**3.1 Lead (Pb)**

Lead (Pb) is accumulated in plant organs, namely, leaves, stems, roots, and tubers (shallots), and the transfer depends on the soil composition and pH. High Pb concentrations (100–1000 mg kg−1) have a toxic effect on photosynthesis and growth [27]. Pb is one of the nonessential heavy metals that are toxic to living organisms. It causes stunted growth, irritates the eyes, and contributes to lung [28] and kidney [29] damage. The highest accumulation in roots was proven by [30] through a study of Pb in kale (*Brassica oleracea* var. *sabellica*). In the 6-week-old kale plant, Pb concentration in the roots reached about 3360 mg kg−1, and in other parts of the plant, it reached 2090 mg kg−1. In 3-week-old kale, the Pb content in the roots was 1.860 mg kg−1 in the sample but 1.130 mg kg−1 in other parts. These data indicate that most Pb in water spinach is accumulated in the roots.

The largest Pb pollution comes from burning gasoline, which produces PbBrCl and PbBrCl2PbO. The pollution can come from Pb components in dissolved air or water, such as PbCO3 [31]. According to [32], heavy metals in the media are rapidly absorbed by plants at very low concentrations. The mechanism of absorption and accumulation can be divided into three continuous processes: (a) Absorption by roots: metals should be brought into the solution around the roots (rhizosphere) in several ways to be absorbed. Water-soluble compounds are usually taken up by the roots with water, while the surface absorbs hydrophobic compounds. (b) Translocation of metals from roots to other plant parts: After penetrating the root endodermis, metal or other foreign compounds follow the transpiration flow through the transport tissue (xylem and phloem) to other parts. (c) Metal localization in cells and tissues: This aims to keep metals from inhibiting plant metabolism. Plants have detoxification mechanisms in certain organs, such as roots, to prevent metal poisoning of cells. Metals in the root cells are transported to other plant parts through the xylem and phloem network when translocation occurs in the plant body. At low concentrations, heavy metals do not affect plant growth but cause damage to the soil, water, and plant at high concentrations.

Satpathy et al. [33] argued that Pb originating from air/atmosphere pollution is in the form of dust particles, which will stay on the plant's surface. Clouds and rain can cause Pb to be dissolved and enter the plant through the stomata, which can cause damage and contaminate food. Air pollution by Pb mainly comes from exhaust fumes from motor vehicles, and this metal is the remnant of combustion between the fuel and the vehicle engine. The presence of Pb in motor vehicle fuel functions as an anti-knock agent. The Pb element is released into the air through the exhaust of the vehicle's gasoline. Some will form particulates in the free air with other elements, while others will stick and be absorbed by the leaves of plants along the way. Soil contamination by Pb is more extensive than other heavy metals. This is because the largest contribution is from anthropogenic sources. The research results [34] showed that the Mn, Co, Cr, and Ni on the soil surface come from lithogenic and anthropogenic sources. These results indicated a significant need for developing pollution prevention and reduction strategies for heavy metal pollution. Accumulation of heavy metals can degrade soil quality, reduce crop yields and agricultural product quality, and negatively impact humans, animals, and the ecosystem. The solution can be achieved by identifying the source and measuring the concentration of heavy metals and the spatial variability in the soil. The results revealed could be used to determine the increase in Cd and Pb concentrations [35].

### **3.2 Cadmium (Cd)**

Soil Cd in igneous, metamorphic, and sedimentary rocks is 0.100–0.300, 0.100–1.00, and 0.300–11 mg kg−1. In general, the Cd content in the soil from the weathering process of rocks is 1.00 mg kg−1 or lower. The elements Cd and Zn have almost similar chemical properties, and only their function in the plant body is different. Cd levels in plant tissues range from 0.100 to 1.00 mg kg−1. Excessive Cd accumulation can occur from other materials, with a detrimental effect on plant growth. This is because it breaks down nitrate absorption and inhibits the activity of the enzyme nitrate reductase. The critical limit of Cd in plants is 5–30 mg kg−1 [36], and the content in the 0–20 cm layer, on average, is close to 0.5 mg kg−1, which is the critical limit concentration of the metal [13]. Cadmium in the soil is an anthropogenic byproduct of fertilizer and garbage dumps. Most of the soil's Cd is affected by pH, organic materials, metallic oxides, clay, and organic and inorganic substances [28]. The average level of natural Cd in the earth's crust is 0.1–0.5 mg kg−1.

The Cd content is influenced by the reaction of the soil and fractions capable of binding the ions. Due to the rise in the hydrolysis process, the adsorption complex, and the charge of the soil colloid, Cd concentration in soil solution reduces with increasing pH. Sarwar et al. [37] stated that there was a reduction in root and shoot length of about 45 and 35% in maize plants grown on media containing 28.1 and 11.2 mg kg−1 Cd(II) ions, respectively, at the age of 18 days. The contribution from atmospheric deposits occurs in industrial areas that use coal and oil as fuel. Cd is added to the soil through phosphate fertilizers, manure, incinerator waste (furnace), and sewage sludge [23, 36]. In addition, the increase in Cd can occur through phosphate fertilizers, whose levels vary greatly depending on the type of phosphorite as an industrial material for phosphate fertilizers [38]. Cadmium has chemical properties similar to those of Zn, especially in the process of absorption by plants and soil. However, Cd is more toxic, which can interfere with enzyme activity. Excessive levels of Cd in food can damage kidney function, interfere with Ca and P metabolism, and cause bone disease [39].
