The sources of heavy metals include both natural processes and human activities. Over past decades, more and more heavy metals of anthropogenic origin have been discharged into the environment, most of which have increasingly accumulated to potentially harmful levels in soils [1]. In addition, several human activities (such as wastewater irrigation, pesticides, chemical fertilizers, urban wastes, and metal mining) have led to the accumulation and contamination of heavy metals in agricultural soils [2]. Therefore, the accumulation of these metals in agricultural soils has become a vital problem worldwide as they can transfer into the food chain and threaten human health [1]. Moreover, when the heavy metal accumulation in soil is excessive, it can lead to crop loss and environmental and ecological deterioration [3]. Among heavy metals, zinc (Zn) and lead (Pb) are common soil co-pollutants from anthropogenic activities, such as severe soil degradation, automobile emissions, mining, and others [4]. Pb is one of the most toxic and widely reported metals in farmlands. Shi et al. [5] stated that more than 800,000 t of Pb had been released into the environment globally over five decades, most of which has accumulated in soil. Pb accumulation in soils affects environmental health and can impact human health and food quality. Furthermore, Pb affects the diversity of the biological population in soils. Biochemical processes, including nutrient cycling and soil organic matter breakdown, have also been influenced by high concentrations of Pb [6]. Another widely reported metal in soils is Zn. While Zn is an essential nutrient for the growth and development of plants, Zn at high concentrations in soils may cause metabolic disorders, become phytotoxic, and lead to a threat to human health from the food chain [7].

Consequently, the uptake and toxicity of Pb and Zn in plants were considered in this study. In addition, cereal crops (such as maize) are the major dietary sources of metal accumulation (such as of Pb and Zn) in humans, and therefore, reducing the metal transfer from soil to grains is a key issue for the food safety [8].

Maize is one of the main cereals produced worldwide and represents a basic food crop in human alimentation [9]. Chen et al. [10] stated that the production of maize (Zea mays L.) surpasses that of either wheat or rice. Furthermore, Wang et al. [11] stated that maize is an important and common agricultural crop worldwide that has been applied in several studies about metal pollution. Zampieri et al. [12] expressed that the global production of maize is estimated to be more than 1 × 10⁹ t. Hence, from the perspective of evaluating the uptake of Pb and Zn by plants, it would be valuable to give attention to the toxicity and mode of action of Pb and Zn in maize.

