Adaptations to oxidative stress in Zea mays roots under short-term Pb2+ exposure

Lead (Pb), a widespread contaminant in terrestrial landscape, is highly detrimental to plant and animal life. Specifically, Pb-contaminated soils cause a sharp decrease in crop productivity, thereby posing a serious risk to agriculture. A study was planned to investigate the toxic effect of Pb2+ (0, 16, 40 and 80 mg L-1) in the seedlings of maize (Zea mays), in terms of induced physiological and biochemical changes at initial hours of treatment (0-8 h). Increased accumulation of malondialdehyde (MDA) served as an indicator of cellular peroxidation. At 80 mg L-1 Pb2+, MDA content enhanced over the control by 175% after 2 h of exposure and increased further to 461% greater over the control after 8 h of exposure. Elevated superoxide ion (O-2(-center dot)) and H2O2 contents suggested oxidative damage to the plants. The level of H2O2 increased over control by 70%, 80%, 135% and 182% at 2, 4, 6, and 8 h after exposure to 16 mg L-1 Pb2+, respectively. In situ histochemical localization confirmed the level of lipid peroxides, increased accumulation of O-2(-center dot) and loss of membrane integrity upon Pb2+ treatment. Pb2+-induced oxidative stress triggered significant changes in the activities of antioxidant enzymes. A concentration-dependent increase was observed in the activities of the superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and glutathione reductase (GR) in response to Pb2+, whereas catalases (CAT) was not able to provide protection against oxidative stress. These observations imply that Pb2+ -induced oxidative stress during initial period (0-8 h) of exposure involved ROS accumulation and upregulation of scavenging enzymes except CAT as a defense against Pb2+-toxicity.