Accumulation of Heavy Metals in Selected Medicinal Plants - Reviews of Environmental Contamination and Toxicology - page 68

3 Heavy Metal Accumulation in Selected Medicinal Plants:

Cause and Threat

Many papers have been published that address the analysis of heavy metals in medicinal
plants; the results presented in these papers have improved insights into what the heavy
metal accumulation levels of some medicinal plants are, or may be (Ajasa et al. 2004 ;
Koe and Sari 2009 ; Sharma et al. 2009 ; Sheded et al. 2006 ; Wong et al. 1993 ). In Table 1 ,
we have summarized data from the literature on the metal concentrations found in 88
medicinal plants. These data clearly illustrate that zinc is the metal that is most com-
monly found in medicinal plants, and is present in all species tested. These data also
reveal that metals vital for human health are present in many plant species at variable
concentrations. For example, Fe and Cu were found in 87 of the surveyed plant species,
Cl in 23 species, whereas Se and Au were found in 21 and 9 species, respectively.
However, many medicinal herbs accumulate heavy metals (e.g., As, Cd, Pb, and Hg),
which are variably hazardous to humans, in ways that are dependent on their oxidation
states and their concentrations (Lekouch et al . 2001 ).
Certain metals (e.g., Cr, Mn, Zn, and Cu) usually appear in green plants at low
concentrations, and the health of organisms consuming such plants is unaffected.
However, under certain environmental conditions, the concentration of metals may
be higher. For example, copper plant levels increase when the soil pH is low and
when organic fertilizers are used (Elekes et al. 2010 ). Similarly, in wet soils having
an acidic pH, half of the zinc present is bound to organic matter (Domany et al.
1996 ), binding increases and zinc levels can reach 5,000 mg/kg (Elekes et al. 2010 ).
Some plant species that absorb high metal levels have developed the ability to
detoxify them by forming metal-binding peptides called phytochelatin (PCs). Such
plants retain great capacity to absorb metals from the soil and transfer these metals
to plant-consuming biomass. Heavy metal ions present in plants are absorbed by
roots, taken to aerial plant parts and are bioaccumulated, along with any essential
metals that are present, such as Mn, Fe, Cu, Zn, and Se (Maiga et al. 2005 ). This is
an active physiological process, which requires energy (Fox and Guerinot 1998 ).
Depending on the plant species involved, plants base their tolerance to heavy metals
on two basic strategies: exclusion and accumulation (De Vas et al. 1991 ). The accu-
mulation strategy involves physiological processes that require the cells to maintain
the intracellular heavy metal ions, but in a nontoxic form (Cobbet 2000 ). Exclusion
occurs when any stored heavy metal ions or complexes are later removed by leaf fall
(Ernst et al. 1992 ). Accumulation of heavy metals in plant tissues is affected by the
characteristics of soil and atmosphere, and the uptake ability of plants (Bin et al.
2001 ). As Table 1 shows, medicinal plants do accumulate heavy metals in consider-
able quantities. In some marketed medicinal plants, the concentrations increase to
hazardous levels. Despite the presence of high metal levels, medicinal plants are
still used in phytopharmaceuticals. Therefore, one may assume that some propor-
tion of medicinal plants that are grown in metal contaminated soils are fated to suf-
fer phytotoxicity, and HM residues from them may reach the food chain and pose a
threat to human health (Oliver 1997 ; Maharia et al. 2010 ).