Lead Induced Toxicity, Detoxification and Bioremediation 191

11.2.2 Lead Uptake and Toxicity in Plant System

Pb is available in different exchangeable and non-exchangeable forms in soil. Among these, Pb (II) is

the only form that can be absorbed by plants. Higher concentrations of Pb in soil may lead to its entry

inside the plant system. Soil pH is also one of the essential factors which affect the bioavailability

of Pb to plants. Pb is adsorbed by plant roots at low pH, as high pH favors the formation of stable

and insoluble covalent Pb compounds such as acetates, carbonates and hydroxides (Shahid et al.

2017). The most common mechanism by which Pb enters the plant body is through roots. However,

Pb is translocated through the apoplast pathway and Ca channels on the plasma membrane to above

ground parts of the plant. Nevertheless, the level of Pb that reaches the shoot region is very low as

most of it is sequestered through chelation with the mucilage and galacturonic acids of the cell walls

of roots. Even lower concentrations of Pb that enter the plant system causes significant disturbances

in plant fundamental processes.

Pb being a toxic heavy metal, has no biological function in plant primary as well secondary

processes. Excessive amounts of Pb in growing environments may negatively influence plant

activities from seed germination to crop yield. Pb contamination disrupts seed germination by

inhibiting activities of various enzymes such as amylases, proteases thereby leading to retarded

seedling development. Decreased root, shoot length, leaf area and impaired photosynthesis are other

symptoms reflected in plants grown in Pb prone areas.

Pb pollution inhibited root growth, root hair differentiation, water and essential divalent cations

absorption such as Mg, Ca, Mn, Fe, etc., by roots, which resulted in damage to normal machinery

of plants (Rucińska-Sobkowiak 2016). Pb may also directly interact with many functional groups

such as -OH, -COOH, -SH, CO, CHO, etc., thereby, leading to conformational changes in primary

biomolecules, i.e., lipids, proteins, carbohydrates and nucleic acids. In addition to this, the production

of Reactive Oxygen Species (ROS) under Pb stress has also been reported in many research studies.

These ROS may oxidize vital plant metabolites and hence, hamper the overall efficiency of various

metabolic processes such as photosynthesis, respiration, transpiration, etc. (Ghori et al. 2019,

Hasanuzzaman et al. 2020).

Pb is also shown to inhibit the action of enzymes involved in the production of chlorophyll

synthesis and upregulates the activity of chlorophyllase, chlorophyll degrading enzyme which results

in decreased chlorophyll content in plants (Yang et al. 2020). In certain cases, the replacement of

the central Mg atom of chlorophyll by Pb has also been found. Other processes which are disturbed

include hill reaction, Calvin cycle and grana stacking in the chloroplast. All these factors lead to

impaired photosynthesis, one of the most indispensable processes influencing crop productivity

(Santos et al. 2015). The toxic effects of Pb on plant health have been summarized in Figure 11.1.

A large amount of nuclear damage resulting from Pb toxicity has also been reported in different

plants, including polyploidy, replication errors leading to single stranded DNA formation, incomplete

cell cycle and chromosome stickiness (Pizzaia et al. 2019). In addition to these, modifications in

the level of water potential, membrane integrity and hormonal signaling may also occur due to Pb­

induced induced plant damage. All these alterations assertively result in dwindled crop productivity

and hence create a serious threat to the world’s health and economy.

11.3 Lead Tolerance and Detoxification Mechanism Inside Plant System

Pb enters the environment as a result of a number of anthropogenic activities. Plants being sessile

organisms, cannot change their habitat in response to such conditions. Therefore, they adapt to such

an environment by stimulating intracellular mechanisms that ensure the normal survival of plants.

These mechanisms may include Pb accumulation in plant parts without any visible symptoms,

sequestration of Pb into vacuoles, exudation from roots into soils in the insoluble precipitated form

that cannot be reabsorbed. All such mechanisms are referred to as tolerance and detoxification

mechanisms (Figure 11.2). Usually, entry of Pb stimulate either or all the following detoxification

processes: