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Fig. 1.7.1. Influence of ion availability in the rhizosphere on yield. (After Wallnofer and Engelhardt 1984)

Negative effect

Fig. 1.7.1. Influence of ion availability in the rhizosphere on yield. (After Wallnofer and Engelhardt 1984)

Some heavy metals are micronutrients (Fe, Mn, Zn, Cu, Ni, Mo, Co), others are potent cell toxins: Hg, Pb, Cd, Cr and As. However, also micronutrients may become toxic if they accumulate in the organelles of plant cells to higher concentrations. For heavy metals the concentration ranges of deficiency, optimal supply and toxicity are very close together (Fig. 1.7.1 and Table 1.7.1).

While the thresholds of symptoms of heavy metal deficiency are more or less independent of the plant species and within a relatively narrow concentration range, toxicity of heavy metals shows a broad concentration range (Table 1.7.1): Many plants are able to diminish uptake, detoxify or sequester (the so-called hyperaccumulators) heavy metal ions and thus avoid metabolic problems. Certain plants growing on soils with a surplus of heavy metals are called metallophytes. For example, Silene vulgaris is able to tolerate cadmium, cobalt, copper, lead, manganese, nickel and zinc in the substrate, Festuca ovina, Agrostis tenuis and Minuartia verna have ecotypes with similar metal resistance as Silene vulgaris, Viola calaminaria grows on soils rich in zinc and the fern Pteris vittata accumulates arsenic.

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