Heavy metal-mediated toxicity to maize: oxidative damage, antioxidant defence response and metal distribution in plant organs

Heavy metals are serious environmental threats that, after accumulation inside edible organs of the food crops and following consumption, pose major human health problems. Maize is one of the most important food grain crops and so is the changes caused by heavy metals. Realizing this, the toxic impact of cadmium, chromium and nickel on biological characteristics, seed attributes, antioxidant enzymes and metal distribution in maize was assessed. Growth and yields of maize plants declined regularly with progressively increasing concentrations of metals. Among all metals, cadmium had most lethal effects and the onset of vegetative and reproductive growth stages was delayed. Cadmium at 36 mg/kg maximally declined the root and shoot length by 65 and 32%, respectively, while chromium (204 mg/kg) reduced the total chlorophyll content, grain yield and grain protein by 77, 84 and 16%, respectively, over control. The severity of oxidative stress increased with increasing rates of metals. Proline and malondialdehyde were enhanced by 59 and 72%, respectively, over control. The expression of antioxidant enzymes was superior in foliage of maize grown under metal stress. Cadmium maximally reduced the total P content in roots and shoots. Roots in general had more metals than shoots and grains. The scanning electron microscopy and confocal laser scanning microscopy images showed metal distortive impact while energy-dispersive X-ray spectroscopy confirmed the location of metals inside plant organs. The results suggest alarming consequences of metal toxicity to maize and the accumulation of heavy metals within grains raises disturbing public health concerns.