxicity and/or Ca deficiency. Besides enhancement of lipid peroxidation in the crude extracts of root tips due to Al, the activities of superoxide dismutase (EC 22.214.171.124) and peroxidase (EC 126.96.36.199) increased, whereas catalase (EC 188.8.131.52) activity decreased. This indicates a greater generation of oxygen free radicals and related tissue damage. The results suggest that lipid peroxidation is part of the overall expression of Al toxicity in roots and that enhanced lipid peroxidation by oxygen free radicals is a consequence of primary effects of Al on membrane structure.( Ismail Cakmak, et al,2006). Lead toxicity induces lipid peroxidation and alters the activities of antioxidant enzymes in growing rice plants: When seedlings of two rice (Oryza sativa L.) cultivars were raised in sand cultures under 500 and 1000 μM Pb(NO3)2 in the medium, lengths as well as weights of roots and shoots decreased with increase in Pb concentration. Pb-treated seedlings showed elevated levels of lipid peroxides with a concomitant increase in the activities of the enzymes superoxide dismutase (SOD), guaiacol peroxidase, ascorbate peroxidase and glutathione reductase compared to controls. Though Pb was readily absorbed by growing seedlings, its localization was greater in roots than shoots. The level of Pb accumulation in seedlings was far higher than the supplied one. Seedlings grown for 5–20 days in presence of 1000 μM Pb(NO3)2 showed about 21–177% increase in the level of thiobarbituric acid reacting substances (TBARS) in shoots indicating enhanced lipid peroxidation compared to controls. With increase in the level of Pb treatment in situ peroxidases showed more increase in activity than SOD. Under both controls as well as Pb treatments roots maintained higher activity of these enzymes than shoots. About 87–100% increase in SOD activity, 1.2–5.6 times increase in guaiacol peroxidase activity and 1.2–1.9 times increase in ascorbate peroxidase activity was observed in the roots of seedlings grown for 15 days in presence of 1000 μM Pb in the medium. (Shalini Verma, et al,2003). Cadmium and zinc induction of lipid peroxidation and effects on antioxidant enzyme activities in bean (Phaseolus vulgaris L.) The relationship between Cd and Zn phytotoxicities and oxidative reactions in bean plants was studied. Ten-day-old bean (Phaseolus vulgaris L. cv. Morgan) plants treated for 96 h by 5 μM Cd and 100 μM Zn, separately, showed the same reduction of growth. In response to each metal, lipid peroxidation was enhanced in all plant organs, and catalase (CAT) activity was decreased in both roots and leaves, but not in stems. However, Cd and Zn stimulated the activity of guaiacol- dependent peroxidase only in stems, where native electrophoresis revealed, at least, two new anionic isozymes. The induction of one of these iso-guaiacol peroxidase (iso-GPX) was Zn- specific. The exposure to metals did not modify the activity of ascorbate-specific peroxidase either in roots or in stems. In leaves, where the glutathione reductase (GR) activity was assayed, increases of ascorbate peroxidase (APX) and glutathione reductase (GR) activities were recorded. These results suggest that some antioxidant enzymes can be activated, notably in upper plant parts, in response to oxidative stress induced by Cd and Zn.( Abdelilah Chaoui, et al,1996). Peroxidase Activity and Lipid Peroxidation in Strawberry (Fragaria X ananassa) Plants Under Low Temperature The activity of peroxidase (PRX) isozyme, lipid peroxidation (Malondialdehyde, MDA content) and cell membrane injury were studied during low temperature treatment for different periods in strawberry (Fragaria x ananassa cv. Camarosa) leaf tissues. Seedlings were grown for six weeks (plants had 4-5 leaves) in a greenhouse then the plants were transferred to a climate chamber with constant 5oC, 60% relative humidity, 14/10 h (light/dark) photoperiod regime and 4 LS light intensity for 1, 4, 7 or 10 days to impose a low temperature stress. In general, low temperature application during 10 days caused a linear increase in MDA content. Native polyacrylamide gel electrophoresis (PAGE) of both acidic and basic peroxidase (PRX) isozymes yielded a single sharp protein band with Rf=0.23 and Rf=0.17, respectively. In addition data indicated a strong relationship between band intensities and the duration of the low temperature treatment. However, the considerable increase of PRX activities could not stop the deleterious effects of low temperature, but reduced severity of stress, thus showing a reduction in the percentage of injury on the 7th day which is correlated with cold-acclimation of strawberry leaf tissues under low temperature. THE ROLE OF GLUTATHIONE REDUCTASE To study the role of glutathione reductase in lipid peroxidation, bean leaves (Phaseolus vulgaris) cv Fori were treated with the herbicide acifluorfen-sodium (sodium 5-12-chloro4- (trifluoromethyl)phenoxy-2- nitrobenzoic acid). Acifluorfen is a potent inducer of lipid peroxidation. In beans, decrease of acid-soluble SH-compounds and lipid peroxidation, measured as ethane evolution, were the toxic events after treatment of leaves with acifluorfen. As a primary response to peroxidation, increased production of antioxidants, such as vitamin C and glutathione, was found. This was followed by elevation of glutathione reductase activity. Enhanced activity of the enzyme prevented both further decline of acidsoluble SH-compounds and lipid peroxidation. Increased production of antioxidants and elevated activity of antioxidative enzymes, like glutathione reductase, seem to be a general strategy to limit toxic peroxidation in plants.( ARNO SCHMIDT, et al,1986).