Contamination of soils by heavy metals is of widespread occurrence as a result of human, agricultural and industrial activities. Among heavy metals, lead is a potential pollutant that readily accumulates in soils and sediments. Although lead is not an essential element for plants, it gets easily absorbed and accumulated in different plant parts. Uptake of Pb in plants is regulated by pH, particle size and cation exchange capacity of the soils as well as by root exudation and other physico-chemical parameters. Excess Pb causes a number of toxicity symptoms in plants e.g. stunted growth, chlorosis and blackening of root system. Pb inhibits photosynthesis, upsets mineral nutrition and water balance, changes hormonal status and affects membrane structure and permeability. This review addresses various morphological, physiological and biochemical effects of Pb toxicity and also strategies adopted by plants for Pb-detoxification and developing tolerance to Pb. Mechanisms of Pb-detoxification include sequestration of Pb in the vacuole, phytochelatin synthesis and binding to glutathione and aminoacids etc. Pb tolerance is associated with the capacity of plants to restrict Pb to the cell walls, synthesis of osmolytes and activation of antioxidant defense system. Remediation of soils contaminated with Pb using phytoremediation and rhizofiltration technologies appear to have great potential for cleaning of Pb-contaminated soils.

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Lead toxicity in plants

Plants are the target of a wide range of pollutants that vary in concentration, speciation, and toxicity. Such pollutants mainly enter the plant system through the soil (Arshad et al. 2008) or via the atmosphere (Uzu et al. 2010). Among common pollutants that affect plants, lead is among the most toxic and frequently encountered (Cecchi et al. 2008; Grover et al. 2010; Shahid et al. 2011). Lead continues to be used widely in many industrial processes and occurs as a contaminant in all environmental compartments (soils, water, the atmosphere, and living organisms). The prominence of environmental lead contamination results both from its persistence (Islam et al. 2008; Andra et al. 2009; Punamiya et al. 2010) and from its present and past numerous sources. These sources have included smelting, combustion of leaded gasoline, or applications of lead-contaminated media (sewage sludge and fertilizers) to land (Piotrowska et al. 2009; Gupta et al. 2009; Sammut et al. 2010; Grover et al. 2010). In 2009, production of recoverable lead from mining operations was 1690, 516, and 400 thousand metric tons by China, Australia, and the USA, respectively (USGS 2009).

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Lead Uptake, Toxicity, and Detoxification in Plants

Using the examples of cadmium and lead, the review considers the various toxic effects exerted by these heavy metals. Putative specific and nonspecific mechanisms of the toxic effects of the heavy metals and plant responses are discussed together with the issue of Cd and Pb accumulation in various plant organelles, cells, tissues, and organs. The basic mechanisms providing for plant resistance to excess Cd and Pb are elucidated. These data are used to schematically outline the changes in plant metabolism produced by these heavy metals.

Physiological Aspects of Cadmium and Lead Toxic Effects on Higher Plants

A critical review on speciation, mobilization and toxicity of lead in soil-microbe-plant system and bioremediation strategies.

The effect of lead on seed imbibition and germination in different plant species

SUMMARY
A histochemical method was used to investigate the distribution of applied lead in the seed and seedling stage of Raphanus sativus. Lead contamination of the embryo was inhibited by the testa until it ruptured during imbibition. In seedlings, lead moved into the root cortex predominantly along the cell walls as far as the endodermis. Beyond the endodermis lead was localized extensively in the vascular tissues. There was little movement of lead from the vascular tissue into the surrounding tissue of the stem and cotyledons.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1977.tb02206.x

E. S. Martin

Papers

A histochemical investigation of lead uptake in raphanus sativus