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Abstract Plants can mobilize iron (Fe) in the rhizosphere by non‐specific and specific (adaptive) mechanisms. Non‐specific mechanisms are, for example, rhizosphere acidification related to high cation‐anion uptake ratios, or citric acid excretion. The specific mechanisms are root responses to Fe deficiency and can be classified into two different strategies. The Strategy I is typical for dicots and monocots except for grasses (graminaceous species) and is characterized by increased plasma membrane‐bound reductase activity, enhanced net excretion of protons and enhanced release of reducing compounds, mainly phenolics. The reductase activity is stimulated by low pH, and with supply of Fe III chelates, ferric reduction at the plasma membrane takes place prior to uptake. In contrast, in graminaceous species (Strategy II) these root responses are absent, but enhancement of release of Fe III chelating compounds ‐ phytosiderophores ‐ takes place. These phytosiderophores are very efficient in mobilizing Fe III from artificially prepared sparingly soluble inorganic compounds (e.g. Fe III hydroxide) and from calcareous soils. The ferrated phytosiderophores are taken up by grasses at rates 10 2 to 10 3 times higher than Fe supplied either as synthetic chelate or microbial siderophores (e.g. ferrioxamine B), indicating a specific membrane transport system for ferrated phytosiderophores in roots of grasses. In calcareous soils phytosiderophores not only mobilize Fe, but also Zn, Mn, and Cu by chelation. However, only the Fe III phytosiderophores are taken up preferentially by Fe deficient grasses. The ecological advantages and disadvantages of Strategy I and Strategy II for Fe acquisition from calcareous soils are discussed.
Marschner et al. (Sun,) studied this question.