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Iron Toxicity

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Tiny brown spots on leaves (IRRI).

Diagnostic summary

  • increased polyphenol oxidase activity, leading to the production of oxidized polyphenols
  • caused leaf bronzing
  • reduced root oxidation power

  • lower leaves with tiny brown spots from tip and spread toward the base or whole leaf is orange-yellow to brown
  • spots combine on leaf interveins and leaves turn orange-brown and die
  • leaves narrow but often remain green
  • leaf tips become orange-yellow and dry up in some varieties
  • leaves appear purple-brown if Fe toxicity is severe
  • stunted growth, extremely limited tillering
  • dark brown to black coating on the root surface and many dead roots
  • fresh uprooted rice hills have many black roots

  • occur on a wide range of soils, but generally in lowland rice soils with permanent flooding during crop growth
  • occur on soils that remain waterlogged
  • can affect the rice crop throughout its growth cycle

 

Full fact sheet

  • Tiny brown spots on lower leaves starting from tip and spread toward the leaf base or whole leaf colored orange-yellow to brown
  • Spots combine on leaf interveins and leaves turn orange-brown and die
  • Leaves narrow but often remain green
  • In some varieties, leaf tips become orange-yellow and dry up
  • Leaves appear purple-brown if Fe toxicity is severe
  • Stunted growth, extremely limited tillering.
  • Coarse, sparse, damaged root system with a dark brown to black coating on the root surface and many dead roots
  • Freshly uprooted rice hills often have poor root systems with many black roots

 

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Leaf browning (IRRI).

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Bronzing (IRRI).

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)
Tillering-PI Y leaf 100-150 >300-500

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:

  • Large Fe2+ concentration in soil solution because of strongly reducing conditions in the soil and/or low pH.
  • Low and unbalanced crop nutrient status. Poor root oxidation and Fe2+ exclusion power because of P, Ca, Mg, or K deficiency. K deficiency is often associated with low soil base content and low soil pH, which result in a large concentration of Fe in the soil solution.
  • Poor root oxidation (Fe2+ exclusion) power because of the accumulation of substances that inhibit respiration (e.g., H2S, FeS, organic acids,).
  • Application of large amounts of undecomposed organic matter.
  • Continuous supply of Fe into soil from groundwater or lateral seepage from hills.
  • Application of urban or industrial sewage with a high Fe content

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:

  • Poorly drained soils (Aquents, Aquepts, Aquults) in inland valleys receiving inflow from acid upland soils (Philippines, Sri Lanka)
  • Kaolinitic soils with low CEC and small amounts of available P and K (Madagascar)
  • Alluvial or colluvial acid clayey soils (Indonesia, Philippines)
  • Young acid sulfate soils (Sulfaquepts in Senegal, Thailand)
  • Acid lowland or highland peat (swamp) soils (Burundi, Liberia, Madagascar)

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 stress avoidance because of Fe2+ oxidation in the rhizosphere. The precipitation of Fe3+ hydroxide in the rhizosphere by healthy roots (indicated by reddish brown coatings on the roots) prevents excessive Fe2+ uptake. In strongly reduced soils containing very large amounts of Fe, however, there may be insufficient oxygen at the root surface to oxidize Fe2+. In such cases, Fe uptake is excessive and roots appear black because of the presence of Fe sulfide. Root oxidation power includes the excretion of O2 (transported from the shoot to the root through aerenchyma) from roots and oxidation mediated by enzymes such as peroxidase or catalase. An inadequate supply of nutrients (K, Si, P, Ca, and Mg) and excessive amounts of toxic substances (H2S) reduce root oxidation power. Rice varieties differ in their ability to release O2 from roots to oxidize Fe2+ in the rhizosphere and protect the plant from Fe toxicity.
  • Fe stress tolerance may be due to the avoidance or tolerance of toxin accumulation. Another mechanism involves the retention of Fe in root tissue (oxidation of Fe2+ and precipitation as Fe3+).
  • Fe toxicity is related to multiple nutritional stress, which leads to reduced root oxidation power. The root of plants deficient in K, P, Ca, and/or Mg exude more low molecular weight metabolites (soluble sugars, amides, amino acids) than plants with an adequate nutrient supply. In periods of intense metabolic activity (e.g., tillering), this results in an increased rhizoflora population, which in turn leads to increased demand for electron acceptors. Under such conditions, facultative and obligate anaerobic bacteria reduce Fe3+ to Fe2+. The continuous reduction of Fe3+ contained in Fe2O3 root coatings may result in a breakdown in Fe oxidation, leading to an uncontrolled influx of Fe2+ into the rice plant roots. A black stain of Fe sulfide (a diagnostic indication of excessively reduced conditions and Fe toxicity) may then form on the root surface.

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:

  • Varieties: Plant rice varieties tolerant of Fe toxicity (e.g., IR8192-200, IR9764-45, Kuatik Putih, Mahsuri). If nutrients are supplied in sufficient amounts, hybrid rice varieties have a more vigorous root system and higher root oxidation power, and do not tend to absorb excessive amounts of Fe from Fe-toxic soils.
  • Seed treatment: In temperate climates where direct seeding is practiced, coat seeds with oxidants (e.g., Ca peroxide at 50-100% of seed weight) to improve germination and seedling emergence by increasing the O2 supply.
  • Crop management: Delay planting until the peak in Fe2+ concentration has passed (i.e., not less than 10-20 d after flooding).
  • Water management: Use intermittent irrigation and avoid continuous flooding on poorly drained soils containing a large concentration of Fe and organic matter.
  • Fertilizer management: Balance the use of fertilizers (NPK or NPK + lime) to avoid nutrient stress. Apply sufficient K fertilizer. Apply lime on acid soils. Do not apply excessive amounts of organic matter (manure, straw) on soils containing large amounts of Fe and organic matter and where drainage is poor. Use urea (less acidifying) instead of ammonium sulfate (more acidifying).
  • Soil management: Carry out dry tillage after the rice harvest to enhance Fe oxidation during the fallow period. This reduces Fe2+ accumulation during the subsequent flooding period, but will require machinery (tractor).

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:

  • Applying additional K, P, and Mg fertilizers.
  • Incorporating lime in the topsoil to raise pH in acid soils.
  • Incorporating about 100-200 kg MnO2 ha-1 in the topsoil to decrease Fe3+ reduction.
  • Carrying out midseason drainage to remove accumulated Fe2+. At the midtillering stage (25-30 d after planting/sowing), drain the field and keep it free of floodwater (but moist) for about 7-10 d to improve oxygen supply during tillering.

Source:

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.