Document Type : Original Article
Authors
1
MSc. Student, Department of Environment, University of Birjand, Birjand, Iran
2
Associate Professor, Department of Environment Sciences, Faculty of Natural Resources and Environment, University of Birjand, Birjand, Iran
3
Associate Professor, Department of Plant Production and Genetics engineering, Faculty of Agriculture, University of Birjand, Birjand, Iran
4
Member of the Plant and Environmental Stresses Research Group, University of Birjand, Birjand, Iran
Abstract
Introduction
Heavy metals (HMs) are commonly defined as metals with a density greater than 5 g cm⁻³. Lead (Pb), zinc (Zn), copper (Cu), cadmium (Cd), nickel (Ni), arsenic (As), iron (Fe), manganese (Mn), molybdenum (Mo), cobalt (Co), magnesium (Mg), mercury (Hg), chromium (Cr), silver (Ag), and selenium (Se) are among the metals commonly classified as HMs. Some HMs, such as copper (Cu), selenium (Se), and zinc (Zn), are essential in trace amounts for maintaining normal physiological functions but can be toxic at higher concentrations. Cadmium is a non-essential heavy metal that is highly toxic to humans, animals, and plants. Cadmium exhibits relatively high solubility in water, allowing it to enter plants through contaminated soil and water and subsequently propagate through the food chain when herbivores consume contaminated plant biomass. Lead is a heavy metal used in diverse sectors, including the military, chemical, healthcare, agricultural, petroleum, and mining industries. Consequently, lead is regarded as one of the most significant and pervasive contaminants globally. In light of the hazards posed by soil contamination with cadmium and lead, remediation of affected areas is essential to mitigate the risks associated with these pollutants. Phytoremediation, or bioremediation using plants, improves environmental quality and is regarded as a sustainable bioengineering approach for remediating contaminated ecosystems. During phytoremediation, plants can sequester or stabilize contaminants, thereby inhibiting their release into the environment. This study was conducted to examine the growth characteristics of sorghum and assess its capacity to absorb and transport cadmium and lead at different soil concentrations, given the importance of phytoremediation and the substantial growth potential of sorghum under adverse environmental conditions.
Materials and methods
A pot factorial experiment with three replications was conducted in the research greenhouse of the Faculty of Agriculture, University of Birjand, to assess the effects of cadmium and lead on sorghum growth and its potential for phytoremediation in a completely randomized design. The experiment included two metal types (cadmium and lead) and six soil metal concentrations: 0, 25, 50, 100, 200, and 400 mg kg⁻¹. The treatment soils were prepared by adding cadmium chloride and lead chloride. After harvesting, the plant parts were powdered using an electric grinder. Then, 0.5 g of each shoot and root sample was digested with 10 mL of a 9:4 mixture of HNO₃ and HClO₄. The samples were placed in a sand bath at 100 °C and heated until they became translucent. After digestion, the samples were diluted to a final volume of 20 mL with deionized water and filtered through Whatman No. 1 filter paper. The cadmium and lead concentrations in the samples were determined using an atomic absorption spectrometer (Perkin-Elmer AAnalyst 700) at the Pollution Laboratory, Faculty of Environment, University of Birjand. Cadmium and lead concentrations were determined using calibration curves. Finally, the bioconcentration factor, bioaccumulation coefficient, transfer factor, transfer efficiency, and tolerance index were calculated.
Results and discussion
The comparison of means for plant height, greenness index, root dry weight, and shoot dry weight revealed reductions of 38.3%, 0.51%, 41.4%, and 38.9%, respectively, at the highest heavy metal concentration (400 mg kg⁻¹). The comparison of means regarding the effects of heavy metals on sorghum growth characteristics revealed that cadmium had a greater effect on plant height and root dry weight than lead, with corresponding effects of 29.6% and 30.8%, respectively. Increasing the concentrations of Cd and Pb resulted in increased levels of these metals in the roots and shoots. An increase in the concentration from 25 to 400 mg kg⁻¹ resulted in a 74.6% increase in heavy metal concentration in the roots and a 49.4% increase in the shoots. The bioconcentration factors, root bioaccumulation coefficients, and shoot bioaccumulation coefficients decreased as the concentrations of both cadmium and lead increased. The transfer factor and transfer efficiency for lead were higher than those for cadmium. Sorghum plants appear to mitigate the toxicity of cadmium and lead by predominantly sequestering these metals in their roots and transferring only a small fraction to the shoots.
Conclusion
The results of this study indicated that the presence of heavy metals, including cadmium and lead, in the soil resulted in reduced sorghum growth and biomass production, particularly at elevated concentrations. The effect of cadmium on the growth parameters of sorghum was significantly greater than that of lead. Moreover, elevated concentrations of cadmium and lead in the soil were associated with increased levels of these elements in the roots and shoots of sorghum. The bioconcentration factors, root bioaccumulation coefficients, and shoot bioaccumulation coefficients decreased with increasing concentrations of both cadmium and lead. The transfer factor and transfer efficiency were higher for lead than for cadmium. Despite the higher rate of lead translocation from roots to shoots compared with cadmium, the observed reduction in absorption and translocation with increasing concentrations of cadmium and lead in the soil suggests that sorghum functions as a stabilizing agent for these elements in bioremediation
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