Multiple nutritional soil stress as the prerequisite for iron toxicity of wetland rice (Oryza sativa L.)
Keywords:Oryza sativa L., Soils, Iron, Toxicity
AbstractSoil samples of different iron-toxic soils were collected from various sites in Africa and southeast Asia and compared over a wide range of physico-chemical properties. Rice leaves, clearly showing bronzing, were investigated for their nutrient content. The general characteristics shared by the iron-toxic soils examined were a relatively low CEC (cation exchange capacity), an extremely small amount of exchangeable K and Ca (in most cases) as well as a deficiency in available P. The pH (H2O) of soils varied greatly (pH 3 - 7.2) as did the active Fe (Feo) content, suggesting that neither the acidity nor the amount of active Fe should be regarded as an essential prerequisite for iron toxicity. The soil stresses were clearly reflected by the mineral composition of the leaves. Except for an excessive content of Fe (290 - 1000 ppm) and Mn (often > 1000 ppm), all iron-toxic leaves revealed low or deficient amounts of K and P and often of Zn and sometimes of Ca and Mg. Iron toxicity is ascribed to a multiple nutritional soil stress (insufficient supply of K, P, Ca and Mg) rather than to a high level of active Fe under acid conditions. A mechanism of iron toxicity is proposed in which an adequate supply of P, K and Ca is regarded as the essential prerequisites in maintaining an effective protective oxidized Fe root-coating and, thereby, an Fe-excluding mechanism.