PROTECTIVE ROLE OF EXOGENOUS NITRIC OXIDE AGAINST ZINC TOXICITY IN PLANTAGO MAJOR L.

Zinc is one of the heavy metal in plants that causes toxicity at high concentration via producing Reactive oxygen species. Nitric oxide can protect cells from oxidative stress produced by reactive oxygen Species. Effect of different concentrations of Zn (0, 100, 300 and 500 mu M) and interaction with sodium nitroprusside (SNP, a donor of NO) (100 and 200 mu M) were studied on growth parameter (total dry Weight) and some physiological factors (chlorophyll a, chlorophyll b, hydrogen peroxide and malondialdehyde contents), antioxidant enzymes (catalase, peroxidase, ascorbate peroxidase and Superoxide dismutase) in Plantago major L. Excess Zn reduced dry weight and chlorophyll content, resulting a decrease in photosynthesis. Zn stress induced the production of hydrogen peroxide (H2O2), leading to malondialdehyde (MDA) accumulation. Furthermore, it was found that the activities of antioxidant enzymes in Zn-treated plants such as superoxide dismutase (SOD) and ascorbate peroxidase (APX) were decreased but peroxidase (POD) and catalase (CAT) were increased. The use of NO especially in low concentration reversed Zn-induced negative effects whereas high concentration of NO had no obvious alleviating effect on Zn toxicity in Plantago major L. In particular, application of 100 mu M SNP could mitigate Zn stress, as a defense mechanism of the plant against Zn toxicity.