2. Zn and Pb Accumulation in Farmlands Worldwide

Zn is an essential micronutrient for plants, and several plant species have developed strategies for securing or maximizing the utilization of Zn [13]. Intensive fertilizer use, wastewater or sewage sludge, and agricultural and animal wastes can cause the accumulation of Zn in many agricultural soils [14]. Zn accumulation in soil can affect soil fertility with phytotoxicity, microbial biomass, and soil macronutrient shortage (such as of phosphorous) [15]. Pb is naturally occurring in soils but mostly accumulates through anthropogenic activities, such as atmospheric deposition, mining, and gasoline use. Furthermore, the addition of Pb to soils via herbicides/pesticides has been frequently reported in the past [14]. Nyiramigisha et al. [15] expressed that the accumulation of Pb in soil can cause abnormalities in the metabolic function of microorganisms, shortages of soil macronutrients (such as phosphorus), decreases in urease, invertase, catalase, and acid phosphatase activity, and interruptions in water balance, mineral nutrition, and enzyme activity. As described by Leštan et al., there are four main reactions that control the fractionation of heavy metals in soil [16], including: (1) adsorption/desorption because of ion-exchange and the formation of complexes and chemical bounds; (2) precipitation, usually with anions such as carbonate, phosphate, and sulfate, and participating as hydroxides; (3) penetration into the crystal structure of minerals and isomorphic exchange with cations; and (4) biological immobilization and mobilization. Zunaidi et al. [17] stated that valence, the speciation and charge of metal ions, and soil properties (such as clay, redox potential, pH, and organic matter content) can influence the behavior of metals in contaminated soils. The type of agricultural soil is one of the most important factors that can affect the fate of heavy metals and their transfer in soils. Li et al. [18] expressed that soil minerals are key components of solid soil matrices. Clay minerals are important active components of soils that meaningfully affect the fixation and migration of metals within soils. It has generally been reported that clay plays a vital role in the accumulation of heavy metals. The adsorption of heavy metals with clay constituents is one of the important processes that defines the mobility and bioavailability of heavy metals in environments [19]. Ou et al. [20] stated that clay minerals commonly decrease the fractions of bioavailable/extractable heavy metals in soil. Clay minerals frequently have small particle sizes and high specific surface areas and contribute to the quantity of electric charge. Moreover, clay can adsorb heavy metals over inner-sphere complexation reactions [21]. In addition, clay particles contain commonly negative charge, which is a vital factor affecting the sorption properties of soil [22]. Two main types of clay minerals, based on the arrangement of tetrahedral and octahedral sheets, include 1:1 and 2:1 [23]. The 2:1 clays have a much greater surface area than the 1:1 clays due to the existence of an internal surface area. The 2:1 clays also have a greater cation exchange capacity (CEC) than the nonexpanding types; thus, the 2:1 clays have a much greater propensity for immobilizing metal ions [22]. Many studies ^[23][24][23,24] have shown that Zn and Pb can be fixed by sorption onto specific clay minerals. Soil pH is another important factor that has a vital effect on Zn and Pb dynamics in soil and their uptake by plants [25]. Zwolka et al. [25] stated that the acidic pH of soil can be considered as one of the most vital factors affecting the mobility of metals in soil and their absorption by plants. Adamczyk-Szabela et al. [26] reported that a significant decrease in the Zn content of plants was observed with increasing soil pH levels up to 10. This may be a result of the increased Zn adsorption to soil with a high pH as the adsorption capacity of a solid soil surface that is usually enhanced by an increasing pH-dependent negative charge, chemisorption on calcite, co-precipitation in ferric oxides, and the formation of hydrolyzed forms of Zn [27]. However, the adsorption of metals on soil colloids is decreased at very acidic pH levels due to the competition of metals cations with H⁺ in adsorbing to colloids [28]. Leštan et al. [16] stated that the adsorption reactions of Pb and Zn are vital in soil at pH 3 to 5 and pH 5 to 6.5, respectively. Complexation and precipitation reactions of both Zn and Pb are dominant at pH 6 to 7. Soil organic matter (SOM) plays a vital role in the mobility and uptake of Zn and Pb in soil and plants. Commonly, the solid phase of SOM is associated with the retention, decreased mobility, and bioavailability of trace metals; however, cationic metals, which would ordinarily precipitate at certain pH values, are sometimes maintained in solution via complexation with soluble organics [29]. In one study, the extractability of Pb was shown to be low in organic matter-rich soil, and the retention of Pb by SOM can be explained by the formation of organic complexes [30]. Oudeh et al. [31] found that SOM provides binding sites for metals. In another study, SOM strongly inhibited the precipitation of Pb at an acidic pH (3 to 4) [32]. Rutkowska et al. [27] stated that SOM has a dual effect on the concentration of Zn is soil solution. SOM enhances the adsorption of Zn to a solid phase; thus, it can decrease the Zn concentration in soil. However, high SOM levels can generate high dissolved organic carbon content, which can help form Zn complexes and result in higher concentrations of Zn in soil solutions. The study by Rutkowska et al. [27] also showed that the Zn activity in soil can increase with an increase in dissolved organic matter (DOM). DOM is a complex mixture of various molecules and is generally defined as the organic matter that can pass through a 0.45 μm filter. DOM can strongly bind Pb and Zn and play a vital role in controlling these metals in soil [33]. Table 1 shows the concentrations of Pb and Zn in agricultural soils worldwide, demonstrating that the accumulation and pollution of these metals in agricultural soils can be considered as a global issue. The greatest Pb concentration (up to 3015 mg/Kg) was reported in Namibia, whereas the greatest Zn concentration (1140 mg/Kg) was detected in Guilin (China).

