Changes in chlorophyll-carotenoid contents, antioxidant enzyme activities and lipid peroxidation levels in Zn-stressed Mentha pulegium

The relationships between chlorophyll-carotenoid contents, the antioxidant enzyme activities, such as superoxide dismutase (SOD), catalase (CAT), ascorbate-dependent peroxidase (AsA-dep POD) and guaiacol-dependent peroxidase (Gua-dep POD), and lipid peroxidation (LPO) levels were investigated in leaves at different positions of M. pulegium grown in nutrient medium containing 0-1 muM Zn. The maximum chlorophyll-carotenoid content's SOD and CAT activities and the minimum LPO level were observed in leaf 6 at 7.6 10(-2) muM Zn (as control). While chlorophyll-carotenoid contents in the absence of Zn were lower than those of the control, SOD and CAT activities significantly increased with decreasing Zn concentration. With excess Zn, chlorophyll-carotenoid content exhibited a trend similar to that of the control. However, SOD, CAT and Gua-dep POD activities and also AsA-dep POD activities after leaf position 4 were inhibited by excess Zn. Absence of Zn in the nutrient medium caused significantly higher LPO levels than in the control. Excess Zn showed its toxic effects after leaf position 4. The effects of decreasing Zn concentrations in the range 7.67-1.8 10(-2) muM on the same parameters were also investigated with respect to time. The sharp decreases in antioxidant enzyme activities caused increasing LPO levels after the 12(th) day. According to the results, the highest antioxidative capacity of M. pulegium is in the range 0.38-7.67 10(-2) muM, which is the optimum for preventing Fe2+-initiated lipid peroxidation