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Rusty reed roots reveal a complex response to iron
There is increasing interest in the use of biological systems to remediate contaminated land. This relies on the ability of particular organisms either to metabolize or to take up and immobilize the contaminant in question. ‘Low-tech’ but nevertheless effective versions of this are constructed wetlands and reed-beds used to treat land contaminated by heavy metals from mines and other industrial operations. However, even the most tolerant of plants eventually reach their limits. This is well illustrated in the paper by Batty and Younger (University of Newcastle, UK, pp. 801-806) on the iron tolerance of Phragmites australis (common reed). The authors grew P. australis in culture solution augmented with iron (as FeSO4.7H2O) at various concentrations. Concentrations of Fe above 1.0 mg L-1 increasingly inhibited plant growth whereas no ill-effects were observed at lower concentrations. Indeed, Fe at 0.5 mg L-1 or less actually stimulated root elongation growth. The question then arises as to whether the toxicity of higher concentrations of iron is direct, perhaps by interacting with membranes or enzymes, or indirect, for example by inhibition of uptake of other nutrients. The authors observed that at concentrations of 1.0 mg L-1 and above, a red plaque of iron oxyhydroxide formed on the roots and they considered it possible that this impeded uptake of phosphate. However, this idea was not born out by their data. Increasing iron concentrations led to a slight increase in the phosphate content of rhizomes and shoots but a very marked increase, especially above 2.0 mg L-1 Fe, in the phosphate content of roots. Much of this increase was not mobilized to rhizomes and shoots and the authors suggest that the excess phosphate was complexed with or trapped in the iron oxyhydroxide plaque. Nevertheless, even at these high Fe concentrations the phosphate content of all organs was adequate for growth. The search for understanding of this complex situation continues.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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