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b Differences between control and ispolin-fertilized samples are significant at P < 0.05.

b Differences between control and ispolin-fertilized samples are significant at P < 0.05.

SO42- increased Cd uptake by plants; this may be explained as a result of Cl- and SO42- complexing with Cd2+ and thereby increasing the bioavailability of Cd2+ in soil. Similar results were reported by Norvell et al. [30], who showed that accumulation of Cd in wheat may be enhanced by presence of Cl- ions in soil. Although the mechanism of the ion interactions was not clear, the authors suggested that it is likely to involve increased solubility or availability of soil Cd resulting from the formation of chlorocomplexes in soil solution.

Potassium also has an effect on plant uptake of Cd. In particular, Cd concentrations in shoots increased two times with increasing K addition [24]. Lorenz et al. [107] reported that an excess addition of such cations as K+ and NH4+ to soil caused a substantial increase of major and heavy metal ions in soil solution as well as their uptake by plants. This may be explained by the fact that NH4+ ions have an acidifying effect in soil, due to the nitrification processes or the release of H+ ions to soil solution during plant uptake of NH4+ ions.

An application of amendments such as lime, apatite, and zeolite will increase Ca content in soil and also increase soil pH [49]. Because the increase of pH may lead to decrease of metal mobility in soil, these Ca fertilizers may be used for metal stabilization in contaminated soils. It is also known that many enzymatic and physiological processes in plant cells are under the regulatory control of Ca2+ [108]. In particular, externally supplied Ca2+ can affect the toxicity of some elements, perhaps by competing with metal uptake [104].

Kim et al. [36] studied the effects of the presence of K+, Ca2+, or Mg2+ in soil on transport and toxicity of Pb2+ and Cd2+ in rice. In contrast to the data cited previously, in this experiment K+ had little effect on uptake or toxicity of Pb2+ and Cd2+. Ca2+ and Mg2+ blocked Cd2+ transport into rice roots and Cd2+ toxicity for the root growth, which suggested that detoxification effect of these ions is directly related to their blocking the entry of the metal. Similarly, Ca2+ blocked Pb2+ transport into the roots and Pb2+ toxicity for the root growth. The authors suggested that the protective effect of Ca2+ on Pb2+ toxicity may be related to inhibition of the metal accumulation in the root tip, a potential target site of Pb2+ toxicity. Mg2+ did not ameliorate the Pb2+ toxicity for the root growth as much as Ca2+ did, though it decreased Pb2+ uptake by roots in the same manner as Ca2+. These results suggest that the protective effect of Ca2+ on Pb2+ toxicity may involve multiple mechanisms,

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