Heavy metal pollution is not only a hazard to living organisms but also an important worldwide environmental concern.
These metals occur naturally in the biosphere as a result of volcanic eruptions and weathering of rocks. Recently, more heavy metals have been released into the environment due to various human activities such as mining, fossil fuel combustion and poor sewerage systems. Agricultural soils are usually contaminated by heavy metals such as lead (Pb), cadmium (Cd), zinc (Zn) and copper (Cu) which are readily available for uptake by plants.
Plants support the ecological system and provide food for the human population. Some plants can tolerate and serve as potential pathways for integrating the undesirable heavy metals into plant tissues. The tolerance to heavy metals can lead to a greater risk to human health. Various plants and crops have been shown to be hyperaccumulators of heavy metals. Studies have shown that over 80% of heavy metals could be ingested via the food crops. According to, during
1998–2001, 36–50% of the Cd consumed by Japanese was from rice. Some heavy metals (e.g., Mn and Zn) are beneficial to microorganisms, plants, and animals. However, these metals at high concentrations in contaminated soils and water can be toxic to plants (primary producers).
Many soil factors such as pH and cation exchange capacity as well as plant species, cultivars and age, could influence heavy metal uptake by plants. Plants are at a higher risk of lipid peroxidation if they are exposed to heavy metals, which is usually tested through its end product, Malondialdehyde (MDA). Under stressful conditions, MDA level in plant tissues is usually elevated in plants.
In previous reports, higher plants exposed to heavy metals have shown increased MDA content. Studies with Cd, Pb, and Cu had enhanced lipid peroxidation and increased reactive oxygen species (ROS) in various plants. However, plants have evolved protective enzymatic mechanisms like superoxide dismutase (SOD) and peroxidases (POD) that can scavenge the ROS and alleviate the resultant negative effects.
To maintain cell membrane stability, SOD can sequester O2, reducing lipid membrane peroxidation, while POD can reduce H2O2 accumulation and eliminate MDA, hence keeping the integrity of cell membrane lipids.
China is the most populous country in the world with about 1.3 billion people. About 67% of this population is living in rural areas and more than 300 million people are directly involved in agricultural activities. The arable land is only 7% of the global amount and more than 75% of the total cultivated land is used for producing food crops. Currently, China produces 18% and 50%, respectively, of the cereal grains and vegetables in the world. However, agricultural development in China is confronted with many challenges, among them soil contamination. Studies have shown that 13,330 ha of farmland have been contaminated by heavy metals from phosphate fertilizer application, industrial emission and municipal wastes in 11 provinces. Reliable approaches are desired to prevent heavy metal accumulation in crops and consequently protect living organisms including human from related health hazards.
Nanotechnology has been growing in importance as a potential technology that could be used to clean the environment. Nanoparticles, often characterized by a significant amount of surface area, have unique properties and potential applications in reducing the negative effects of heavy metals on the natural resources. Few studies have utilized nanotechnology to investigate plant phytotoxicity caused by heavy metals in various environmental mediums. The remediation of polluted soils and water using nanoscale materials continues to gain relevance especially in light of new environmental molecular science and engineering techniques. Although nanoparticles can be cost effective in reducing heavy metal toxicity in plants, alleviation of heavy metal-induced root growth inhibition and oxidative stress in plant has been hardly studied. Magnetic (Fe3O4) nanoparticles were successfully used to adsorb heavy metal ions. In our previous study (data not published), we evaluated the alleviation of Cd-induced root growth inhibition and oxidative stress in the seedlings of cucumber and wheat using different sizes (6 nm, 50 nm, and 100 nm) of magnetic (Fe3O4) nanoparticles and bulk-Fe3O4. Four concentrations (0, 50, 500, and 2000 mg/L) of nano-Fe3O4 or bulk and 25 mg/L (Cd) were used in that experiment. The addition of nano-Fe3O4 (6 nm) significantly decreased the Cd accumulation, Cd-induced seedling growth inhibition and oxidative stress in the seedlings of both cucumber and wheat with increasing concentration. Therefore, we hypothesized that different heavy metal (Pb, Zn, Cd and Cu) treatments may cause inhibition in root growth, and induce oxidative stress in the wheat seedlings. Under this stress, the inhibitory effects of the selected heavy metals would be reduced and the antioxidant mechanisms activated with the addition of magnetic (Fe3O4) nanoparticles (6 nm). We also hypothesized that the reducing effects of nano-Fe3O4 would vary with the heavy metal types. To test these hypotheses, root elongation, accumulation of heavy metals, activities of SOD and POD, and MDA content were measured in this study. To better understand the effects of nanoparticles on reducing the phytotoxicity of heavy metals, we conducted study with magnetic (Fe3O4) nanoparticles on the toxicity and oxidative stress induced by four heavy metals (Pb, Zn, Cd and Cu) to early seedling growth of wheat (Triticum aestivum L.). Wheat is common model that has been extensively used in previous studies. Moreover, wheat species have great economic and ecological relevance and are among the widely consumed plant crops in the world. Furthermore, we provide a basis for developing approaches to reduce risks associated with the toxicity of heavy metals and maintaining sustainable plant production. The growth of seedlings, the accumulation of heavy metals and the oxidative stress were investigated at 1mMand 10mMof the selected heavy metals with or without nano-Fe3O4 (2000 mg/L). According to the US EPA guidelines, 2000 mg/L of nano-Fe3O4 is the highest concentration of NPs that can be regarded to have minimal phytotoxicity if no adverse effect on root growth are observed in tested plants. In addition, based on our previous study (data not published), this concentration of nano-Fe3O4 had greater effect in reducing toxicity of Cd (25 mg/L) in wheat and cucumber seedlings. Similarly, copper (Cu), lead (Pb) and zinc (Zn) affected radicle elongation of wheat at 1 mM and 10 mM. Our results could be helpful in improving the protection of plant growth in polluted areas using magnetic (Fe3O4) nanoparticles.
This article is collectively authored by Fahad I. Virk and Amir I. Virk .