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Manganese Toxicity

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Damage on whole plant (Dobermann & Fairhurst).

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Brown spots on leaves.

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Damage on leaf.

Diagnostic summary

  • affects metabolic processes such as enzyme activities and organic compounds
  • sterility
  • symptoms similar to Fe chlorosis
  • Mn toxicity often occurs with Al toxicity

  • yellowish brown spots between leaf veins, extending to the whole interveinal area
  • brown spots on veins of lower leaf blades and leaf sheaths
  • drying of leaf tips eight weeks after planting
  • chlorosis of younger leaves
  • stunting
  • reduced tillering
  • reduced grain yield

  • important throughout the growth cycle
  • relatively rare especially in irrigated rice systems
  • occurs in acid upland soils, lowland soils, acid sulfate soils, and areas affected by Mn mining

 

Full fact sheet

  • Yellowish brown spots between leaf veins, extending to the whole interveinal area
  • Brown spots on veins of lower leaf blades and leaf sheaths
  • Leaf tips dry out eight weeks after planting
  • Chlorosis of younger (upper) leaves, with symptoms similar to those of Fe chlorosis
  • Stunted plants
  • Reduced tillering
  • Sterility results in reduced grain yield

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Yellowish-brown interveinal spots on leaf (Dobermann & Fairhurst).


Mn toxicity can be determined by testing the plant and soil. For plant, the optimal ranges and critical levels for occurrence of Mn toxicity are:

 

Growth stage Plant part Optimum (mg kg-1) Critical level for toxicity (mg kg-1)
Tillering

Y leaf, shoot

40-700

>800-2,500

Tillering

Shoot

50-150

-

Maturity

Straw

0.10-0.15

<0.06

 

 

The chlorosis of younger or upper leaves is similar to those of Fe chlorosis.

Mn toxicity is relatively rare especially in irrigated rice systems.

Mn toxicity can be caused by one or more of the following:

  • Large concentration of Mn2+ in the soil solution because of low soil pH (<5.5) and/or low redox potential.
  • Poor and unbalanced crop nutrient status. Low root oxidation and Fe2+ excluding power because of:
    • deficiencies of Si, K, P, Ca, or Mg, and
    • substances that inhibit respiration (e.g., H2S, FeS, organic acids).
  • Application of urban or industrial waste with large Mn content.

Mn toxicity rarely occurs in lowland rice. Despite high Mn concentrations in solution, Mn toxicity is uncommon because rice is comparatively tolerant of large Mn concentrations. Rice roots are able to exclude Mn and rice has a high internal tolerance for high tissue Mn concentrations. Soils where Mn toxicity can occur are as follows:

  • Acid upland soils (pH <5.5); Mn toxicity often occurs with Al toxicity
  • Lowland soils containing large amounts of easily reducible Mn
  • Acid sulfate soils
  • Areas affected by Mn mining (e.g., Japan)

The Mn concentration in soil solution can increase at low soil pH or when redox potential is low due to flooding. Excessive amounts of Mn in solution can lead to excess Mn uptake in cases where exclusion or tolerance mechanisms in roots are not functioning adequately. A large concentration of Mn in plant tissue changes metabolic processes (e.g., enzyme activities and organic compounds) that lead to visible Mn toxicity symptoms such as chlorosis (photo-oxidation of chlorophyll) or necrosis (accumulation of oxidized phenolic compounds, e.g., anthocyanin).

Varieties differ in their susceptibility to Mn toxicity. The major adaptive mechanisms by which rice plants overcome Mn toxicity are as follows:

  • Mn stress avoidance: Release of O2 from roots (root oxidation power) to oxidize Mn2+ in the rhizosphere. Differences in root anatomy and morphology, and the supply of K, Si, P, Ca, and Mg as well as toxic substances (H2S), affect root oxidation power.
  • Mn stress tolerance: Retention of Mn in root tissue (oxidation and accumulation of Mn2+ in cell walls). Concentration of excess Mn in metabolically inactive forms.

The damage is important throughout the growth cycle.

Mn toxicity is not very common in rice.

There are general measures to prevent Mn toxicity. These are as follows:

  • Seed treatment: In a temperate climate, coat seeds with oxidants (e.g., Ca peroxide) to improve germination and seedling emergence by increasing the supply of O2.
  • Water management: Mn absorption may be accelerated under conditions of surface drainage.
  • Fertilizer management: Balance the use of fertilizers (NPK or NPK + lime) to avoid nutrient stress as a source of Mn toxicity. Apply sufficient K fertilizer. Apply lime on acid soils to reduce the concentration of active Mn. Do not apply excessive amounts of organic matter (manure, straw) on soils containing large concentrations of Mn and organic matter, and on poorly drained soils. Use less-acidifying ammonium fertilizers (e.g., urea as N source). Mn uptake is reduced in the presence of ammonia, unlike with nitrate as an N source.
  • Straw management: Recycle straw or ash to replenish Si and K removed from the field. An adequate Si supply prevents Mn toxicity of rice plants by decreasing plant Mn uptake (increased root oxidation) and by increasing the internal tolerance for an excessive amount of Mn in plant tissue.

The following are options for treating Mn toxicity:

  • Apply lime to alleviate soil acidity on upland soils.
  • Apply silica slags (1.5 to 3 t ha-1) to alleviate Si deficiency

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.