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

The concentrations in plant parts depend both on intrinsic and extrinsic factors and vary greatly for different species and for different metals. Responses by plant species to exposure to metalliferous soils can range from phytotoxicity to survival by exclusion, with only small elevations of metal concentration to survival with accumulated metal constituting a significant percentage of the plant dry matter. The importance of nonvascular transport through laticifers, for example, is also obscure. The extent to which secondary xylem tissues become a long-term repository for accumulated metals is not known. The high degree of endemism to metal-rich soils shown by hyperaccumulating taxa has been related to their survival. The success of many metallophytes in overcoming the adverse edaphic conditions is such that they can exist relatively free from competition. Metal hyperaccumulators have been recognized on the basis of concentrations of metals in leaf dry matter.

Metal hyperaccumulator plants: a review of the ecology and physiology of a biological resource for phytoremediation of metal-polluted soils

Lead phytoextraction, using plants to extract Pb from contaminated soils, is an emerging technology. Calculations of soil Pb mass balance suggest that this technology will be economically feasible only if systems can be developed to employ high biomass plants that can accumulate greater than 1% Pb in their shoots. In this study, we investigated the potential of adding chelates to Pb-contaminated soils to increase Pb accumulation in plants. The addition of chelates to a Pb-contaminated soil (total soil Pb 2500 mg kg-1) increased shoot Pb concentrations of corn (Zea mays L. cv. Fiesta) and pea (Pisum sativum L. cv. Sparkle) from less than 500 mg kg-1 to more than 10000 mg kg-1. The surge of Pb accumulation in these plants was associated with the surge of Pb level in the soil solution due to the addition of chelates to the soil. For the chelates tested, the order of the effectiveness in increasing Pb desorption from the soil was EDTA > HEDTA > DTPA > EGTA >EDDHA. We also found that EDTA significantly increas...

Phytoremediation of Lead-Contaminated Soils: Role of Synthetic Chelates in Lead Phytoextraction

Heavy metal (HM) toxicity is one of the major abiotic stresses leading to hazardous effects in plants. A common consequence of HM toxicity is the excessive accumulation of reactive oxygen species (ROS) and methylglyoxal (MG), both of which can cause peroxidation of lipids, oxidation of protein, inactivation of enzymes, DNA damage and/or interact with other vital constituents of plant cells. Higher plants have evolved a sophisticated antioxidant defense system and a glyoxalase system to scavenge ROS and MG. In addition, HMs that enter the cell may be sequestered by amino acids, organic acids, glutathione (GSH), or by specific metal-binding ligands. Being a central molecule of both the antioxidant defense system and the glyoxalase system, GSH is involved in both direct and indirect control of ROS and MG and their reaction products in plant cells, thus protecting the plant from HM-induced oxidative damage. Recent plant molecular studies have shown that GSH by itself and its metabolizing enzymes—notably glutathione S-transferase, glutathione peroxidase, dehydroascorbate reductase, glutathione reductase, glyoxalase I and glyoxalase II—act additively and coordinately for efficient protection against ROS- and MG-induced damage in addition to detoxification, complexation, chelation and compartmentation of HMs. The aim of this review is to integrate a recent understanding of physiological and biochemical mechanisms of HM-induced plant stress response and tolerance based on the findings of current plant molecular biology research.

https://downloads.hindawi.com/archive/2012/872875.pdf

Molecular Mechanism of Heavy Metal Toxicity and Tolerance in Plants: Central Role of Glutathione in Detoxification of Reactive Oxygen Species and Methylglyoxal and in Heavy Metal Chelation

summary
Lead transport has been characterized in corn (Zea mays L. cv. Fiesta) and ragweed (Ambrosia artemisiifolia L.), and the Pb phytoextraction efficiency of these species has been compared with that of Tritiaim aestivum, Thlaspi rotundifolium, Thlaspi caerulescens and Brassica juncea, using both nutrient solutions and Pb-contaminated soils. Our results demonstrated that plant species differ significantly in Pb uptake and translocation. In short term (60 min) experiments, Pb uptake by ragweed roots was threefold higher than that by corn roots. After 2 wk of Pb (100/yM) exposure in hydroponics, root-Pb concentration was 24000 mg kg−1 for ragweed and 4900 mg kg−1 for corn. In contrast to root-Pb concentration, shoot-Pb concentration was significantly higher in corn (560 mg kg−1) than in ragweed (30 mg kg-1). At an external Pb concentration of 20/IM, corn concentrated Pb in shoots by 90-fold, and ragweed concentrated Pb in shoots by 20-fold over the solution Pb concentration. Of the 11 species/cultivars tested using both nutrient solutions and Pb-contaminated soils, corn accumulated the highest shoot-Pb concentration. Using this corn cultivar, we investigated the role of synthetic chelates in Pb phytoextraction. Addition of HEDTA (2.0 g kg−1 soil) to a Pb-contaminated soil (total soil Pb 2500 mg kg−1) resulted in a surge of Pb accumulation in corn. The shoot Pb-concentration was increased from 40 mg kg−1 for the control (-HEDTA) to 10600 mg kg−1 for the HEDTA-treated soil. To our knowledge, this is the highest shoot Pb concentration reported in the literature for plants grown on Pb-contaminated soils. Our results suggest that in combination with sou amendment, some agronomic crops, such as corn, might be used for the clean-up of Pb-contaminated soil.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1996.tb01147.x

Lead phytoextraction: species variation in lead uptake and translocation

In laboratory experiments, the growth characteristics of the submerged species Elodea nuttallii (Planch.) St. John and Potamogeton crispus L. were assessed in the presence and absence of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth. Light penetration and the development of pH and dissolved oxygen differences were monitored. The growth of P. crispus was suppressed much more than that of E. nuttallii and the effects of A. filiculoides were more severe than those of L. minuta. Findings are related to possible field responses of submerged plants under floating mats, especially their abilities to compensate for the potential suppressive effects of floating mats under natural conditions.

The effects of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth on the growth of submerged macrophytes.

It has been suggested that submerged aquatic plants can influence the nutritional quality of the periphyton which grows on their surfaces, making it more nutritious for grazing invertebrates, particularly snails. In return, these grazers might preferentially feed on the periphyton and clear the plants of a potential competitor, with the plants and grazers both gaining from this mutualistic relationship. A highly replicated experiment was conducted, in which the nature of the plant (isoetid and elodeid types compared with similar-shaped inert substrata), the nutrient loading, and the influence of periphyton grazers (the bladder snail, Physa fontinalis) of similar size and history were controlled. Plant growth and survival significantly increased in the presence of the periphyton grazer. Whilst the presence of the grazers had the largest influence on periphyton abundance, nutrient availability and plant type also had effects. Plant type had little influence on the nutritional quality of the periphyton measured as carbohydrate, protein and C:N. Effects of treatment on snail growth, and the timing and extent of snail reproduction disappeared when they were compared with the quantity of periphyton available. There was no evidence of enhanced grazer success in the presence of the live plants compared with inert substrata. Although submerged plants affect the growth and reproduction of the grazers which feed on their surfaces, through differences in the amount of periphyton which grows there, we found no evidence that they manipulate the periphyton to encourage such grazers.

K. Hardwick

Papers

The effects of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth on the growth of submerged macrophytes.

Do submerged aquatic plants influence their periphyton to enhance the growth and reproduction of invertebrate mutualists

The influence of periphyton on boundary layer conditions: a pH microelectrode investigation

The effect of changing environmental variables in the surrounding water on the physiology of Elodea nuttallii

Responses of three invasive aquatic macrophytes to nutrient enrichment do not explain their observed field displacements