Manganese (Mn) toxicity in plants is often not a clearly identifiable disorder. Symptoms of Mn toxicity as well as the concentration of Mn that causes toxicity vary widely among plant species and varieties within species, perhaps because the phytotoxic mechanisms of Mn involve different biochemical pathways in different plant genotypes. Plant availability of Mn depends on soil adsorption and on root exudates for Mn chelation or reduction. Soils with higher Mn sorption capacity have lower potential for plant absorption of Mn. Great Mn tolerance is associated with restricted absorption, restricted translocation of excess Mn to the shoots, or great tolerance to high Mn levels within the plant tissue. The ability to escape Mn toxicity through limited absorption or translocation is low in plants grown in wet soils rich in organic matter, or grown under high temperature or high light intensity. Manganese toxicity is likely with plants that are fertilized with acid-forming fertilizers, high rates of superphosphate, or nitrate (NO3-) as source of nitrogen (N), or plants that are low in silicon (Si) or deficient in calcium (Ca), iron (Fe), magnesium (Mg), or phosphorus (P). Comparative studies of Mn toxicity among different genotypes of the same species and a multidisciplinary approach are needed for a full understanding of Mn toxicity disorder in plants.
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