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Both the plant and soil can be tested for Fe toxicity.
The optimal ranges and critical levels for occurrence of Fe toxicity in plants are:
|Growth stage||Plant part||Optimum (mg kg-1)||Critical level for toxicity (mg kg-1)|
Fe content in affected plants is usually (but not always) high (300-2,000 mg Fe kg-1), but the critical Fe content depends on plant age and general nutritional status. The critical threshold is lower in poor soils where nutrition is not properly balanced.
Fe-toxic plants have low K content in leaves (often <1% K). A K:Fe ratio of <17-18:1 in straw and <1.5:1 in roots may indicate Fe toxicity.
The critical concentration for the occurrence of Fe toxicity is >300 mg Fe L-1soil. Critical Fe solution concentrations for the occurrence of Fe toxicity vary widely. Reported values range from 10 to 1,000 mg Fe L-1, which implies that Fe toxicity is not related to the Fe concentration in soil solution alone. The difference between critical solution Fe concentrations is caused by differences in the potential of rice roots to resist the effects of Fe toxicity, depending on crop growth stage, physiological status of the plant, and variety grown (root oxidation power).
No critical levels for soil test results have been established, but soils with pH <5.0 (in H2O) are prone to Fe toxicity. Similarly, soils containing small amounts of available K, P, Ca, and Mg contents are prone to Fe toxicity.
Only Fe-toxic plants exhibit these symptoms.
Fe toxicity tends to occur on soils that remain waterlogged. The principal causes of Fe toxicity are as follows:
Fe toxicity occurs on a wide range of soils, but generally in lowland rice soils with permanent flooding during crop growth. The common features of Fe-toxic sites are poor drainage and low soil CEC and macronutrient content, whereas Fe toxicity occurs over a wide range of soil pH (4 to 7)
Soils, which are prone to Fe toxicity, include the following types:
Iron toxicity is primarily caused by the toxic effect of excessive Fe uptake due to high solution Fe concentrations. Recently transplanted rice seedlings may be affected when large amounts of Fe2+ accumulate immediately after flooding. In later growth stages, excessive Fe2+ uptake due to increased root permeability and enhanced microbial Fe reduction in the rhizosphere affects rice plants. Excessive Fe uptake results in increased polyphenol oxidase activity, leading to the production of oxidized polyphenols, the cause of leaf bronzing. Large amounts of Fe in plants can give rise to the formation of oxygen radicals, which are highly phytotoxic and responsible for protein degradation and peroxidation of membrane lipids.
Varieties differ in susceptibility to Fe toxicity. The major adaptive mechanisms by which rice plants overcome Fe toxicity are as follows:
Fe toxicity can affect the rice crop throughout its growth cycle.
Fe toxicity occurs on a wide range of soils, but generally in lowland rice soils with permanent flooding during crop growth.
The following are general measures to prevent Fe toxicity:
Preventive management strategies (see above) should be followed because treatment of Fe toxicity during crop growth is difficult. The following are options for treating Fe toxicity:
Dobermann A, Fairhurst T. 2000. Rice. Nutrient disorders & nutrient management. Handbook series. Potash & Phosphate Institute (PPI), Potash & Phosphate Institute of Canada (PPIC) and International Rice Research Institute. 191 p.