Metal Distribution Between Roots And Upper Plant Parts

Uptake of metals by root cells, the point of entry into living tissues, is a step of major importance for the process of metal phytoextraction. However, for phytoextraction to occur, metals must also be transported from the roots to the upper plant parts. It is quite possible that a plant species exhibiting a significant metal accumulation into the roots may have a limited capacity for phytoex-traction. For example, many publications have reported that concentrations of trace metals in roots may be several times higher than in shoots [61,62]. Kim et al. [36] found that Pb content in roots may increase with increased concentrations of Pb2+ applied to soil, but Pb content in shoots will remain unchanged. In addition, the mechanisms of metal translocation from root to shoot may be different for different metals.

As an example, Table 28.3 shows concentrations of several elements in different parts of three cereal crops (oats, barley, and wheat) grown simultaneously on the same soil. Concentrations of all the elements (except calcium in barley) were higher in roots than in leaves. It is also important to remember that only a part of the total amount of ions associated with the root is absorbed into cells. A significant ion fraction is just physically adsorbed at the extracellular negatively charged sites of the root cell walls [1]. The cell wall-bound fraction cannot be translocated to the shoots and, therefore, cannot be removed by harvesting shoot biomass (phytoextraction).

Binding to the cell wall is not the only plant mechanism responsible for metal immobilization into roots and subsequent inhibition of ion translocation to the shoots. Metals can also be complexed and sequestered in cellular structures (e.g., vacuoles), thus becoming unavailable for translocation to the shoot [63]. In addition, some plants, so-called excluders, possess specialized mechanisms to restrict metal uptake by roots. Movement of metal-containing sap from root to shoot is primarily controlled by two processes: root pressure and leaf transpiration. Following translocation to leaves, part of metals can be reabsorbed from the sap into leaf cells. However, certain amounts of metals may evaporate during transpiration together with water (the water will carry dissolved mineral salts). As has been reported, elemental composition of transpiration solutions correlates well with elemental composition of soil water [64].

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