3. Uptake of Pb and Zn by Maize and Effects of Their Toxicity on Maize

As mentioned above, the uptake of metals by maize roots depends on the metals’ availability in the soil solution, and this is related to several factors, mainly soil pH, presence and quantity of hydrous ferric oxide, soil properties, types of clay, and other factors. The reported accumulations of Pb and Zn in different parts of maize are shown in Table 2. The maximum Pb levels reported in roots, shoots, and grains was 27,870, 4180 (in China), and 245 (in India) mg/Kg, respectively, whereas the maximum Zn levels reported in roots, shoots, and grains was 6320, 2020 (in China), and 39.17 (in India) mg/Kg, respectively. Toxic levels of heavy metals have been reported to affect normal plant functions, disrupting metabolic procedures by modifying the permeability and enzymatic activity of the cell membranes in maize. Moreover, metals negatively interact with vital cellular biomolecules (such as nuclear DNA and proteins, which results in an increase in reactive oxygen species (ROS)) and disrupt the essential metal functionality in biomolecules (such as enzymes or pigments). A high Zn concentration in soil has been found to decrease initial chlorophyll fluorescence [63]. Furthermore, Zn toxicity can cause a blockage of xylem elements and inhibition of photosynthesis through the change in electron transport and the capacity of rubisco to fix CO₂ [64] or through the cellular debris [65]. Apart from that, Rout and Das [66] stated, in high concentrations of Zn (7.5 mM of zinc), root cortical cells were obviously damaged. Moreover, they stated that necrosis can occur in mesophyll cells at high concentrations of Zn. In a study [67], inhibition of growth was reported after five weeks in high concentrations of Zn (400–1600 mM). High concentrations of Zn can significantly reduce growth rate and biomass, and inhibit cell elongation and division [67]. In another study [68], growth of maize was notably reduced in Zn toxicity conditions. In addition, a higher concentration of Zn causes higher accumulation of Zn in grains [69]. Islam et al. [70] stated that Zn, in high concentrations, may interfere with chlorophyll synthesis, which causes reduced photosynthesis and inhibition of plant growth. Pb toxicity reduces root and plant growth and causes chlorosis and the blackening of roots. Pb can inhibit photosynthesis and reduce mineral nutrition and enzyme activities [71]. Pb toxicity causes an inhibition of seed generation and seedling growth and a decrease in the percent and index of germination [72]. Furthermore, Pb can be harmful to the cell membrane, and it alters its permeability, causes a reaction of sulphydryl (-SH) groups with cations, and reacts with phosphate groups and active groups of ADP and ATP [71]. In a study [73] on corn, the seed germination, length of roots and shoots, dry weights of roots and shoots, and total protein content were reduced at high concentrations of Pb. Sofy et al. [74] stated that the toxicity of Pb can negatively affect plant metabolism; thus, inhibition of plant growth can be caused by high concentrations of Pb in soils. The uptake of Pb and Zn increases with an increase in the availability of Pb and Zn in the soil [86]. Plants are capable of the uptake of metals (such as Zn and Pb) primarily through the plant roots via passive absorption, and some specific proteins facilitate metal transport in movement across the membrane (Soliman et al., 2019). The root cell walls first bind metal ions from the soil, and then the metal ions are taken up across the plasma membrane. The uptake of metal ions occurs via the secondary transporters (such as channel proteins and/or H⁺-coupled carrier proteins) [87]. With an increase in heavy metals concentration, the transportation and accumulation of metals in shoots and leaves are increased. In addition, several genes and proteins are involved in transporting Zn and Pb in maize.