Effect of cadmium stress on photosynthesis,lipid peroxidation and antioxidant enzyme activities in maize(Zea mays L.) seedlings

To gain better insight into the biological mechanisms of reactive oxygen species in cadmium-stressed plants,hydroponics culture experiment was conducted to examine the effects of cadmium stress on photosynthesis,lipid peroxidation and antioxidant enzyme activities in maize plants.Maize seedlings were treated with 5,20 and 100 μmol/L Cd2+ for periods of 12 to 168 h.One week of Cd addition decreased shoot biomass,but did not affect root dry matter weight yet.The photosynthesis rate in leaves began to be inhibited by 24 h exposure to 100 μmol/L Cd2+ and decreased with increasing Cd concentrations and prolonged Cd treatment.The reduction of potential maximal quantum yield of PSII due to Cd stress occurred later than that of photosynthesis rate.After 96 h treatments of 20 and 100 μmol/L Cd2+,the levels of malondialdehyde(MDA) in roots increased,but MDA concentrations in leaves did not increase even after 7 days of Cd treatment.The activities of antioxidant enzymes such as superoxide dismutase(SOD,EC1.15.1.1),ascorbate peroxidase(AsA-POD,EC1.11.1.11),guaiacol peroxidase (EC1.11.1.11),catalase(CAT,EC 1.11.1.6) and glutathione reductase(GR,EC1.6.4.2) in leaves and roots increased first by Cd treatments,then declined or did not change.This trend was more obvious with higher concentrations of Cd and extended Cd treatment.Cd had little effect on dehydroascorate reductase(DHAR,EC1.8.5.1) activities in leaves and roots during periods of 12-168 h.Early inhibition of CO2 assimilation by Cd stress was suggested to result in increased production of reactive oxygen species in maize seedlings,which would induce the increased activities of some antioxidant enzymes.