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Trace Elements in Abiotic and Biotic Environments
(Migaszewski et al. 2009). Fairly similar Cr contents of mosses (mean 2.6 mg/kg)
collected in Norway, in 1990–1995 yr, to its contents (mean 1.2 mg/kg) collected in
Wisconsin State (United States), in about similar period, indicated a rather stable Cr
emissions to the atmosphere (Berg and Steinnes 1997; Bennett and Wetmore 2003).
Hexavalent Cr in the atmosphere is reduced to Cr 3+ , mainly by various compounds
of V 2+ , Fe 2+ , and As 2+ . However, oxidation processes are also observed (ATSDR 2002b).
12.5 PLANTS
Until now there are not any clear evidences that Cr is essential in plant metabolism.
Some descriptions of positive effects of Cr on plant growth have not been confirmed,
whereas its phytotoxicity has been often observed and described (Fendorf et al.
2004; Laborga et al. 2007; Sharma et al. 2005; Singh et al. 2007; Stoecker 2004).
Chromium is slightly available to plants, as well as slightly translocated within
plants (Vernay et al. 2007). Contents of Cr in plants are controlled by the mobile Cr
contents of soils, and thus its uptake by plants is highly limited (Zayed and Terry
2003). Hexavalent Cr is easier taken up by plants than trivalent Cr. However, it is
readily reduced and as Cr 3+ is easily bind to cell walls (Zayed et al. 1998). Therefore,
Cr is accumulated mainly in roots. The ratio shoot/roots of Cr contents varies highly,
between 0.005 and 0.0027. The highest Cr content (in both oxidation states) was
found in roots of plants of Brassicaceae family, where it was present as ionic complexes in xylem fluid. The high Cr accumulation in roots, up to 160  mg/kg, was
noticed in fodder radish grown in Cr polluted soil (247 mg Cr/kg), whereas its content of shoots did not exceed 10 mg/kg (Siebielec et al. 2012). All these data clearly
indicate that Cr is concentrated mainly in roots (Becquer et al. 2003).
Contents of Cr in plants have recently received much attention due to its importance as an essential micronutrient in human metabolic processes, and also because
of its carcinogenic impact. Thus, an adequate Cr supply in the diet, especially plant
diet, has been recently broadly investigated.
Mean Cr contents of cereal grains do not exit 90 mg/kg, and is the highest in oats
grains, up to 600 mg/kg (Eriksson 2001a). Very high Cr amounts are present in some
seeds and root vegetables. Grass and clover contain up to 4200 mg Cr/kg.
Toxic effects of excess Cr in plants resulted mainly in the poor protein formation, due to the disruption of N metabolisms, decreases of photosynthesis processes,
and a lower S uptake. Toxic effects of excess Cr on spinach plants depend on the
soil texture: 40 mg Cr/kg is toxic in sandy soils, whereas 320 mg Cr/kg is toxic in
clay loamy soils (Sharma et al. 2005). The phytotoxic Cr concentrations in tops of
plants are reported as follows (in mg/kg): tobacco, 18–24; corn, 4–8; and barley
seedlings, 10–100. In each case, Cr 6+ was more toxic than Cr 3+ . Some plants grown
on Cr-contaminated soils may develop Cr-tolerance mechanism, connected with
increased activities of some enzymes, superoxide dismutase, and peroxidase (Dong
et al. 2007; Pacha and Galimska-Stypa 1988).
The range of average Cr contents of mushroom bay bolete (Xerocomus badius)
sampled in 1993–1998, from the northern part of Poland, were similar in caps and
stalks, 0.22–0.75 and 0.22–0.71  mg/kg, respectively (Malinowska et al. 2004).
Common chanterelles (Cantharellus cibarius) grown in mountains contained a bit