Metal species, translocation

OSC essentially occur on smectitic clays (e.g., montmorillonite, bedeillite, nontronite...) and, therefore, these unstable metal species are quantitatively more abundant in clay-containing soils. The fraction of heavy metals taken up by this mechanism in soils can be greater than ten percent in contaminated acid soils (Roberts et al. 2002), but in the vast majority of soils it amounts to less than a few percent. This highly mobile pool can be easily leached or translocated to another form by increasing the pH or amending the soil with sorbent minerals (Vangronsveld et al. 1995 Mench et al. 2002). 

Ionophores constitute a large collection of structurally diverse substances that share the ability to complex cations and to assist in the translocation of cations through a lipophilic interface.1 Using numerous Lewis-basic heteroatoms, an ionophore organizes itself around a cationic species such as an inorganic metal ion. This arrangement maximizes favorable ion-dipole interactions, while simultaneously exposing a relatively hydrophobic (lipophilic) exterior. 

The mobility of metals in soil solutions is controlled by several processes (1) desorption or dissolution (rate depends on the solubility of metal-mineral form) (2) diffusion (depends on speciation of metal, soil oxidation/reduction potential, and pH) (3) sorption or precipitation (depends on soil solution concentration and rhi-zosphere effects) and (4) translocation in the plants (depends on plant species, soil solution concentration, and competing ions) (McBride... 

Much of the work on the identification of carboxylic add complexation has been pioneered by Tiffen. He was the first to positively identify an iron-citrate complex in plant xylem.22 Iron-dtrate complexes have since been identified in a number of plants. The complex formed in these plant fluids is anionic, and Tiffen has shown that a number of other metal ions (Cr, Cu, Ni, Mn and Zn) are also present as anionic complexes.23 Although the neutrality of complexes may be considered a prerequisite for metal ion penetration of membranes, this has not been demonstrated with plant roots or with leaf-cell membranes. Involvement of negatively charged dtrate complexes of Ni11 has been confirmed both for nickel uptake and for translocation in plant species.24 Trisoxalatochro-mate(III) anion has been found in the leaf tissues of a plant species.25... 

Lead is considered to be a non-essential metal to plants, and only a small proportion of the lead in soils is biovailable to plants (Alloway, 1990). Visible symptoms of toxicity, though unspecific to Pb, are smaller leaves and a stunted growth. Leaves may become chlorotic and reddish with necrosis and the roots may turn black. Several plant species, ecotypes and bacterial strains have been known to develop Pb tolerance. The phytotoxicity of Pb is low as it has very limited availability and uptake from soil and soil solutions. However, plant roots are usually able to take up and accumulate large quantities of Pb2+ in soil and culture solutions but translocation to aerial shoots is generally limited due to binding at root surfaces and cell walls (Lagerwerff, 1971 Jones et al., 1973 Lane and Martin, 1977). 

Fig. 12. The distribution diagram of the species present at the equilibrium in a solution containing equimolar amounts of Ni(II) and of the fluorescent heteroditopic ligand 7 (% concentration in the left vertical axis). Relevant species to the translocation process are [Ni(II)(LH2)]2+, 80% at pH = 7.0, in which the Ni(II) center stays in compartment B, and [Ni(II)(L)], 100% at pH > 8.5, in which Ni(II) has moved to occupy the doubly deprotonated A2- compartment. Open triangles indicate the fluorescence intensity of the anthracene fragment covalently linked to the framework of 7. When the Ni(II) center stays in compartment B, the anthracene subunit is not perturbed and discloses its full fluorescent emission. When the metal moves to compartment A2-, an electron transfer process takes places from Ni(II) to the excited fluorophore which induces complete fluorescence quenching...

Studies have found many natural plant hyperaccumulators tend to have a higher density of metal transporters at the root-cell plasma membrane (Pence et al., 2000). The higher density of metal transporters allows these plants to readily take up metal cations from the soil solution. Once metals are accumulated, hyperaccumulating plant species usually exhibit a rapid translocation of accumulated metals from roots to shoots (Kramer, 2000). Translocated metals are then stored in vacuoles of the epidermal or mesophyllic cells of the stem to decrease toxicity to the plant (Mathys, 1977). 

Although the mechanism is uncertain, like most monocots, root to shoot metal translocation does not readily occur in Festuca species. The lack of a translocation mechanism may be due to low concentrations of histidine in the xylem (Kramer et al., 1996), but this inhibition in Festuca requires further investigation. In many cases, Festuca species have also developed a symbiotic relationship with endophytic fungi, which appears to aid in reducing insect and some herbivore grazing. The low transport of metals into shoot tissues and symbiosis with endophytic fungi further reduce the possibility of metal introduction into the food chain, and increase the attractiveness of Festuca as a phytostabilizer. 

It has been known for some time that tolerance towards high levels of both essential and toxic metals in a local soil environment is exhibited by species and clones of plants that colonize such sites. Tolerance is generally achieved by a combination of exclusion and poor uptake and translocation. Some species can accumulate large quantities of metals in their leaves and shoots at potentially toxic levels, but without any harmful effects. These metal-tolerant species have been used in attempts to reclaim and recolonize metal-contaminated wastelands. More recently such species have attracted the attention of inorganic chemists. There is abundant evidence that the high metal levels are associated with carboxylic acids, particularly with nickel-tolerant species such as Allysum bertolonii. The main carboxylic acids implicated are citric, mahc and malonic acids (see refs. 30 and 31 and literature cited therein). Complexation of zinc by malic and oxalic acids has been reported in the zinc-tolerant Agrostis tenuis and oxalic acid complexation of chromium in the chromium-accumulator species Leptospermum scoparium ... 

The mechanisms by which the chelates enter plants are not well understood. The chelating substances keep the metals soluble in the soil and are usually taken up by plants with the metals (Holmes and Brown, 1955). In some cases both the metal and chelate are absorbed in corresponding amounts but often the two components are separated in the roots, and the metal enters without the equivalent amount of the carrier. There is also evidence that some of the benefits derived from chelating agents are the result of increased translocation of the metal from the roots to the shoots. Many factors, such as pH, plant species, and metal involved affect the process. In fact, some plants do not respond to additions of chelate metals and the reasons are still in doubt. 

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