Conclusions

It is generally accepted that the rhizosphere plays an important role in the bioavailability of trace elements. A range of mechanisms exists, such as excretion of H+ or HCO3-; respiration and release of CO2; exudation; desorption and adsorption of trace elements; dissolution-precipitation; redox reaction; and chelation, by which plant roots can modify the chemical conditions in the rhizosphere. These factors can affect dramatically the behavior of root in the rhizosphere and substantially the bioavailability of trace elements. The mechanisms involved in chemical modifications in the rhizo-sphere as well as on uptake of trace elements differ among plant species and soil conditions.

The extent to which siderophores alter the ecology of the rhizosphere is a question that remains to be investigated. Scientists need to have a better understanding of how siderophores can function in the rhizosphere and under different soil conditions rather than in laboratory experiments. Research efforts must be focused on how these compounds influence heavy metal transport and bioavailability in soils. In the future, we hope these compounds may have application and use siderophores as iron fertilizers. The ability to manipulate siderophore production in the rhizosphere to improve plant trace metal nutrition will remain a significant challenge.

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