Exogenous application of calcium and silica alleviates cadmium toxicity by suppressing oxidative damage in rice seedlings

The present study was undertaken to examine the possible roles of calcium (Ca2+) and silica (Si) in protection against oxidative damage due to Cd2+ toxicity in rice (Oryza sativa L.) seedlings grown in hydroponics. Rice seedlings raised for 12 days in hydroponics containing Cd(NO3)(2) (75 mu M) showed reduced growth; increase in the level of reactive oxygen species (ROS) (O-2 (center dot-) and H2O2), thiobarbituric acid reactive substances (TBARSs) and protein carbonylation; and increase in the activity of antioxidant enzymes-superoxide dismutase (SOD), catalase (CAT) and guaiacol peroxidase (GPX) compared to untreated controls. Exogenously added Ca2+ (2 mM) and Si (200 mu M) significantly alleviated negative effect of Cd2+ by restoration of growth of the seedlings, suppression of Cd2+ uptake and restoration of root plasma membrane integrity. The levels of O-2 (center dot-), H2O2, lipid peroxidation and protein carbonyls were much lower when Ca2+ and Si were added in the growth medium along with Cd2+ as compared to Cd-alone-treated seedlings. Ca2+ and Si lowered Cd-induced increase in SOD, GPX and APX activities while they elevated Cd-induced decline in CAT activity. Using histochemical staining of O-2 (center dot-) and H2O2 in leaf tissues, it was further confirmed that added Ca2+ and Si suppressed Cd-induced accumulation of O-2 (center dot-) and H2O2 in the leaves. The results suggest that exogenous application of Ca2+ and Si appears to be advantageous for rice plants in alleviating Cd2+ toxicity effects by reducing Cd2+ uptake, decreasing ROS production and suppressing oxidative damage. The observations indicate that Ca2+ and Si treatments can help in reducing Cd2+ toxicity in rice plants.