Trace uptake into plants

Trace elements on the surfaces of fly ash particles that are accessible to humans through air, soil, water, can affect health in several ways. The pathways by which metals from CCP may cause harm include (1) soil deposition and resulting plant uptake of metals and subsequent movement into the food chain (2) direct ingestion of soil by animals or humans (3) leaching of metals from CCP to water systems and uptake by plants, animals, or humans and (4) inhalation of dust (from soil) or respirable ash particles (Ryan Bryndzia 1997). 

Roots modify their environment quite extensively in many ways. The most important of these are pH change, exudation and microbiological activity in the rhizosphere. Root exudates contain compounds such as hydroxycarboxylic acids and amino acids and these are capable of complexing trace metals. Bowling (1976), Farago (1986) and Streit and Stumm (1993) have discussed the theories of mineral uptake by plant roots the first suggests that there are four links in the uptake chain movement of ions or complexes in the soil to the roots uptake into the root transport across the root to the vascular system and movement to the shoot. 

The addition of even moderate dressings of municipal compost to rural soils (say, 25 tonnes per ha) can therefore be expected to have a pronounced effect on their trace-element content. What evidence we have suggests that, with the exception of boron, once these elements are introduced into the soil, they are not readily leached out again. Since contamination of the soil with trace elements will lead to increased uptakes by plant roots, with the possibility of ill-effects on the plants or on the animals eating them, it is important to consider the consequences of the agricultural use of this material. 

Most trace metals may be precipitated with phosphate into insoluble metal phosphates (Table 7.5). Most metal phosphates have low solubility. High localization of phosphates reduces the bioavailability of Zn in arid soils. The banded application of P near the seeds depresses Zn uptake by com (Adriano and Murphy, 1970 Grant and Bailey, 1993), causing Zn deficiency. However, both N and P fertilizers increase Cd concentration in plants. Cadmium and Zn are antagonistic in root uptake and distribution within plants. 

An important feature of the biogeochemistry of trace elements in the rhizosphere is the interaction between plant root surfaces and the ions in the soil solution. These ions may accumulate in the aqueous phases of cell surfaces external to the plasma membranes (PMs). In addition, ions may bind to cell wall (CW) components or to the PM surface with variable strength. In this chapter, we shall describe the distribution of ions among the extracellular phases using electrostatic models (i.e. Gouy-Chapman-Stem and Donnan-plus-binding models) for which parameters are now available. Many plant responses to ions correlate well with computed PM-surface activities, but only poorly with activities in the soil solution. These responses include ion uptake, ion-induced intoxication, and the alleviation of intoxication by other ions. We illustrate our technique for the quantitative resolution of multiple ion effects by inserting cell-surface activities into nonlinear equations. 

The first ever application of a radiotracer in a biological experiment dates back to 1923 when George de Hevesy used Pb to study plant uptake of lead from solution [5]. His seminal work was honored by the Nobel Prize in Chemistry in 1943 and made him the father of isotope tracing, a tool that is still indispensable in virtually any area of scientific research. The first use of a stable isotope to study mineral metabolism was reported in 1963, when Lowman and Krivit injected stable Fe together with radioactive Fe into a human subject to compare the plasma clearance of the two isotopes [6]. However, it was not until the 1980s that stable isotope techniques were explored systematically to study mineral and trace element metabolism in humans. This was not only due to the increasing recognition of health hazards associated with the use of radioisotopes. Mass spectro-metric techniques had to be refined to measure isotope ratios of the heavier elements at a precision suitable for the exploitation of isotopically enriched elements as tracers. Stable isotopic labels are made up from the same isotopes as the natural element, from which they differ only in terms of composition, that is, in the relative abundances of their isotopes. 

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