The role of phytochelatins and antioxidants in tolerance to Cd accumulation in Brassica juncea L.

A hydroponics experiment using Indian mustard (Brassica juncea L.) was conducted to investigate the effect of different concentrations (10-160 microM) of cadmium (Cd) and a fixed concentration (500 microM) of ethylene diamine tetra acetic acid (EDTA) on Cd accumulation and its toxicity for 14 and 28 days (d). The results showed that Cd alone and Cd+EDTA increased total dry biomass production, photosynthetic pigments and total protein content of B. juncea up to 160 microM with respect to control for 14d (hormesis effect). Further, on treatment with Cd at 160 microM for 28d, dry biomass of root and shoot, total protein content and total chlorophyll decreased up to 73%, 58%, 67% and 53% respectively, while in the case of Cd+EDTA, the decrease in the above parameters was 38%, 50%, 57% and 46% with respect to their control. It was observed that the maximum Cd accumulation after 28d in the root and shoot was 1925 and 977 mg kg(-1) dry weight (dw), respectively, while in the case of Cd+EDTA it was 1013 and 2316 mg kg(-1)dw, respectively. Levels of phytochelatins (PCs), glutathione reductase (GR; EC 1.6.4.2), non-protein thiols (NP-SH) and glutathione (GSH) were monitored as plants primary and secondary metal detoxifying responses. Glutathione reductase showed three-fold increased activity for Cd and 2.2-fold for Cd+EDTA at 160 microM after 14d followed by decreased activity after 28d with respect to control. Maximum synthesis of PCs was found at 10 microM of Cd exposure followed by a gradual decline after 28d. This may be correlated with reduced level of GSH, probably due to reduced GR activity, resulting in enhanced oxidative stress as also proved by phenotypic changes in plants such as browning of roots and yellowing of leaves. Thus, the capacity of B. juncea to accumulate and tolerate high concentrations of Cd, through enhanced level of PCs, GSH, NP-SH and GR suggests its applicability for phytoremediation.