Lead Acquisition And Transport

22.2.1 Factors Governing Lead Uptake by Plants

Accumulation of Pb by plants depends on several factors, the most important of which are the genetic capability of the plant, as in a hyperaccumulator, and Pb bioavailability. The limiting factor for metal accumulation is the amount of Pb readily available for uptake. The solubility of Pb in soil is limited due to complexation with organic matter, sorption on clays and oxides, and precipitation as carbonates, hydroxides, and phosphates [27]. The application of a synthetic chelator has been used to increase Pb solubility in soil solution and EDTA has been found to be a most effective chelating agent for Pb desorption [7,8,28].

The chemistry and physiology of EDTA-Pb complex is understood better than that of any other Pb-chelate complex. When EDTA is added to Pb-contaminated soil, it complexes with soluble Pb. Due to high affinity of EDTA for Pb, the Pb-EDTA complex formation dominates other metal-EDTA complexation in most soils between pH 5.2 and 7.7 [29]. The soluble soil Pb concentration is not the only factor that influences its uptake in plants. Epstein et al. [30] observed substantially higher Pb accumulations in plants grown in soil containing 4.8 mmol Pb/kg soil and amended with 1 mmol EDTA/kg soil than in plants grown in soil containing 1.5 mmol Pb and 5 mmol EDTA/kg soil. This indicates that a higher ratio of Pb:EDTA in the soil solution will result in enhanced Pb uptake by the plant.

Thus, only a careful consideration of chelate application, taking into account factors like soil type and its total Pb content, can enhance the efficacy of a phytoextraction strategy. EDTA has been shown not only to enhance Pb desorption from the soil components to the soil solution but also to increase its transport into the xylem and its translocation from roots to shoots [7,30-32]. Vassil et al. [32] demonstrated that Pb accumulation in shoots is correlated with the formation of Pb-EDTA in the hydroponic solution and that Pb-EDTA is the major form of Pb taken up and translocated by the plant via xylem stream. The physiological basis of the uptake of Pb-EDTA and, particularly, the possibility of this large molecule to cross the cell membrane are unknown. However, using extended x-ray absorption fine structure spectroscopy, Sarret et al. [16] confirmed the presence of a substantial amount of Pb-EDTA in Phaseolus vulgaris leaves, when this plant was grown in a solution of Pb-EDTA.

22.2.2 Mode of Lead Transport

Electron microscopic techniques have mapped out Pb transport via various plant tissues [31,33-35]. Patterns of Pb migration and deposition in tissues differ depending on whether the supply of Pb is in chelated or unchelated form. In Pinus radiata, it has been shown in ultrathin sections that Pb grains were exclusively distributed in the outermost layer of the root cell wall and in negligible amounts in the needle when Pb was supplied in unchelated form. Conversely, when Pb was supplied in chelated (EDTA or HEDTA) form, higher intensities of Pb fine grains were detected in needle and the least in the root sections [31]. These ultramicroscopic observations are in conformity with the earlier uptake results in Indian mustard [6], where most lead was accumulated in roots on supply of Pb in the unchelated form.

In P. radiata roots, Pb supplied in unchelated form was not found in vacuoles, dictyosomes, or intercellular spaces, but it was often found embedded in or adjacent to cell wall — in many cases, near plasmodesmata [31]. This situation in P. radiata suggests that, in the case of Pb deposition between the cell wall and the cell membrane, it is probable that it was transported apoplastically. However, in the case of its aggregation within plasmodesmata, the mode of transport might have been symplastic. Occurrence of two simultaneous modes of Pb transport has also been advocated in other cases [33,36]. Transmission electron microscopic examination coupled with x-ray microanalysis of Sesbania root showed Pb deposits along the plasma membrane of the cells. The smaller granules appeared to coat the surface of the plasma membrane while larger deposits, looking like globules, extended deeper into the cell wall [9]. A large deposit was observed in high magnification within the tonoplast of the vacuole [9,18]. This is an indication of symplastic mode of transport in Sesbania, though not excluding the possibility of apoplastic migration.

This brings up the question of how Sesbania cells capture Pb particles. The fact that it was not detected in the ground cytoplasm but only in the plasma membrane and vacuole may suggest that endocytosis also plays a role in Pb uptake in this species. Reports of pinocytosis or endocytosis as a device for Pb entrapment are also available in the literature [31,35].

Whether following apoplastic or symplastic routes, Pb must cross the Casparian strip-barrier of the root in order to reach the xylem stream; this is difficult for the large Pb particles due to their size and charge characteristics. However, once they have formed a complex with a chelator such as EDTA, their solubility increases and thus the particle size may also decrease. As a result, the complex may become partially invisible to those processes that would normally prevent unrestricted movement, such as precipitation with phosphates or carbonates or binding to cell walls through mechanisms such as cation exchange.

Thus, the general agreement is that synthetic chelates overcome barriers to translocation. Now, how does Pb enter the vessels or tracheids of the xylem having moved symplastically to the parenchyma cells of the vascular cylinder? The mechanism by which transfer of Pb particles from vascular cylinder parenchyma to vessels or tracheids occurs may be a type of highly selective active-carrier transport, as opposed to facilitated diffusion [37]. Translocation of Pb from root to shoot via xylem has been shown in many studies [6,38,39].

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