# Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review
## 2 Lead Toxic Effects  
Lead toxic effects on human health are linked to lead's capability to interfere with many enzymes and to bind biologically important molecules. The damage can involve a large number of organs, affecting the hematopoietic, gastrointestinal, kidney, muscle, neurological, reproductive, and genetic systems [11]. Children are more vulnerable than adults because of the frequency of pica and hand-to-mouth activity coupled with a higher rate of intestinal absorption and retention [12]. The severity of the adverse effects on human health depends both on the dose and on the length of the exposure to lead, which can occur by inhalation or through the consumption of polluted water, vegetables, and animals farmed in soils rich in this heavy metal [13]. For these reasons, the assessment of the adsorption capacity by vegetables and fruits growing in lead polluted soils and destined for human consumption is of great relevance, especially considering the increasing development of horticulture in urban areas more exposed to anthropic pollution sources. Lead is not essential for plants. It has no nutritional or biological function, but it is conveyed in plant tissues through foliar uptake, being trapped by the cuticular wax and then diffused into the leaf tissue [14] or moving through the roots and crossing the root-cell plasma membrane. In the last case, a possible transport pathway of Pb appears to be the plasma membrane cation channels, such as Ca-channels [15; 16]. Then, Pb retention in the roots is based on the binding of metal to ion-exchangeable sites placed on the cell wall and on extracellular precipitation, mainly in the form of Pb carbonate deposited on the cell wall [17]. Inside plants, lead causes an increased production of reactive oxygen species (ROS), with the consequent damage to cellular lipid membranes with a resultant alteration of the photosynthetic processes [18]. [PERSON] [19] established that high concentrations of lead affect the dry weight of roots and shoots: in general, as a common response to stress posed by heavy metals, the above-ground biomass is strongly reduced, causing growth inhibition. This can be related to a reduction in meristematic cells and to the inhibition of the proton pump responsible for the cells' elongation process [20]. Likewise, earlier studies showed that Pb could bind to nucleic acids, resulting in an inhibition of cell division and plant growth. The exposure to Pb negatively affects several important plant stress indicators, such as (1) the composition of free amino acids and other nitrogen compounds, (2) the relative water content (RWC), and (3) the chlorophyll yield. High concentrations of the metal reduce the action of enzymes involved in chlorophyll pigments biosynthesis, with a consequent decrease in the photosynthetic potential and in plant growth [20], and a reduction of RWC, probably due to stomatal closure. In addition, the potential hazard to human health of edible species grown in the Pb-contaminated soils is linked not only to the initial concentration of lead in the soil, but also to the relationship between the Pb concentration in soil and plants that depends on soil chemical and physical properties (pH, phosphorus content, organic matter, cation exchange capacity, texture) and crop age and species [21; 22]. [PERSON] et al. [23] noted that lead content in soil and roots are well correlated, whereas the correspondence is generally less predictable in the case of leaves and edible parts. This can be justified considering the contribution of air pollution: the deposition of dust containing the metal can be responsible for most of the contamination of broad-leaved plants [21]. Conversely, [PERSON] et al. highlighted that Pb content in vegetables (lettuce, carrots, beans, and tomatoes) grown in soils with a detected lead amount equal to 400 mg/kg did not exceed the limits settled for the metal by international health standards (European Union has set the limit at 0.1 mg/kg f.w. for fruits and roots and at 0.3 mg/kg f.w. for leafy greens) [21].